Working with Engines and Connections

This section details direct usage of the Engine, Connection, and related objects. Its important to note that when using the SQLAlchemy ORM, these objects are not generally accessed; instead, the Session object is used as the interface to the database. However, for applications that are built around direct usage of textual SQL statements and/or SQL expression constructs without involvement by the ORM’s higher level management services, the Engine and Connection are king (and queen?) - read on.

Basic Usage

Recall from Engine Configuration that an Engine is created via the create_engine() call:

engine = create_engine("mysql://scott:tiger@localhost/test")

The typical usage of create_engine() is once per particular database URL, held globally for the lifetime of a single application process. A single Engine manages many individual DBAPI connections on behalf of the process and is intended to be called upon in a concurrent fashion. The Engine is not synonymous to the DBAPI connect function, which represents just one connection resource - the Engine is most efficient when created just once at the module level of an application, not per-object or per-function call.

tip

When using an Engine with multiple Python processes, such as when using os.fork or Python multiprocessing, it’s important that the engine is initialized per process. See Using Connection Pools with Multiprocessing or os.fork() for details.

The most basic function of the Engine is to provide access to a Connection, which can then invoke SQL statements. To emit a textual statement to the database looks like:

from sqlalchemy import text

with engine.connect() as connection:
    result = connection.execute(text("select username from users"))
    for row in result:
        print("username:", row["username"])

Above, the Engine.connect() method returns a Connection object, and by using it in a Python context manager (e.g. the with: statement) the Connection.close() method is automatically invoked at the end of the block. The Connection, is a proxy object for an actual DBAPI connection. The DBAPI connection is retrieved from the connection pool at the point at which Connection is created.

The object returned is known as CursorResult, which references a DBAPI cursor and provides methods for fetching rows similar to that of the DBAPI cursor. The DBAPI cursor will be closed by the CursorResult when all of its result rows (if any) are exhausted. A CursorResult that returns no rows, such as that of an UPDATE statement (without any returned rows), releases cursor resources immediately upon construction.

When the Connection is closed at the end of the with: block, the referenced DBAPI connection is released to the connection pool. From the perspective of the database itself, the connection pool will not actually “close” the connection assuming the pool has room to store this connection for the next use. When the connection is returned to the pool for re-use, the pooling mechanism issues a rollback() call on the DBAPI connection so that any transactional state or locks are removed, and the connection is ready for its next use.

Our example above illustrated the execution of a textual SQL string, which should be invoked by using the text() construct to indicate that we’d like to use textual SQL. The Connection.execute() method can of course accommodate more than that, including the variety of SQL expression constructs described in SQL Expression Language Tutorial (1.x API).

Using Transactions

Note

This section describes how to use transactions when working directly with Engine and Connection objects. When using the SQLAlchemy ORM, the public API for transaction control is via the Session object, which makes usage of the Transaction object internally. See Managing Transactions for further information.

The Connection object provides a Connection.begin() method which returns a Transaction object. Like the Connection itself, this object is usually used within a Python with: block so that its scope is managed:

with engine.connect() as connection:
    with connection.begin():
        r1 = connection.execute(table1.select())
        connection.execute(table1.insert(), {"col1": 7, "col2": "this is some data"})

The above block can be stated more simply by using the Engine.begin() method of Engine:

# runs a transaction
with engine.begin() as connection:
    r1 = connection.execute(table1.select())
    connection.execute(table1.insert(), {"col1": 7, "col2": "this is some data"})

The block managed by each .begin() method has the behavior such that the transaction is committed when the block completes. If an exception is raised, the transaction is instead rolled back, and the exception propagated outwards.

The underlying object used to represent the transaction is the Transaction object. This object is returned by the Connection.begin() method and includes the methods Transaction.commit() and Transaction.rollback(). The context manager calling form, which invokes these methods automatically, is recommended as a best practice.

Nesting of Transaction Blocks

Deprecated since version 1.4: The “transaction nesting” feature of SQLAlchemy is a legacy feature that is deprecated in the 1.4 release and will be removed in SQLAlchemy 2.0. The pattern has proven to be a little too awkward and complicated, unless an application makes more of a first-class framework around the behavior. See the following subsection Arbitrary Transaction Nesting as an Antipattern.

The Transaction object also handles “nested” behavior by keeping track of the outermost begin/commit pair. In this example, two functions both issue a transaction on a Connection, but only the outermost Transaction object actually takes effect when it is committed.

# method_a starts a transaction and calls method_b
def method_a(connection):
    with connection.begin():  # open a transaction
        method_b(connection)


# method_b also starts a transaction
def method_b(connection):
    with connection.begin():  # open a transaction - this runs in the
        # context of method_a's transaction
        connection.execute(text("insert into mytable values ('bat', 'lala')"))
        connection.execute(mytable.insert(), {"col1": "bat", "col2": "lala"})


# open a Connection and call method_a
with engine.connect() as conn:
    method_a(conn)

Above, method_a is called first, which calls connection.begin(). Then it calls method_b. When method_b calls connection.begin(), it just increments a counter that is decremented when it calls commit(). If either method_a or method_b calls rollback(), the whole transaction is rolled back. The transaction is not committed until method_a calls the commit() method. This “nesting” behavior allows the creation of functions which “guarantee” that a transaction will be used if one was not already available, but will automatically participate in an enclosing transaction if one exists.

Arbitrary Transaction Nesting as an Antipattern

With many years of experience, the above “nesting” pattern has not proven to be very popular, and where it has been observed in large projects such as Openstack, it tends to be complicated.

The most ideal way to organize an application would have a single, or at least very few, points at which the “beginning” and “commit” of all database transactions is demarcated. This is also the general idea discussed in terms of the ORM at When do I construct a Session, when do I commit it, and when do I close it?. To adapt the example from the previous section to this practice looks like:

# method_a calls method_b
def method_a(connection):
    method_b(connection)


# method_b uses the connection and assumes the transaction
# is external
def method_b(connection):
    connection.execute(text("insert into mytable values ('bat', 'lala')"))
    connection.execute(mytable.insert(), {"col1": "bat", "col2": "lala"})


# open a Connection inside of a transaction and call method_a
with engine.begin() as conn:
    method_a(conn)

That is, method_a() and method_b() do not deal with the details of the transaction at all; the transactional scope of the connection is defined externally to the functions that have a SQL dialogue with the connection.

It may be observed that the above code has fewer lines, and less indentation which tends to correlate with lower cyclomatic complexity. The above code is organized such that method_a() and method_b() are always invoked from a point at which a transaction is begun. The previous version of the example features a method_a() and a method_b() that are trying to be agnostic of this fact, which suggests they are prepared for at least twice as many potential codepaths through them.

Migrating from the “nesting” pattern

As SQLAlchemy’s intrinsic-nested pattern is considered legacy, an application that for either legacy or novel reasons still seeks to have a context that automatically frames transactions should seek to maintain this functionality through the use of a custom Python context manager. A similar example is also provided in terms of the ORM in the “seealso” section below.

To provide backwards compatibility for applications that make use of this pattern, the following context manager or a similar implementation based on a decorator may be used:

import contextlib


@contextlib.contextmanager
def transaction(connection):
    if not connection.in_transaction():
        with connection.begin():
            yield connection
    else:
        yield connection

The above contextmanager would be used as:

# method_a starts a transaction and calls method_b
def method_a(connection):
    with transaction(connection):  # open a transaction
        method_b(connection)


# method_b either starts a transaction, or uses the one already
# present
def method_b(connection):
    with transaction(connection):  # open a transaction
        connection.execute(text("insert into mytable values ('bat', 'lala')"))
        connection.execute(mytable.insert(), {"col1": "bat", "col2": "lala"})


# open a Connection and call method_a
with engine.connect() as conn:
    method_a(conn)

A similar approach may be taken such that connectivity is established on demand as well; the below approach features a single-use context manager that accesses an enclosing state in order to test if connectivity is already present:

import contextlib


def connectivity(engine):
    connection = None

    @contextlib.contextmanager
    def connect():
        nonlocal connection

        if connection is None:
            connection = engine.connect()
            with connection:
                with connection.begin():
                    yield connection
        else:
            yield connection

    return connect

Using the above would look like:

# method_a passes along connectivity context, at the same time
# it chooses to establish a connection by calling "with"
def method_a(connectivity):
    with connectivity():
        method_b(connectivity)


# method_b also wants to use a connection from the context, so it
# also calls "with:", but also it actually uses the connection.
def method_b(connectivity):
    with connectivity() as connection:
        connection.execute(text("insert into mytable values ('bat', 'lala')"))
        connection.execute(mytable.insert(), {"col1": "bat", "col2": "lala"})


# create a new connection/transaction context object and call
# method_a
method_a(connectivity(engine))

The above context manager acts not only as a “transaction” context but also as a context that manages having an open connection against a particular Engine. When using the ORM Session, this connectivty management is provided by the Session itself. An overview of ORM connectivity patterns is at Managing Transactions.

See also

Migrating from the “subtransaction” pattern - ORM version

Library Level (e.g. emulated) Autocommit

Deprecated since version 1.4: The “autocommit” feature of SQLAlchemy Core is deprecated and will not be present in version 2.0 of SQLAlchemy. DBAPI-level AUTOCOMMIT is now widely available which offers superior performance and occurs transparently. See Library-level (but not driver level) “Autocommit” removed from both Core and ORM for background.

Note

This section discusses the feature within SQLAlchemy that automatically invokes the .commit() method on a DBAPI connection, however this is against a DBAPI connection that is itself transactional. For true AUTOCOMMIT, see the next section Setting Transaction Isolation Levels including DBAPI Autocommit.

The previous transaction example illustrates how to use Transaction so that several executions can take part in the same transaction. What happens when we issue an INSERT, UPDATE or DELETE call without using Transaction? While some DBAPI implementations provide various special “non-transactional” modes, the core behavior of DBAPI per PEP-0249 is that a transaction is always in progress, providing only rollback() and commit() methods but no begin(). SQLAlchemy assumes this is the case for any given DBAPI.

Given this requirement, SQLAlchemy implements its own “autocommit” feature which works completely consistently across all backends. This is achieved by detecting statements which represent data-changing operations, i.e. INSERT, UPDATE, DELETE, as well as data definition language (DDL) statements such as CREATE TABLE, ALTER TABLE, and then issuing a COMMIT automatically if no transaction is in progress. The detection is based on the presence of the autocommit=True execution option on the statement. If the statement is a text-only statement and the flag is not set, a regular expression is used to detect INSERT, UPDATE, DELETE, as well as a variety of other commands for a particular backend:

conn = engine.connect()
conn.execute(text("INSERT INTO users VALUES (1, 'john')"))  # autocommits

The “autocommit” feature is only in effect when no Transaction has otherwise been declared. This means the feature is not generally used with the ORM, as the Session object by default always maintains an ongoing Transaction.

Full control of the “autocommit” behavior is available using the generative Connection.execution_options() method provided on Connection and Engine, using the “autocommit” flag which will turn on or off the autocommit for the selected scope. For example, a text() construct representing a stored procedure that commits might use it so that a SELECT statement will issue a COMMIT:

with engine.connect().execution_options(autocommit=True) as conn:
    conn.execute(text("SELECT my_mutating_procedure()"))

Setting Transaction Isolation Levels including DBAPI Autocommit

Most DBAPIs support the concept of configurable transaction isolation levels. These are traditionally the four levels “READ UNCOMMITTED”, “READ COMMITTED”, “REPEATABLE READ” and “SERIALIZABLE”. These are usually applied to a DBAPI connection before it begins a new transaction, noting that most DBAPIs will begin this transaction implicitly when SQL statements are first emitted.

DBAPIs that support isolation levels also usually support the concept of true “autocommit”, which means that the DBAPI connection itself will be placed into a non-transactional autocommit mode. This usually means that the typical DBAPI behavior of emitting “BEGIN” to the database automatically no longer occurs, but it may also include other directives. SQLAlchemy treats the concept of “autocommit” like any other isolation level; in that it is an isolation level that loses not only “read committed” but also loses atomicity.

Tip

It is important to note, as will be discussed further in the section below at Understanding the DBAPI-Level Autocommit Isolation Level, that “autocommit” isolation level like any other isolation level does not affect the “transactional” behavior of the Connection object, which continues to call upon DBAPI .commit() and .rollback() methods (they just have no effect under autocommit), and for which the .begin() method assumes the DBAPI will start a transaction implicitly (which means that SQLAlchemy’s “begin” does not change autocommit mode).

SQLAlchemy dialects should support these isolation levels as well as autocommit to as great a degree as possible. The levels are set via family of “execution_options” parameters and methods that are throughout the Core, such as the Connection.execution_options() method. The parameter is known as Connection.execution_options.isolation_level and the values are strings which are typically a subset of the following names:

# possible values for Connection.execution_options(isolation_level="<value>")

"AUTOCOMMIT"
"READ COMMITTED"
"READ UNCOMMITTED"
"REPEATABLE READ"
"SERIALIZABLE"

Not every DBAPI supports every value; if an unsupported value is used for a certain backend, an error is raised.

For example, to force REPEATABLE READ on a specific connection, then begin a transaction:

with engine.connect().execution_options(isolation_level="REPEATABLE READ") as connection:
    with connection.begin():
        connection.execute(<statement>)

Note

The return value of the Connection.execution_options() method is a so-called “branched” connection under the SQLAlchemy 1.x series when not using create_engine.future mode, which is a shallow copy of the original Connection object. Despite this, the isolation_level execution option applies to the original Connection object and all “branches” overall.

When using create_engine.future mode (i.e. 2.0 style usage), the concept of these so-called “branched” connections is removed, and Connection.execution_options() returns the same Connection object without creating any copies.

The Connection.execution_options.isolation_level option may also be set engine wide, as is often preferable. This is achieved by passing it within the create_engine.execution_options parameter to create_engine():

from sqlalchemy import create_engine

eng = create_engine(
    "postgresql://scott:tiger@localhost/test",
    execution_options={"isolation_level": "REPEATABLE READ"},
)

With the above setting, the DBAPI connection will be set to use a "REPEATABLE READ" isolation level setting for each new transaction begun.

An application that frequently chooses to run operations within different isolation levels may wish to create multiple “sub-engines” of a lead Engine, each of which will be configured to a different isolation level. One such use case is an application that has operations that break into “transactional” and “read-only” operations, a separate Engine that makes use of "AUTOCOMMIT" may be separated off from the main engine:

from sqlalchemy import create_engine

eng = create_engine("postgresql://scott:tiger@localhost/test")

autocommit_engine = eng.execution_options(isolation_level="AUTOCOMMIT")

Above, the Engine.execution_options() method creates a shallow copy of the original Engine. Both eng and autocommit_engine share the same dialect and connection pool. However, the “AUTOCOMMIT” mode will be set upon connections when they are acquired from the autocommit_engine.

The isolation level setting, regardless of which one it is, is unconditionally reverted when a connection is returned to the connection pool.

Note

The Connection.execution_options.isolation_level parameter necessarily does not apply to statement level options, such as that of Executable.execution_options(). This because the option must be set on a DBAPI connection on a per-transaction basis.

See also

SQLite Transaction Isolation

PostgreSQL Transaction Isolation

MySQL Transaction Isolation

SQL Server Transaction Isolation

Setting Transaction Isolation Levels / DBAPI AUTOCOMMIT - for the ORM

Using DBAPI Autocommit Allows for a Readonly Version of Transparent Reconnect - a recipe that uses DBAPI autocommit to transparently reconnect to the database for read-only operations

Understanding the DBAPI-Level Autocommit Isolation Level

In the parent section, we introduced the concept of the Connection.execution_options.isolation_level parameter and how it can be used to set database isolation levels, including DBAPI-level “autocommit” which is treated by SQLAlchemy as another transaction isolation level. In this section we will attempt to clarify the implications of this approach.

If we wanted to check out a Connection object and use it “autocommit” mode, we would proceed as follows:

with engine.connect() as connection:
    connection.execution_options(isolation_level="AUTOCOMMIT")
    connection.execute(<statement>)
    connection.execute(<statement>)

Above illustrates normal usage of “DBAPI autocommit” mode. There is no need to make use of methods such as Connection.begin() or Connection.commit() (noting the latter applies to 2.0 style usage).

What’s important to note however is that the above autocommit mode is persistent on that particular Connection until we change it directly using isolation_level again. The isolation level is also reset on the DBAPI connection when we release the connection back to the connection pool. However, calling upon Connection.begin() will not change the isolation level, meaning we stay in autocommit. The example below illustrates this:

with engine.connect() as connection:
    connection = connection.execution_options(isolation_level="AUTOCOMMIT")

    # this begin() does nothing, isolation stays at AUTOCOMMIT
    with connection.begin() as trans:
        connection.execute(<statement>)
        connection.execute(<statement>)

When we run a block like the above with logging turned on, the logging will attempt to indicate that while a DBAPI level .commit() is called, it probably will have no effect due to autocommit mode:

INFO sqlalchemy.engine.Engine BEGIN (implicit)
...
INFO sqlalchemy.engine.Engine COMMIT using DBAPI connection.commit(), DBAPI should ignore due to autocommit mode

Similarly, when using 2.0 style create_engine.future mode, the Connection will use autobegin behavior, meaning that the pattern below will raise an error:

engine = create_engine(..., future=True)

with engine.connect() as connection:
    connection = connection.execution_options(isolation_level="AUTOCOMMIT")

    # "transaction" is autobegin (but has no effect due to autocommit)
    connection.execute(<statement>)

    # this will raise; "transaction" is already begun
    with connection.begin() as trans:
        connection.execute(<statement>)

This is all to demonstrate that the autocommit isolation level setting is completely independent from the begin/commit behavior of the SQLAlchemy Connection object. The “autocommit” mode will not interact with Connection.begin() in any way and the Connection does not consult this status when performing its own state changes with regards to the transaction (with the exception of suggesting within engine logging that these blocks are not actually committing). The rationale for this design is to maintain a completely consistent usage pattern with the Connection where DBAPI-autocommit mode can be changed independently without indicating any code changes elsewhere.

Isolation level settings, including autocommit mode, are reset automatically when the connection is released back to the connection pool. Therefore it is preferable to avoid trying to switch isolation levels on a single Connection object as this leads to excess verbosity.

To illustrate how to use “autocommit” in an ad-hoc mode within the scope of a single Connection checkout, the Connection.execution_options.isolation_level parameter must be re-applied with the previous isolation level. We can write our above block “correctly” as (noting 2.0 style usage below):

# if we wanted to flip autocommit on and off on a single connection/
# which... we usually don't.

engine = create_engine(..., future=True)

with engine.connect() as connection:

    connection.execution_options(isolation_level="AUTOCOMMIT")

    # run statement(s) in autocommit mode
    connection.execute(<statement>)

    # "commit" the autobegun "transaction" (2.0/future mode only)
    connection.commit()

    # switch to default isolation level
    connection.execution_options(isolation_level=connection.default_isolation_level)

    # use a begin block
    with connection.begin() as trans:
        connection.execute(<statement>)

Above, to manually revert the isolation level we made use of Connection.default_isolation_level to restore the default isolation level (assuming that’s what we want here). However, it’s probably a better idea to work with the architecture of of the Connection which already handles resetting of isolation level automatically upon checkin. The preferred way to write the above is to use two blocks

engine = create_engine(..., future=True)

# use an autocommit block
with engine.connect().execution_options(isolation_level="AUTOCOMMIT") as connection:

    # run statement in autocommit mode
    connection.execute(<statement>)

# use a regular block
with engine.begin() as connection:
    connection.execute(<statement>)

To sum up:

1. “DBAPI level autocommit” isolation level is entirely independent of the Connection object’s notion of “begin” and “commit”

2. use individual Connection checkouts per isolation level. Avoid trying to change back and forth between “autocommit” on a single connection checkout; let the engine do the work of restoring default isolation levels

Using Server Side Cursors (a.k.a. stream results)

Some backends feature explicit support for the concept of “server side cursors” versus “client side cursors”. A client side cursor here means that the database driver fully fetches all rows from a result set into memory before returning from a statement execution. Drivers such as those of PostgreSQL and MySQL/MariaDB generally use client side cursors by default. A server side cursor, by contrast, indicates that result rows remain pending within the database server’s state as result rows are consumed by the client. The drivers for Oracle generally use a “server side” model, for example, and the SQLite dialect, while not using a real “client / server” architecture, still uses an unbuffered result fetching approach that will leave result rows outside of process memory before they are consumed.

What we really mean is “buffered” vs. “unbuffered” results

Server side cursors also imply a wider set of features with relational databases, such as the ability to “scroll” a cursor forwards and backwards. SQLAlchemy does not include any explicit support for these behaviors; within SQLAlchemy itself, the general term “server side cursors” should be considered to mean “unbuffered results” and “client side cursors” means “result rows are buffered into memory before the first row is returned”. To work with a richer “server side cursor” featureset specific to a certain DBAPI driver, see the section Working with the DBAPI cursor directly.

From this basic architecture it follows that a “server side cursor” is more memory efficient when fetching very large result sets, while at the same time may introduce more complexity in the client/server communication process and be less efficient for small result sets (typically less than 10000 rows).

For those dialects that have conditional support for buffered or unbuffered results, there are usually caveats to the use of the “unbuffered”, or server side cursor mode. When using the psycopg2 dialect for example, an error is raised if a server side cursor is used with any kind of DML or DDL statement. When using MySQL drivers with a server side cursor, the DBAPI connection is in a more fragile state and does not recover as gracefully from error conditions nor will it allow a rollback to proceed until the cursor is fully closed.

For this reason, SQLAlchemy’s dialects will always default to the less error prone version of a cursor, which means for PostgreSQL and MySQL dialects it defaults to a buffered, “client side” cursor where the full set of results is pulled into memory before any fetch methods are called from the cursor. This mode of operation is appropriate in the vast majority of cases; unbuffered cursors are not generally useful except in the uncommon case of an application fetching a very large number of rows in chunks, where the processing of these rows can be complete before more rows are fetched.

For database drivers that provide client and server side cursor options, the Connection.execution_options.stream_results and Connection.execution_options.yield_per execution options provide access to “server side cursors” on a per-Connection or per-statement basis. Similar options exist when using an ORM Session as well.

Streaming with a fixed buffer via yield_per

As individual row-fetch operations with fully unbuffered server side cursors are typically more expensive than fetching batches of rows at once, The Connection.execution_options.yield_per execution option configures a Connection or statement to make use of server-side cursors as are available, while at the same time configuring a fixed-size buffer of rows that will retrieve rows from the server in batches as they are consumed. This parameter may be to a positive integer value using the Connection.execution_options() method on Connection or on a statement using the Executable.execution_options() method.

New in version 1.4.40: Connection.execution_options.yield_per as a Core-only option is new as of SQLAlchemy 1.4.40; for prior 1.4 versions, use Connection.execution_options.stream_results directly in combination with Result.yield_per().

Using this option is equivalent to manually setting the Connection.execution_options.stream_results option, described in the next section, and then invoking the Result.yield_per() method on the Result object with the given integer value. In both cases, the effect this combination has includes:

• server side cursors mode is selected for the given backend, if available and not already the default behavior for that backend

• as result rows are fetched, they will be buffered in batches, where the size of each batch up until the last batch will be equal to the integer argument passed to the Connection.execution_options.yield_per option or the Result.yield_per() method; the last batch is then sized against the remaining rows fewer than this size

• The default partition size used by the Result.partitions() method, if used, will be made equal to this integer size as well.

These three behaviors are illustrated in the example below:

with engine.connect() as conn:
    result = conn.execution_options(yield_per=100).execute(text("select * from table"))

    for partition in result.partitions():
        # partition is an iterable that will be at most 100 items
        for row in partition:
            print(f"{row}")

The above example illustrates the combination of yield_per=100 along with using the Result.partitions() method to run processing on rows in batches that match the size fetched from the server. The use of Result.partitions() is optional, and if the Result is iterated directly, a new batch of rows will be buffered for each 100 rows fetched. Calling a method such as Result.all() should not be used, as this will fully fetch all remaining rows at once and defeat the purpose of using yield_per.

The Connection.execution_options.yield_per option is portable to the ORM as well, used by a Session to fetch ORM objects, where it also limits the amount of ORM objects generated at once. See the section Fetching Large Result Sets with Yield Per - in the ORM Querying Guide for further background on using Connection.execution_options.yield_per with the ORM.

New in version 1.4.40: Added Connection.execution_options.yield_per as a Core level execution option to conveniently set streaming results, buffer size, and partition size all at once in a manner that is transferrable to that of the ORM’s similar use case.

Streaming with a dynamically growing buffer using stream_results

To enable server side cursors without a specific partition size, the Connection.execution_options.stream_results option may be used, which like Connection.execution_options.yield_per may be called on the Connection object or the statement object.

When a Result object delivered using the Connection.execution_options.stream_results option is iterated directly, rows are fetched internally using a default buffering scheme that buffers first a small set of rows, then a larger and larger buffer on each fetch up to a pre-configured limit of 1000 rows. The maximum size of this buffer can be affected using the Connection.execution_options.max_row_buffer execution option:

with engine.connect() as conn:
    conn = conn.execution_options(stream_results=True, max_row_buffer=100)
    result = conn.execute(text("select * from table"))

    for row in result:
        print(f"{row}")

While the Connection.execution_options.stream_results option may be combined with use of the Result.partitions() method, a specific partition size should be passed to Result.partitions() so that the entire result is not fetched. It is usually more straightforward to use the Connection.execution_options.yield_per option when setting up to use the Result.partitions() method.

See also

Fetching Large Result Sets with Yield Per - in the ORM Querying Guide

Result.partitions()

Result.yield_per()

Connectionless Execution, Implicit Execution

Deprecated since version 2.0: The features of “connectionless” and “implicit” execution in SQLAlchemy are deprecated and will be removed in version 2.0. See “Implicit” and “Connectionless” execution, “bound metadata” removed for background.

Recall from the first section we mentioned executing with and without explicit usage of Connection. “Connectionless” execution refers to the usage of the execute() method on an object which is not a Connection. This was illustrated using the Engine.execute() method of Engine:

result = engine.execute(text("select username from users"))
for row in result:
    print("username:", row["username"])

In addition to “connectionless” execution, it is also possible to use the Executable.execute() method of any Executable construct, which is a marker for SQL expression objects that support execution. The SQL expression object itself references an Engine or Connection known as the bind, which it uses in order to provide so-called “implicit” execution services.

Given a table as below:

from sqlalchemy import MetaData, Table, Column, Integer

metadata_obj = MetaData()
users_table = Table(
    "users",
    metadata_obj,
    Column("id", Integer, primary_key=True),
    Column("name", String(50)),
)

Explicit execution delivers the SQL text or constructed SQL expression to the Connection.execute() method of Connection:

engine = create_engine('sqlite:///file.db')
with engine.connect() as connection:
    result = connection.execute(users_table.select())
    for row in result:
        # ....

Explicit, connectionless execution delivers the expression to the Engine.execute() method of Engine:

engine = create_engine('sqlite:///file.db')
result = engine.execute(users_table.select())
for row in result:
    # ....
result.close()

Implicit execution is also connectionless, and makes usage of the Executable.execute() method on the expression itself. This method is provided as part of the Executable class, which refers to a SQL statement that is sufficient for being invoked against the database. The method makes usage of the assumption that either an Engine or Connection has been bound to the expression object. By “bound” we mean that the special attribute MetaData.bind has been used to associate a series of Table objects and all SQL constructs derived from them with a specific engine:

engine = create_engine('sqlite:///file.db')
metadata_obj.bind = engine
result = users_table.select().execute()
for row in result:
    # ....
result.close()

Above, we associate an Engine with a MetaData object using the special attribute MetaData.bind. The select() construct produced from the Table object has a method Executable.execute(), which will search for an Engine that’s “bound” to the Table.

Overall, the usage of “bound metadata” has three general effects:

• SQL statement objects gain an Executable.execute() method which automatically locates a “bind” with which to execute themselves.

• The ORM Session object supports using “bound metadata” in order to establish which Engine should be used to invoke SQL statements on behalf of a particular mapped class, though the Session also features its own explicit system of establishing complex Engine/ mapped class configurations.

• The MetaData.create_all(), MetaData.drop_all(), Table.create(), Table.drop(), and “autoload” features all make usage of the bound Engine automatically without the need to pass it explicitly.

Note

The concepts of “bound metadata” and “implicit execution” are not emphasized in modern SQLAlchemy. While they offer some convenience, they are no longer required by any API and are never necessary.

In applications where multiple Engine objects are present, each one logically associated with a certain set of tables (i.e. vertical sharding), the “bound metadata” technique can be used so that individual Table can refer to the appropriate Engine automatically; in particular this is supported within the ORM via the Session object as a means to associate Table objects with an appropriate Engine, as an alternative to using the bind arguments accepted directly by the Session.

However, the “implicit execution” technique is not at all appropriate for use with the ORM, as it bypasses the transactional context maintained by the Session.

Overall, in the vast majority of cases, “bound metadata” and “implicit execution” are not useful. While “bound metadata” has a marginal level of usefulness with regards to ORM configuration, “implicit execution” is a very old usage pattern that in most cases is more confusing than it is helpful, and its usage is discouraged. Both patterns seem to encourage the overuse of expedient “short cuts” in application design which lead to problems later on.

Modern SQLAlchemy usage, especially the ORM, places a heavy stress on working within the context of a transaction at all times; the “implicit execution” concept makes the job of associating statement execution with a particular transaction much more difficult. The Executable.execute() method on a particular SQL statement usually implies that the execution is not part of any particular transaction, which is usually not the desired effect.

In both “connectionless” examples, the Connection is created behind the scenes; the CursorResult returned by the execute() call references the Connection used to issue the SQL statement. When the CursorResult is closed, the underlying Connection is closed for us, resulting in the DBAPI connection being returned to the pool with transactional resources removed.

Translation of Schema Names

To support multi-tenancy applications that distribute common sets of tables into multiple schemas, the Connection.execution_options.schema_translate_map execution option may be used to repurpose a set of Table objects to render under different schema names without any changes.

Given a table:

user_table = Table(
    "user",
    metadata_obj,
    Column("id", Integer, primary_key=True),
    Column("name", String(50)),
)

The “schema” of this Table as defined by the Table.schema attribute is None. The Connection.execution_options.schema_translate_map can specify that all Table objects with a schema of None would instead render the schema as user_schema_one:

connection = engine.connect().execution_options(
    schema_translate_map={None: "user_schema_one"}
)

result = connection.execute(user_table.select())

The above code will invoke SQL on the database of the form:

SELECT user_schema_one.user.id, user_schema_one.user.name FROM
user_schema_one.user

That is, the schema name is substituted with our translated name. The map can specify any number of target->destination schemas:

connection = engine.connect().execution_options(
    schema_translate_map={
        None: "user_schema_one",  # no schema name -> "user_schema_one"
        "special": "special_schema",  # schema="special" becomes "special_schema"
        "public": None,  # Table objects with schema="public" will render with no schema
    }
)

The Connection.execution_options.schema_translate_map parameter affects all DDL and SQL constructs generated from the SQL expression language, as derived from the Table or Sequence objects. It does not impact literal string SQL used via the text() construct nor via plain strings passed to Connection.execute().

The feature takes effect only in those cases where the name of the schema is derived directly from that of a Table or Sequence; it does not impact methods where a string schema name is passed directly. By this pattern, it takes effect within the “can create” / “can drop” checks performed by methods such as MetaData.create_all() or MetaData.drop_all() are called, and it takes effect when using table reflection given a Table object. However it does not affect the operations present on the Inspector object, as the schema name is passed to these methods explicitly.

Tip

To use the schema translation feature with the ORM Session, set this option at the level of the Engine, then pass that engine to the Session. The Session uses a new Connection for each transaction:

schema_engine = engine.execution_options(schema_translate_map={...})

session = Session(schema_engine)

...

Warning

When using the ORM Session without extensions, the schema translate feature is only supported as a single schema translate map per Session. It will not work if different schema translate maps are given on a per-statement basis, as the ORM Session does not take current schema translate values into account for individual objects.

To use a single Session with multiple schema_translate_map configurations, the Horizontal Sharding extension may be used. See the example at Horizontal Sharding.

New in version 1.1.

SQL Compilation Caching

New in version 1.4: SQLAlchemy now has a transparent query caching system that substantially lowers the Python computational overhead involved in converting SQL statement constructs into SQL strings across both Core and ORM. See the introduction at Transparent SQL Compilation Caching added to All DQL, DML Statements in Core, ORM.

SQLAlchemy includes a comprehensive caching system for the SQL compiler as well as its ORM variants. This caching system is transparent within the Engine and provides that the SQL compilation process for a given Core or ORM SQL statement, as well as related computations which assemble result-fetching mechanics for that statement, will only occur once for that statement object and all others with the identical structure, for the duration that the particular structure remains within the engine’s “compiled cache”. By “statement objects that have the identical structure”, this generally corresponds to a SQL statement that is constructed within a function and is built each time that function runs:

def run_my_statement(connection, parameter):
    stmt = select(table)
    stmt = stmt.where(table.c.col == parameter)
    stmt = stmt.order_by(table.c.id)
    return connection.execute(stmt)

The above statement will generate SQL resembling SELECT id, col FROM table WHERE col = :col ORDER BY id, noting that while the value of parameter is a plain Python object such as a string or an integer, the string SQL form of the statement does not include this value as it uses bound parameters. Subsequent invocations of the above run_my_statement() function will use a cached compilation construct within the scope of the connection.execute() call for enhanced performance.

Note

it is important to note that the SQL compilation cache is caching the SQL string that is passed to the database only, and not the data returned by a query. It is in no way a data cache and does not impact the results returned for a particular SQL statement nor does it imply any memory use linked to fetching of result rows.

While SQLAlchemy has had a rudimentary statement cache since the early 1.x series, and additionally has featured the “Baked Query” extension for the ORM, both of these systems required a high degree of special API use in order for the cache to be effective. The new cache as of 1.4 is instead completely automatic and requires no change in programming style to be effective.

The cache is automatically used without any configurational changes and no special steps are needed in order to enable it. The following sections detail the configuration and advanced usage patterns for the cache.

Configuration

The cache itself is a dictionary-like object called an LRUCache, which is an internal SQLAlchemy dictionary subclass that tracks the usage of particular keys and features a periodic “pruning” step which removes the least recently used items when the size of the cache reaches a certain threshold. The size of this cache defaults to 500 and may be configured using the create_engine.query_cache_size parameter:

engine = create_engine("postgresql://scott:tiger@localhost/test", query_cache_size=1200)

The size of the cache can grow to be a factor of 150% of the size given, before it’s pruned back down to the target size. A cache of size 1200 above can therefore grow to be 1800 elements in size at which point it will be pruned to 1200.

The sizing of the cache is based on a single entry per unique SQL statement rendered, per engine. SQL statements generated from both the Core and the ORM are treated equally. DDL statements will usually not be cached. In order to determine what the cache is doing, engine logging will include details about the cache’s behavior, described in the next section.

Estimating Cache Performance Using Logging

The above cache size of 1200 is actually fairly large. For small applications, a size of 100 is likely sufficient. To estimate the optimal size of the cache, assuming enough memory is present on the target host, the size of the cache should be based on the number of unique SQL strings that may be rendered for the target engine in use. The most expedient way to see this is to use SQL echoing, which is most directly enabled by using the create_engine.echo flag, or by using Python logging; see the section Configuring Logging for background on logging configuration.

As an example, we will examine the logging produced by the following program:

from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session

Base = declarative_base()


class A(Base):
    __tablename__ = "a"

    id = Column(Integer, primary_key=True)
    data = Column(String)
    bs = relationship("B")


class B(Base):
    __tablename__ = "b"
    id = Column(Integer, primary_key=True)
    a_id = Column(ForeignKey("a.id"))
    data = Column(String)


e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)

s = Session(e)

s.add_all([A(bs=[B(), B(), B()]), A(bs=[B(), B(), B()]), A(bs=[B(), B(), B()])])
s.commit()

for a_rec in s.query(A):
    print(a_rec.bs)

When run, each SQL statement that’s logged will include a bracketed cache statistics badge to the left of the parameters passed. The four types of message we may see are summarized as follows:

• [raw sql] - the driver or the end-user emitted raw SQL using Connection.exec_driver_sql() - caching does not apply

• [no key] - the statement object is a DDL statement that is not cached, or the statement object contains uncacheable elements such as user-defined constructs or arbitrarily large VALUES clauses.

• [generated in Xs] - the statement was a cache miss and had to be compiled, then stored in the cache. it took X seconds to produce the compiled construct. The number X will be in the small fractional seconds.

• [cached since Xs ago] - the statement was a cache hit and did not have to be recompiled. The statement has been stored in the cache since X seconds ago. The number X will be proportional to how long the application has been running and how long the statement has been cached, so for example would be 86400 for a 24 hour period.

Each badge is described in more detail below.

The first statements we see for the above program will be the SQLite dialect checking for the existence of the “a” and “b” tables:

INFO sqlalchemy.engine.Engine PRAGMA temp.table_info("a")
INFO sqlalchemy.engine.Engine [raw sql] ()
INFO sqlalchemy.engine.Engine PRAGMA main.table_info("b")
INFO sqlalchemy.engine.Engine [raw sql] ()

For the above two SQLite PRAGMA statements, the badge reads [raw sql], which indicates the driver is sending a Python string directly to the database using Connection.exec_driver_sql(). Caching does not apply to such statements because they already exist in string form, and there is nothing known about what kinds of result rows will be returned since SQLAlchemy does not parse SQL strings ahead of time.

The next statements we see are the CREATE TABLE statements:

INFO sqlalchemy.engine.Engine
CREATE TABLE a (
  id INTEGER NOT NULL,
  data VARCHAR,
  PRIMARY KEY (id)
)

INFO sqlalchemy.engine.Engine [no key 0.00007s] ()
INFO sqlalchemy.engine.Engine
CREATE TABLE b (
  id INTEGER NOT NULL,
  a_id INTEGER,
  data VARCHAR,
  PRIMARY KEY (id),
  FOREIGN KEY(a_id) REFERENCES a (id)
)

INFO sqlalchemy.engine.Engine [no key 0.00006s] ()

For each of these statements, the badge reads [no key 0.00006s]. This indicates that these two particular statements, caching did not occur because the DDL-oriented CreateTable construct did not produce a cache key. DDL constructs generally do not participate in caching because they are not typically subject to being repeated a second time and DDL is also a database configurational step where performance is not as critical.

The [no key] badge is important for one other reason, as it can be produced for SQL statements that are cacheable except for some particular sub-construct that is not currently cacheable. Examples of this include custom user-defined SQL elements that don’t define caching parameters, as well as some constructs that generate arbitrarily long and non-reproducible SQL strings, the main examples being the Values construct as well as when using “multivalued inserts” with the Insert.values() method.

So far our cache is still empty. The next statements will be cached however, a segment looks like:

INFO sqlalchemy.engine.Engine INSERT INTO a (data) VALUES (?)
INFO sqlalchemy.engine.Engine [generated in 0.00011s] (None,)
INFO sqlalchemy.engine.Engine INSERT INTO a (data) VALUES (?)
INFO sqlalchemy.engine.Engine [cached since 0.0003533s ago] (None,)
INFO sqlalchemy.engine.Engine INSERT INTO a (data) VALUES (?)
INFO sqlalchemy.engine.Engine [cached since 0.0005326s ago] (None,)
INFO sqlalchemy.engine.Engine INSERT INTO b (a_id, data) VALUES (?, ?)
INFO sqlalchemy.engine.Engine [generated in 0.00010s] (1, None)
INFO sqlalchemy.engine.Engine INSERT INTO b (a_id, data) VALUES (?, ?)
INFO sqlalchemy.engine.Engine [cached since 0.0003232s ago] (1, None)
INFO sqlalchemy.engine.Engine INSERT INTO b (a_id, data) VALUES (?, ?)
INFO sqlalchemy.engine.Engine [cached since 0.0004887s ago] (1, None)

Above, we see essentially two unique SQL strings; "INSERT INTO a (data) VALUES (?)" and "INSERT INTO b (a_id, data) VALUES (?, ?)". Since SQLAlchemy uses bound parameters for all literal values, even though these statements are repeated many times for different objects, because the parameters are separate, the actual SQL string stays the same.

Note

the above two statements are generated by the ORM unit of work process, and in fact will be caching these in a separate cache that is local to each mapper. However the mechanics and terminology are the same. The section Disabling or using an alternate dictionary to cache some (or all) statements below will describe how user-facing code can also use an alternate caching container on a per-statement basis.

The caching badge we see for the first occurrence of each of these two statements is [generated in 0.00011s]. This indicates that the statement was not in the cache, was compiled into a String in .00011s and was then cached. When we see the [generated] badge, we know that this means there was a cache miss. This is to be expected for the first occurrence of a particular statement. However, if lots of new [generated] badges are observed for a long-running application that is generally using the same series of SQL statements over and over, this may be a sign that the create_engine.query_cache_size parameter is too small. When a statement that was cached is then evicted from the cache due to the LRU cache pruning lesser used items, it will display the [generated] badge when it is next used.

The caching badge that we then see for the subsequent occurrences of each of these two statements looks like [cached since 0.0003533s ago]. This indicates that the statement was found in the cache, and was originally placed into the cache .0003533 seconds ago. It is important to note that while the [generated] and [cached since] badges refer to a number of seconds, they mean different things; in the case of [generated], the number is a rough timing of how long it took to compile the statement, and will be an extremely small amount of time. In the case of [cached since], this is the total time that a statement has been present in the cache. For an application that’s been running for six hours, this number may read [cached since 21600 seconds ago], and that’s a good thing. Seeing high numbers for “cached since” is an indication that these statements have not been subject to cache misses for a long time. Statements that frequently have a low number of “cached since” even if the application has been running a long time may indicate these statements are too frequently subject to cache misses, and that the create_engine.query_cache_size may need to be increased.

Our example program then performs some SELECTs where we can see the same pattern of “generated” then “cached”, for the SELECT of the “a” table as well as for subsequent lazy loads of the “b” table:

INFO sqlalchemy.engine.Engine SELECT a.id AS a_id, a.data AS a_data
FROM a
INFO sqlalchemy.engine.Engine [generated in 0.00009s] ()
INFO sqlalchemy.engine.Engine SELECT b.id AS b_id, b.a_id AS b_a_id, b.data AS b_data
FROM b
WHERE ? = b.a_id
INFO sqlalchemy.engine.Engine [generated in 0.00010s] (1,)
INFO sqlalchemy.engine.Engine SELECT b.id AS b_id, b.a_id AS b_a_id, b.data AS b_data
FROM b
WHERE ? = b.a_id
INFO sqlalchemy.engine.Engine [cached since 0.0005922s ago] (2,)
INFO sqlalchemy.engine.Engine SELECT b.id AS b_id, b.a_id AS b_a_id, b.data AS b_data
FROM b
WHERE ? = b.a_id

From our above program, a full run shows a total of four distinct SQL strings being cached. Which indicates a cache size of four would be sufficient. This is obviously an extremely small size, and the default size of 500 is fine to be left at its default.

How much memory does the cache use?

The previous section detailed some techniques to check if the create_engine.query_cache_size needs to be bigger. How do we know if the cache is not too large? The reason we may want to set create_engine.query_cache_size to not be higher than a certain number would be because we have an application that may make use of a very large number of different statements, such as an application that is building queries on the fly from a search UX, and we don’t want our host to run out of memory if for example, a hundred thousand different queries were run in the past 24 hours and they were all cached.

It is extremely difficult to measure how much memory is occupied by Python data structures, however using a process to measure growth in memory via top as a successive series of 250 new statements are added to the cache suggest a moderate Core statement takes up about 12K while a small ORM statement takes about 20K, including result-fetching structures which for the ORM will be much greater.

Disabling or using an alternate dictionary to cache some (or all) statements

The internal cache used is known as LRUCache, but this is mostly just a dictionary. Any dictionary may be used as a cache for any series of statements by using the Connection.execution_options.compiled_cache option as an execution option. Execution options may be set on a statement, on an Engine or Connection, as well as when using the ORM Session.execute() method for SQLAlchemy-2.0 style invocations. For example, to run a series of SQL statements and have them cached in a particular dictionary:

my_cache = {}
with engine.connect().execution_options(compiled_cache=my_cache) as conn:
    conn.execute(table.select())

The SQLAlchemy ORM uses the above technique to hold onto per-mapper caches within the unit of work “flush” process that are separate from the default cache configured on the Engine, as well as for some relationship loader queries.

The cache can also be disabled with this argument by sending a value of None:

# disable caching for this connection
with engine.connect().execution_options(compiled_cache=None) as conn:
    conn.execute(table.select())

Caching for Third Party Dialects

The caching feature requires that the dialect’s compiler produces SQL strings that are safe to reuse for many statement invocations, given a particular cache key that is keyed to that SQL string. This means that any literal values in a statement, such as the LIMIT/OFFSET values for a SELECT, can not be hardcoded in the dialect’s compilation scheme, as the compiled string will not be re-usable. SQLAlchemy supports rendered bound parameters using the BindParameter.render_literal_execute() method which can be applied to the existing Select._limit_clause and Select._offset_clause attributes by a custom compiler, which are illustrated later in this section.

As there are many third party dialects, many of which may be generating literal values from SQL statements without the benefit of the newer “literal execute” feature, SQLAlchemy as of version 1.4.5 has added an attribute to dialects known as Dialect.supports_statement_cache. This attribute is checked at runtime for its presence directly on a particular dialect’s class, even if it’s already present on a superclass, so that even a third party dialect that subclasses an existing cacheable SQLAlchemy dialect such as sqlalchemy.dialects.postgresql.PGDialect must still explicitly include this attribute for caching to be enabled. The attribute should only be enabled once the dialect has been altered as needed and tested for reusability of compiled SQL statements with differing parameters.

For all third party dialects that don’t support this attribute, the logging for such a dialect will indicate dialect does not support caching.

When a dialect has been tested against caching, and in particular the SQL compiler has been updated to not render any literal LIMIT / OFFSET within a SQL string directly, dialect authors can apply the attribute as follows:

from sqlalchemy.engine.default import DefaultDialect


class MyDialect(DefaultDialect):
    supports_statement_cache = True

The flag needs to be applied to all subclasses of the dialect as well:

class MyDBAPIForMyDialect(MyDialect):
    supports_statement_cache = True

New in version 1.4.5: Added the Dialect.supports_statement_cache attribute.

The typical case for dialect modification follows.

Example: Rendering LIMIT / OFFSET with post compile parameters

As an example, suppose a dialect overrides the SQLCompiler.limit_clause() method, which produces the “LIMIT / OFFSET” clause for a SQL statement, like this:

# pre 1.4 style code
def limit_clause(self, select, **kw):
    text = ""
    if select._limit is not None:
        text += " \n LIMIT %d" % (select._limit,)
    if select._offset is not None:
        text += " \n OFFSET %d" % (select._offset,)
    return text

The above routine renders the Select._limit and Select._offset integer values as literal integers embedded in the SQL statement. This is a common requirement for databases that do not support using a bound parameter within the LIMIT/OFFSET clauses of a SELECT statement. However, rendering the integer value within the initial compilation stage is directly incompatible with caching as the limit and offset integer values of a Select object are not part of the cache key, so that many Select statements with different limit/offset values would not render with the correct value.

The correction for the above code is to move the literal integer into SQLAlchemy’s post-compile facility, which will render the literal integer outside of the initial compilation stage, but instead at execution time before the statement is sent to the DBAPI. This is accessed within the compilation stage using the BindParameter.render_literal_execute() method, in conjunction with using the Select._limit_clause and Select._offset_clause attributes, which represent the LIMIT/OFFSET as a complete SQL expression, as follows:

# 1.4 cache-compatible code
def limit_clause(self, select, **kw):
    text = ""

    limit_clause = select._limit_clause
    offset_clause = select._offset_clause

    if select._simple_int_clause(limit_clause):
        text += " \n LIMIT %s" % (
            self.process(limit_clause.render_literal_execute(), **kw)
        )
    elif limit_clause is not None:
        # assuming the DB doesn't support SQL expressions for LIMIT.
        # Otherwise render here normally
        raise exc.CompileError(
            "dialect 'mydialect' can only render simple integers for LIMIT"
        )
    if select._simple_int_clause(offset_clause):
        text += " \n OFFSET %s" % (
            self.process(offset_clause.render_literal_execute(), **kw)
        )
    elif offset_clause is not None:
        # assuming the DB doesn't support SQL expressions for OFFSET.
        # Otherwise render here normally
        raise exc.CompileError(
            "dialect 'mydialect' can only render simple integers for OFFSET"
        )

    return text

The approach above will generate a compiled SELECT statement that looks like:

SELECT x FROM y
LIMIT __[POSTCOMPILE_param_1]
OFFSET __[POSTCOMPILE_param_2]

Where above, the __[POSTCOMPILE_param_1] and __[POSTCOMPILE_param_2] indicators will be populated with their corresponding integer values at statement execution time, after the SQL string has been retrieved from the cache.

After changes like the above have been made as appropriate, the Dialect.supports_statement_cache flag should be set to True. It is strongly recommended that third party dialects make use of the dialect third party test suite which will assert that operations like SELECTs with LIMIT/OFFSET are correctly rendered and cached.

See also

Why is my application slow after upgrading to 1.4 and/or 2.x? - in the Frequently Asked Questions section

Using Lambdas to add significant speed gains to statement production

Deep Alchemy

This technique is generally non-essential except in very performance intensive scenarios, and intended for experienced Python programmers. While fairly straightforward, it involves metaprogramming concepts that are not appropriate for novice Python developers. The lambda approach can be applied to at a later time to existing code with a minimal amount of effort.

Python functions, typically expressed as lambdas, may be used to generate SQL expressions which are cacheable based on the Python code location of the lambda function itself as well as the closure variables within the lambda. The rationale is to allow caching of not only the SQL string-compiled form of a SQL expression construct as is SQLAlchemy’s normal behavior when the lambda system isn’t used, but also the in-Python composition of the SQL expression construct itself, which also has some degree of Python overhead.

The lambda SQL expression feature is available as a performance enhancing feature, and is also optionally used in the with_loader_criteria() ORM option in order to provide a generic SQL fragment.

Synopsis

Lambda statements are constructed using the lambda_stmt() function, which returns an instance of StatementLambdaElement, which is itself an executable statement construct. Additional modifiers and criteria are added to the object using the Python addition operator +, or alternatively the StatementLambdaElement.add_criteria() method which allows for more options.

It is assumed that the lambda_stmt() construct is being invoked within an enclosing function or method that expects to be used many times within an application, so that subsequent executions beyond the first one can take advantage of the compiled SQL being cached. When the lambda is constructed inside of an enclosing function in Python it is then subject to also having closure variables, which are significant to the whole approach:

from sqlalchemy import lambda_stmt


def run_my_statement(connection, parameter):
    stmt = lambda_stmt(lambda: select(table))
    stmt += lambda s: s.where(table.c.col == parameter)
    stmt += lambda s: s.order_by(table.c.id)

    return connection.execute(stmt)


with engine.connect() as conn:
    result = run_my_statement(some_connection, "some parameter")

Above, the three lambda callables that are used to define the structure of a SELECT statement are invoked exactly once, and the resulting SQL string cached in the compilation cache of the engine. From that point forward, the run_my_statement() function may be invoked any number of times and the lambda callables within it will not be called, only used as cache keys to retrieve the already-compiled SQL.

Note

It is important to note that there is already SQL caching in place when the lambda system is not used. The lambda system only adds an additional layer of work reduction per SQL statement invoked by caching the building up of the SQL construct itself and also using a simpler cache key.

Quick Guidelines for Lambdas

Above all, the emphasis within the lambda SQL system is ensuring that there is never a mismatch between the cache key generated for a lambda and the SQL string it will produce. The LambdaElement and related objects will run and analyze the given lambda in order to calculate how it should be cached on each run, trying to detect any potential problems. Basic guidelines include:

• Any kind of statement is supported - while it’s expected that select() constructs are the prime use case for lambda_stmt(), DML statements such as insert() and update() are equally usable:

  def upd(id_, newname):
      stmt = lambda_stmt(lambda: users.update())
      stmt += lambda s: s.values(name=newname)
      stmt += lambda s: s.where(users.c.id == id_)
      return stmt
  
  
  with engine.begin() as conn:
      conn.execute(upd(7, "foo"))

• ORM use cases directly supported as well - the lambda_stmt() can accommodate ORM functionality completely and used directly with Session.execute():

  def select_user(session, name):
      stmt = lambda_stmt(lambda: select(User))
      stmt += lambda s: s.where(User.name == name)
  
      row = session.execute(stmt).first()
      return row

• Bound parameters are automatically accommodated - in contrast to SQLAlchemy’s previous “baked query” system, the lambda SQL system accommodates for Python literal values which become SQL bound parameters automatically. This means that even though a given lambda runs only once, the values that become bound parameters are extracted from the closure of the lambda on every run:

  >>> def my_stmt(x, y):
  ...     stmt = lambda_stmt(lambda: select(func.max(x, y)))
  ...     return stmt
  >>> engine = create_engine("sqlite://", echo=True)
  >>> with engine.connect() as conn:
  ...     print(conn.scalar(my_stmt(5, 10)))
  ...     print(conn.scalar(my_stmt(12, 8)))
  SELECT max(?, ?) AS max_1
  [generated in 0.00057s] (5, 10)
  10
  SELECT max(?, ?) AS max_1
  [cached since 0.002059s ago] (12, 8)
  12

  Above, StatementLambdaElement extracted the values of x and y from the closure of the lambda that is generated each time my_stmt() is invoked; these were substituted into the cached SQL construct as the values of the parameters.

• The lambda should ideally produce an identical SQL structure in all cases - Avoid using conditionals or custom callables inside of lambdas that might make it produce different SQL based on inputs; if a function might conditionally use two different SQL fragments, use two separate lambdas:

  # **Don't** do this:
  
  def my_stmt(parameter, thing=False):
      stmt = lambda_stmt(lambda: select(table))
      stmt += (
          lambda s: s.where(table.c.x > parameter) if thing
          else s.where(table.c.y == parameter)
      return stmt
  
  # **Do** do this:
  
  def my_stmt(parameter, thing=False):
      stmt = lambda_stmt(lambda: select(table))
      if thing:
          stmt += lambda s: s.where(table.c.x > parameter)
      else:
          stmt += lambda s: s.where(table.c.y == parameter)
      return stmt

  There are a variety of failures which can occur if the lambda does not produce a consistent SQL construct and some are not trivially detectable right now.

• Don’t use functions inside the lambda to produce bound values - the bound value tracking approach requires that the actual value to be used in the SQL statement be locally present in the closure of the lambda. This is not possible if values are generated from other functions, and the LambdaElement should normally raise an error if this is attempted:

  >>> def my_stmt(x, y):
  ...     def get_x():
  ...         return x
  ...
  ...     def get_y():
  ...         return y
  ...
  ...     stmt = lambda_stmt(lambda: select(func.max(get_x(), get_y())))
  ...     return stmt
  >>> with engine.connect() as conn:
  ...     print(conn.scalar(my_stmt(5, 10)))
  Traceback (most recent call last):
    # ...
  sqlalchemy.exc.InvalidRequestError: Can't invoke Python callable get_x()
  inside of lambda expression argument at
  <code object <lambda> at 0x7fed15f350e0, file "<stdin>", line 6>;
  lambda SQL constructs should not invoke functions from closure variables
  to produce literal values since the lambda SQL system normally extracts
  bound values without actually invoking the lambda or any functions within it.

  Above, the use of get_x() and get_y(), if they are necessary, should occur outside of the lambda and assigned to a local closure variable:

  >>> def my_stmt(x, y):
  ...     def get_x():
  ...         return x
  ...
  ...     def get_y():
  ...         return y
  ...
  ...     x_param, y_param = get_x(), get_y()
  ...     stmt = lambda_stmt(lambda: select(func.max(x_param, y_param)))
  ...     return stmt

• Avoid referring to non-SQL constructs inside of lambdas as they are not cacheable by default - this issue refers to how the LambdaElement creates a cache key from other closure variables within the statement. In order to provide the best guarantee of an accurate cache key, all objects located in the closure of the lambda are considered to be significant, and none will be assumed to be appropriate for a cache key by default. So the following example will also raise a rather detailed error message:

  >>> class Foo:
  ...     def __init__(self, x, y):
  ...         self.x = x
  ...         self.y = y
  >>> def my_stmt(foo):
  ...     stmt = lambda_stmt(lambda: select(func.max(foo.x, foo.y)))
  ...     return stmt
  >>> with engine.connect() as conn:
  ...     print(conn.scalar(my_stmt(Foo(5, 10))))
  Traceback (most recent call last):
    # ...
  sqlalchemy.exc.InvalidRequestError: Closure variable named 'foo' inside of
  lambda callable <code object <lambda> at 0x7fed15f35450, file
  "<stdin>", line 2> does not refer to a cacheable SQL element, and also
  does not appear to be serving as a SQL literal bound value based on the
  default SQL expression returned by the function.  This variable needs to
  remain outside the scope of a SQL-generating lambda so that a proper cache
  key may be generated from the lambda's state.  Evaluate this variable
  outside of the lambda, set track_on=[<elements>] to explicitly select
  closure elements to track, or set track_closure_variables=False to exclude
  closure variables from being part of the cache key.

  The above error indicates that LambdaElement will not assume that the Foo object passed in will continue to behave the same in all cases. It also won’t assume it can use Foo as part of the cache key by default; if it were to use the Foo object as part of the cache key, if there were many different Foo objects this would fill up the cache with duplicate information, and would also hold long-lasting references to all of these objects.

  The best way to resolve the above situation is to not refer to foo inside of the lambda, and refer to it outside instead:

  >>> def my_stmt(foo):
  ...     x_param, y_param = foo.x, foo.y
  ...     stmt = lambda_stmt(lambda: select(func.max(x_param, y_param)))
  ...     return stmt

  In some situations, if the SQL structure of the lambda is guaranteed to never change based on input, to pass track_closure_variables=False which will disable any tracking of closure variables other than those used for bound parameters:

  >>> def my_stmt(foo):
  ...     stmt = lambda_stmt(
  ...         lambda: select(func.max(foo.x, foo.y)), track_closure_variables=False
  ...     )
  ...     return stmt

  There is also the option to add objects to the element to explicitly form part of the cache key, using the track_on parameter; using this parameter allows specific values to serve as the cache key and will also prevent other closure variables from being considered. This is useful for cases where part of the SQL being constructed originates from a contextual object of some sort that may have many different values. In the example below, the first segment of the SELECT statement will disable tracking of the foo variable, whereas the second segment will explicitly track self as part of the cache key:

  >>> def my_stmt(self, foo):
  ...     stmt = lambda_stmt(
  ...         lambda: select(*self.column_expressions), track_closure_variables=False
  ...     )
  ...     stmt = stmt.add_criteria(lambda: self.where_criteria, track_on=[self])
  ...     return stmt

  Using track_on means the given objects will be stored long term in the lambda’s internal cache and will have strong references for as long as the cache doesn’t clear out those objects (an LRU scheme of 1000 entries is used by default).

Cache Key Generation

In order to understand some of the options and behaviors which occur with lambda SQL constructs, an understanding of the caching system is helpful.

SQLAlchemy’s caching system normally generates a cache key from a given SQL expression construct by producing a structure that represents all the state within the construct:

>>> from sqlalchemy import select, column
>>> stmt = select(column("q"))
>>> cache_key = stmt._generate_cache_key()
>>> print(cache_key)  # somewhat paraphrased
CacheKey(key=(
  '0',
  <class 'sqlalchemy.sql.selectable.Select'>,
  '_raw_columns',
  (
    (
      '1',
      <class 'sqlalchemy.sql.elements.ColumnClause'>,
      'name',
      'q',
      'type',
      (
        <class 'sqlalchemy.sql.sqltypes.NullType'>,
      ),
    ),
  ),
  # a few more elements are here, and many more for a more
  # complicated SELECT statement
),)

The above key is stored in the cache which is essentially a dictionary, and the value is a construct that among other things stores the string form of the SQL statement, in this case the phrase “SELECT q”. We can observe that even for an extremely short query the cache key is pretty verbose as it has to represent everything that may vary about what’s being rendered and potentially executed.

The lambda construction system by contrast creates a different kind of cache key:

>>> from sqlalchemy import lambda_stmt
>>> stmt = lambda_stmt(lambda: select(column("q")))
>>> cache_key = stmt._generate_cache_key()
>>> print(cache_key)
CacheKey(key=(
  <code object <lambda> at 0x7fed1617c710, file "<stdin>", line 1>,
  <class 'sqlalchemy.sql.lambdas.StatementLambdaElement'>,
),)

Above, we see a cache key that is vastly shorter than that of the non-lambda statement, and additionally that production of the select(column("q")) construct itself was not even necessary; the Python lambda itself contains an attribute called __code__ which refers to a Python code object that within the runtime of the application is immutable and permanent.

When the lambda also includes closure variables, in the normal case that these variables refer to SQL constructs such as column objects, they become part of the cache key, or if they refer to literal values that will be bound parameters, they are placed in a separate element of the cache key:

>>> def my_stmt(parameter):
...     col = column("q")
...     stmt = lambda_stmt(lambda: select(col))
...     stmt += lambda s: s.where(col == parameter)
...     return stmt

The above StatementLambdaElement includes two lambdas, both of which refer to the col closure variable, so the cache key will represent both of these segments as well as the column() object:

>>> stmt = my_stmt(5)
>>> key = stmt._generate_cache_key()
>>> print(key)
CacheKey(key=(
  <code object <lambda> at 0x7f07323c50e0, file "<stdin>", line 3>,
  (
    '0',
    <class 'sqlalchemy.sql.elements.ColumnClause'>,
    'name',
    'q',
    'type',
    (
      <class 'sqlalchemy.sql.sqltypes.NullType'>,
    ),
  ),
  <code object <lambda> at 0x7f07323c5190, file "<stdin>", line 4>,
  <class 'sqlalchemy.sql.lambdas.LinkedLambdaElement'>,
  (
    '0',
    <class 'sqlalchemy.sql.elements.ColumnClause'>,
    'name',
    'q',
    'type',
    (
      <class 'sqlalchemy.sql.sqltypes.NullType'>,
    ),
  ),
  (
    '0',
    <class 'sqlalchemy.sql.elements.ColumnClause'>,
    'name',
    'q',
    'type',
    (
      <class 'sqlalchemy.sql.sqltypes.NullType'>,
    ),
  ),
),)

The second part of the cache key has retrieved the bound parameters that will be used when the statement is invoked:

>>> key.bindparams
[BindParameter('%(139668884281280 parameter)s', 5, type_=Integer())]

For a series of examples of “lambda” caching with performance comparisons, see the “short_selects” test suite within the Performance performance example.

Engine Disposal

The Engine refers to a connection pool, which means under normal circumstances, there are open database connections present while the Engine object is still resident in memory. When an Engine is garbage collected, its connection pool is no longer referred to by that Engine, and assuming none of its connections are still checked out, the pool and its connections will also be garbage collected, which has the effect of closing out the actual database connections as well. But otherwise, the Engine will hold onto open database connections assuming it uses the normally default pool implementation of QueuePool.

The Engine is intended to normally be a permanent fixture established up-front and maintained throughout the lifespan of an application. It is not intended to be created and disposed on a per-connection basis; it is instead a registry that maintains both a pool of connections as well as configurational information about the database and DBAPI in use, as well as some degree of internal caching of per-database resources.

However, there are many cases where it is desirable that all connection resources referred to by the Engine be completely closed out. It’s generally not a good idea to rely on Python garbage collection for this to occur for these cases; instead, the Engine can be explicitly disposed using the Engine.dispose() method. This disposes of the engine’s underlying connection pool and replaces it with a new one that’s empty. Provided that the Engine is discarded at this point and no longer used, all checked-in connections which it refers to will also be fully closed.

Valid use cases for calling Engine.dispose() include:

• When a program wants to release any remaining checked-in connections held by the connection pool and expects to no longer be connected to that database at all for any future operations.

• When a program uses multiprocessing or fork(), and an Engine object is copied to the child process, Engine.dispose() should be called so that the engine creates brand new database connections local to that fork. Database connections generally do not travel across process boundaries. Use the Engine.dispose.close parameter set to False in this case. See the section Using Connection Pools with Multiprocessing or os.fork() for more background on this use case.

• Within test suites or multitenancy scenarios where many ad-hoc, short-lived Engine objects may be created and disposed.

Connections that are checked out are not discarded when the engine is disposed or garbage collected, as these connections are still strongly referenced elsewhere by the application. However, after Engine.dispose() is called, those connections are no longer associated with that Engine; when they are closed, they will be returned to their now-orphaned connection pool which will ultimately be garbage collected, once all connections which refer to it are also no longer referenced anywhere. Since this process is not easy to control, it is strongly recommended that Engine.dispose() is called only after all checked out connections are checked in or otherwise de-associated from their pool.

An alternative for applications that are negatively impacted by the Engine object’s use of connection pooling is to disable pooling entirely. This typically incurs only a modest performance impact upon the use of new connections, and means that when a connection is checked in, it is entirely closed out and is not held in memory. See Switching Pool Implementations for guidelines on how to disable pooling.

See also

Connection Pooling

Using Connection Pools with Multiprocessing or os.fork()

Working with Driver SQL and Raw DBAPI Connections

The introduction on using Connection.execute() made use of the text() construct in order to illustrate how textual SQL statements may be invoked. When working with SQLAlchemy, textual SQL is actually more of the exception rather than the norm, as the Core expression language and the ORM both abstract away the textual representation of SQL. However, the text() construct itself also provides some abstraction of textual SQL in that it normalizes how bound parameters are passed, as well as that it supports datatyping behavior for parameters and result set rows.

Invoking SQL strings directly to the driver

For the use case where one wants to invoke textual SQL directly passed to the underlying driver (known as the DBAPI) without any intervention from the text() construct, the Connection.exec_driver_sql() method may be used:

with engine.connect() as conn:
    conn.exec_driver_sql("SET param='bar'")

New in version 1.4: Added the Connection.exec_driver_sql() method.

Working with the DBAPI cursor directly

There are some cases where SQLAlchemy does not provide a genericized way at accessing some DBAPI functions, such as calling stored procedures as well as dealing with multiple result sets. In these cases, it’s just as expedient to deal with the raw DBAPI connection directly.

The most common way to access the raw DBAPI connection is to get it from an already present Connection object directly. It is present using the Connection.connection attribute:

connection = engine.connect()
dbapi_conn = connection.connection

The DBAPI connection here is actually a “proxied” in terms of the originating connection pool, however this is an implementation detail that in most cases can be ignored. As this DBAPI connection is still contained within the scope of an owning Connection object, it is best to make use of the Connection object for most features such as transaction control as well as calling the Connection.close() method; if these operations are performed on the DBAPI connection directly, the owning Connection will not be aware of these changes in state.

To overcome the limitations imposed by the DBAPI connection that is maintained by an owning Connection, a DBAPI connection is also available without the need to procure a Connection first, using the Engine.raw_connection() method of Engine:

dbapi_conn = engine.raw_connection()

This DBAPI connection is again a “proxied” form as was the case before. The purpose of this proxying is now apparent, as when we call the .close() method of this connection, the DBAPI connection is typically not actually closed, but instead released back to the engine’s connection pool:

dbapi_conn.close()

While SQLAlchemy may in the future add built-in patterns for more DBAPI use cases, there are diminishing returns as these cases tend to be rarely needed and they also vary highly dependent on the type of DBAPI in use, so in any case the direct DBAPI calling pattern is always there for those cases where it is needed.

See also

How do I get at the raw DBAPI connection when using an Engine? - includes additional details about how the DBAPI connection is accessed as well as the “driver” connection when using asyncio drivers.

Some recipes for DBAPI connection use follow.

Calling Stored Procedures and User Defined Functions

SQLAlchemy supports calling stored procedures and user defined functions several ways. Please note that all DBAPIs have different practices, so you must consult your underlying DBAPI’s documentation for specifics in relation to your particular usage. The following examples are hypothetical and may not work with your underlying DBAPI.

For stored procedures or functions with special syntactical or parameter concerns, DBAPI-level callproc may potentially be used with your DBAPI. An example of this pattern is:

connection = engine.raw_connection()
try:
    cursor_obj = connection.cursor()
    cursor_obj.callproc("my_procedure", ["x", "y", "z"])
    results = list(cursor_obj.fetchall())
    cursor_obj.close()
    connection.commit()
finally:
    connection.close()

Note

Not all DBAPIs use callproc and overall usage details will vary. The above example is only an illustration of how it might look to use a particular DBAPI function.

Your DBAPI may not have a callproc requirement or may require a stored procedure or user defined function to be invoked with another pattern, such as normal SQLAlchemy connection usage. One example of this usage pattern is, at the time of this documentation’s writing, executing a stored procedure in the PostgreSQL database with the psycopg2 DBAPI, which should be invoked with normal connection usage:

connection.execute("CALL my_procedure();")

This above example is hypothetical. The underlying database is not guaranteed to support “CALL” or “SELECT” in these situations, and the keyword may vary dependent on the function being a stored procedure or a user defined function. You should consult your underlying DBAPI and database documentation in these situations to determine the correct syntax and patterns to use.

Multiple Result Sets

Multiple result set support is available from a raw DBAPI cursor using the nextset method:

connection = engine.raw_connection()
try:
    cursor_obj = connection.cursor()
    cursor_obj.execute("select * from table1; select * from table2")
    results_one = cursor_obj.fetchall()
    cursor_obj.nextset()
    results_two = cursor_obj.fetchall()
    cursor_obj.close()
finally:
    connection.close()

Registering New Dialects

The create_engine() function call locates the given dialect using setuptools entrypoints. These entry points can be established for third party dialects within the setup.py script. For example, to create a new dialect “foodialect://”, the steps are as follows:

1. Create a package called foodialect.

2. The package should have a module containing the dialect class, which is typically a subclass of sqlalchemy.engine.default.DefaultDialect. In this example let’s say it’s called FooDialect and its module is accessed via foodialect.dialect.

3. The entry point can be established in setup.py as follows:

   entry_points = """
   [sqlalchemy.dialects]
   foodialect = foodialect.dialect:FooDialect
   """

If the dialect is providing support for a particular DBAPI on top of an existing SQLAlchemy-supported database, the name can be given including a database-qualification. For example, if FooDialect were in fact a MySQL dialect, the entry point could be established like this:

entry_points = """
[sqlalchemy.dialects]
mysql.foodialect = foodialect.dialect:FooDialect
"""

The above entrypoint would then be accessed as create_engine("mysql+foodialect://").

Registering Dialects In-Process

SQLAlchemy also allows a dialect to be registered within the current process, bypassing the need for separate installation. Use the register() function as follows:

from sqlalchemy.dialects import registry

registry.register("mysql.foodialect", "myapp.dialect", "MyMySQLDialect")

The above will respond to create_engine("mysql+foodialect://") and load the MyMySQLDialect class from the myapp.dialect module.

Connection / Engine API

Object Name	Description
Connection	Provides high-level functionality for a wrapped DB-API connection.
CreateEnginePlugin	A set of hooks intended to augment the construction of an Engine object based on entrypoint names in a URL.
Engine	Connects a Pool and Dialect together to provide a source of database connectivity and behavior.
ExceptionContext	Encapsulate information about an error condition in progress.
NestedTransaction	Represent a ‘nested’, or SAVEPOINT transaction.
RootTransaction	Represent the “root” transaction on a Connection.
Transaction	Represent a database transaction in progress.
TwoPhaseTransaction	Represent a two-phase transaction.

class sqlalchemy.engine.Connection(engine, connection=None, close_with_result=False, _branch_from=None, _execution_options=None, _dispatch=None, _has_events=None, _allow_revalidate=True)

Provides high-level functionality for a wrapped DB-API connection.

This is the SQLAlchemy 1.x.x version of the Connection class. For the 2.0 style version, which features some API differences, see Connection.

The Connection object is procured by calling the Engine.connect() method of the Engine object, and provides services for execution of SQL statements as well as transaction control.

The Connection object is not thread-safe. While a Connection can be shared among threads using properly synchronized access, it is still possible that the underlying DBAPI connection may not support shared access between threads. Check the DBAPI documentation for details.

Members

__init__(), begin(), begin_nested(), begin_twophase(), close(), closed, connect(), connection, default_isolation_level, detach(), exec_driver_sql(), execute(), execution_options(), get_execution_options(), get_isolation_level(), get_nested_transaction(), get_transaction(), in_nested_transaction(), in_transaction(), info, invalidate(), invalidated, run_callable(), scalar(), scalars(), schema_for_object(), transaction()

The Connection object represents a single DBAPI connection checked out from the connection pool. In this state, the connection pool has no affect upon the connection, including its expiration or timeout state. For the connection pool to properly manage connections, connections should be returned to the connection pool (i.e. connection.close()) whenever the connection is not in use.

Class signature

class sqlalchemy.engine.Connection (sqlalchemy.engine.Connectable)

method sqlalchemy.engine.Connection.__init__(engine, connection=None, close_with_result=False, _branch_from=None, _execution_options=None, _dispatch=None, _has_events=None, _allow_revalidate=True)

Construct a new Connection.

method sqlalchemy.engine.Connection.begin()

Begin a transaction and return a transaction handle.

The returned object is an instance of Transaction. This object represents the “scope” of the transaction, which completes when either the Transaction.rollback() or Transaction.commit() method is called.

Tip

The Connection.begin() method is invoked when using the Engine.begin() context manager method as well. All documentation that refers to behaviors specific to the Connection.begin() method also apply to use of the Engine.begin() method.

Legacy use: nested calls to begin() on the same Connection will return new Transaction objects that represent an emulated transaction within the scope of the enclosing transaction, that is:

trans = conn.begin()   # outermost transaction
trans2 = conn.begin()  # "nested"
trans2.commit()        # does nothing
trans.commit()         # actually commits

Calls to Transaction.commit() only have an effect when invoked via the outermost Transaction object, though the Transaction.rollback() method of any of the Transaction objects will roll back the transaction.

Tip

The above “nesting” behavior is a legacy behavior specific to 1.x style use and will be removed in SQLAlchemy 2.0. For notes on 2.0 style use, see Connection.begin().

See also

Connection.begin_nested() - use a SAVEPOINT

Connection.begin_twophase() - use a two phase /XID transaction

Engine.begin() - context manager available from Engine

method sqlalchemy.engine.Connection.begin_nested()

Begin a nested transaction (i.e. SAVEPOINT) and return a transaction handle, assuming an outer transaction is already established.

Nested transactions require SAVEPOINT support in the underlying database. Any transaction in the hierarchy may commit and rollback, however the outermost transaction still controls the overall commit or rollback of the transaction of a whole.

The legacy form of Connection.begin_nested() method has alternate behaviors based on whether or not the Connection.begin() method was called previously. If Connection.begin() was not called, then this method will behave the same as the Connection.begin() method and return a RootTransaction object that begins and commits a real transaction - no savepoint is invoked. If Connection.begin() has been called, and a RootTransaction is already established, then this method returns an instance of NestedTransaction which will invoke and manage the scope of a SAVEPOINT.

Tip

The above mentioned behavior of Connection.begin_nested() is a legacy behavior specific to 1.x style use. In 2.0 style use, the Connection.begin_nested() method instead autobegins the outer transaction that can be committed using “commit-as-you-go” style; see Connection.begin_nested() for migration details.

Changed in version 1.4.13: The behavior of Connection.begin_nested() as returning a RootTransaction if Connection.begin() were not called has been restored as was the case in 1.3.x versions; in previous 1.4.x versions, an outer transaction would be “autobegun” but would not be committed.

See also

Connection.begin()

Using SAVEPOINT - ORM support for SAVEPOINT

method sqlalchemy.engine.Connection.begin_twophase(xid=None)

Begin a two-phase or XA transaction and return a transaction handle.

The returned object is an instance of TwoPhaseTransaction, which in addition to the methods provided by Transaction, also provides a TwoPhaseTransaction.prepare() method.

Parameters:

xid – the two phase transaction id. If not supplied, a random id will be generated.

See also

Connection.begin()

Connection.begin_twophase()

method sqlalchemy.engine.Connection.close()

Close this Connection.

This results in a release of the underlying database resources, that is, the DBAPI connection referenced internally. The DBAPI connection is typically restored back to the connection-holding Pool referenced by the Engine that produced this Connection. Any transactional state present on the DBAPI connection is also unconditionally released via the DBAPI connection’s rollback() method, regardless of any Transaction object that may be outstanding with regards to this Connection.

After Connection.close() is called, the Connection is permanently in a closed state, and will allow no further operations.

attribute sqlalchemy.engine.Connection.closed

Return True if this connection is closed.

method sqlalchemy.engine.Connection.connect(close_with_result=False)

Returns a branched version of this Connection.

Deprecated since version 1.4: The Connection.connect() method is considered legacy as of the 1.x series of SQLAlchemy and will be removed in 2.0. (Background on SQLAlchemy 2.0 at: Migrating to SQLAlchemy 2.0)

The Connection.close() method on the returned Connection can be called and this Connection will remain open.

This method provides usage symmetry with Engine.connect(), including for usage with context managers.

attribute sqlalchemy.engine.Connection.connection

The underlying DB-API connection managed by this Connection.

This is a SQLAlchemy connection-pool proxied connection which then has the attribute _ConnectionFairy.dbapi_connection that refers to the actual driver connection.

See also

Working with Driver SQL and Raw DBAPI Connections

attribute sqlalchemy.engine.Connection.default_isolation_level

The initial-connection time isolation level associated with the Dialect in use.

This value is independent of the Connection.execution_options.isolation_level and Engine.execution_options.isolation_level execution options, and is determined by the Dialect when the first connection is created, by performing a SQL query against the database for the current isolation level before any additional commands have been emitted.

Calling this accessor does not invoke any new SQL queries.

New in version 0.9.9.

See also

Connection.get_isolation_level() - view current actual isolation level

create_engine.isolation_level - set per Engine isolation level

Connection.execution_options.isolation_level - set per Connection isolation level

method sqlalchemy.engine.Connection.detach()

Detach the underlying DB-API connection from its connection pool.

E.g.:

with engine.connect() as conn:
    conn.detach()
    conn.execute(text("SET search_path TO schema1, schema2"))

    # work with connection

# connection is fully closed (since we used "with:", can
# also call .close())

This Connection instance will remain usable. When closed (or exited from a context manager context as above), the DB-API connection will be literally closed and not returned to its originating pool.

This method can be used to insulate the rest of an application from a modified state on a connection (such as a transaction isolation level or similar).

method sqlalchemy.engine.Connection.exec_driver_sql(statement, parameters=None, execution_options=None)

Executes a string SQL statement on the DBAPI cursor directly, without any SQL compilation steps.

This can be used to pass any string directly to the cursor.execute() method of the DBAPI in use.

Parameters:

• statement – The statement str to be executed. Bound parameters must use the underlying DBAPI’s paramstyle, such as “qmark”, “pyformat”, “format”, etc.

• parameters – represent bound parameter values to be used in the execution. The format is one of: a dictionary of named parameters, a tuple of positional parameters, or a list containing either dictionaries or tuples for multiple-execute support.

Returns:

a CursorResult.

E.g. multiple dictionaries:

conn.exec_driver_sql(
    "INSERT INTO table (id, value) VALUES (%(id)s, %(value)s)",
    [{"id":1, "value":"v1"}, {"id":2, "value":"v2"}]
)

Single dictionary:

conn.exec_driver_sql(
    "INSERT INTO table (id, value) VALUES (%(id)s, %(value)s)",
    dict(id=1, value="v1")
)

Single tuple:

conn.exec_driver_sql(
    "INSERT INTO table (id, value) VALUES (?, ?)",
    (1, 'v1')
)

Note

The Connection.exec_driver_sql() method does not participate in the ConnectionEvents.before_execute() and ConnectionEvents.after_execute() events. To intercept calls to Connection.exec_driver_sql(), use ConnectionEvents.before_cursor_execute() and ConnectionEvents.after_cursor_execute().

See also

PEP 249

method sqlalchemy.engine.Connection.execute(statement, *multiparams, **params)

Executes a SQL statement construct and returns a CursorResult.

Parameters:

• statement –

  The statement to be executed. May be one of:

  • a plain string (deprecated)

  • any ClauseElement construct that is also a subclass of Executable, such as a select() construct

  • a FunctionElement, such as that generated by func, will be automatically wrapped in a SELECT statement, which is then executed.

  • a DDLElement object

  • a DefaultGenerator object

  • a Compiled object

  Deprecated since version 2.0: passing a string to Connection.execute() is deprecated and will be removed in version 2.0. Use the text() construct with Connection.execute(), or the Connection.exec_driver_sql() method to invoke a driver-level SQL string.

• *multiparams/**params –

  represent bound parameter values to be used in the execution. Typically, the format is either a collection of one or more dictionaries passed to *multiparams:

  conn.execute(
      table.insert(),
      {"id":1, "value":"v1"},
      {"id":2, "value":"v2"}
  )

  …or individual key/values interpreted by **params:

  conn.execute(
      table.insert(), id=1, value="v1"
  )

  In the case that a plain SQL string is passed, and the underlying DBAPI accepts positional bind parameters, a collection of tuples or individual values in *multiparams may be passed:

  conn.execute(
      "INSERT INTO table (id, value) VALUES (?, ?)",
      (1, "v1"), (2, "v2")
  )
  
  conn.execute(
      "INSERT INTO table (id, value) VALUES (?, ?)",
      1, "v1"
  )

  Note above, the usage of a question mark “?” or other symbol is contingent upon the “paramstyle” accepted by the DBAPI in use, which may be any of “qmark”, “named”, “pyformat”, “format”, “numeric”. See pep-249 for details on paramstyle.

  To execute a textual SQL statement which uses bound parameters in a DBAPI-agnostic way, use the text() construct.

  Deprecated since version 2.0: use of tuple or scalar positional parameters is deprecated. All params should be dicts or sequences of dicts. Use exec_driver_sql() to execute a plain string with tuple or scalar positional parameters.

method sqlalchemy.engine.Connection.execution_options(**opt)

Set non-SQL options for the connection which take effect during execution.

For a “future” style connection, this method returns this same Connection object with the new options added.

For a legacy connection, this method returns a copy of this Connection which references the same underlying DBAPI connection, but also defines the given execution options which will take effect for a call to execute(). As the new Connection references the same underlying resource, it’s usually a good idea to ensure that the copies will be discarded immediately, which is implicit if used as in:

result = connection.execution_options(stream_results=True).\
                    execute(stmt)

Note that any key/value can be passed to Connection.execution_options(), and it will be stored in the _execution_options dictionary of the Connection. It is suitable for usage by end-user schemes to communicate with event listeners, for example.

The keywords that are currently recognized by SQLAlchemy itself include all those listed under Executable.execution_options(), as well as others that are specific to Connection.

Parameters:

• autocommit –

  Available on: Connection, statement. When True, a COMMIT will be invoked after execution when executed in ‘autocommit’ mode, i.e. when an explicit transaction is not begun on the connection. Note that this is library level, not DBAPI level autocommit. The DBAPI connection will remain in a real transaction unless the “AUTOCOMMIT” isolation level is used.

  Deprecated since version 1.4: The “autocommit” execution option is deprecated and will be removed in SQLAlchemy 2.0. See Library-level (but not driver level) “Autocommit” removed from both Core and ORM for discussion.

• compiled_cache –

  Available on: Connection. A dictionary where Compiled objects will be cached when the Connection compiles a clause expression into a Compiled object. This dictionary will supersede the statement cache that may be configured on the Engine itself. If set to None, caching is disabled, even if the engine has a configured cache size.

  Note that the ORM makes use of its own “compiled” caches for some operations, including flush operations. The caching used by the ORM internally supersedes a cache dictionary specified here.

• logging_token –

  Available on: Connection, Engine.

  Adds the specified string token surrounded by brackets in log messages logged by the connection, i.e. the logging that’s enabled either via the create_engine.echo flag or via the logging.getLogger("sqlalchemy.engine") logger. This allows a per-connection or per-sub-engine token to be available which is useful for debugging concurrent connection scenarios.

  New in version 1.4.0b2.

  See also

  Setting Per-Connection / Sub-Engine Tokens - usage example

  create_engine.logging_name - adds a name to the name used by the Python logger object itself.

• isolation_level –

  Available on: Connection.

  Set the transaction isolation level for the lifespan of this Connection object. Valid values include those string values accepted by the create_engine.isolation_level parameter passed to create_engine(). These levels are semi-database specific; see individual dialect documentation for valid levels.

  The isolation level option applies the isolation level by emitting statements on the DBAPI connection, and necessarily affects the original Connection object overall, not just the copy that is returned by the call to Connection.execution_options() method. The isolation level will remain at the given setting until the DBAPI connection itself is returned to the connection pool, i.e. the Connection.close() method on the original Connection is called, where an event handler will emit additional statements on the DBAPI connection in order to revert the isolation level change.

  Warning

  The isolation_level execution option should not be used when a transaction is already established, that is, the Connection.begin() method or similar has been called. A database cannot change the isolation level on a transaction in progress, and different DBAPIs and/or SQLAlchemy dialects may implicitly roll back or commit the transaction, or not affect the connection at all.

  Note

  The isolation_level execution option is implicitly reset if the Connection is invalidated, e.g. via the Connection.invalidate() method, or if a disconnection error occurs. The new connection produced after the invalidation will not have the isolation level re-applied to it automatically.

  See also

  create_engine.isolation_level - set per Engine isolation level

  Connection.get_isolation_level() - view current actual level

  SQLite Transaction Isolation

  PostgreSQL Transaction Isolation

  MySQL Transaction Isolation

  SQL Server Transaction Isolation

  Setting Transaction Isolation Levels / DBAPI AUTOCOMMIT - for the ORM

• no_parameters – When True, if the final parameter list or dictionary is totally empty, will invoke the statement on the cursor as cursor.execute(statement), not passing the parameter collection at all. Some DBAPIs such as psycopg2 and mysql-python consider percent signs as significant only when parameters are present; this option allows code to generate SQL containing percent signs (and possibly other characters) that is neutral regarding whether it’s executed by the DBAPI or piped into a script that’s later invoked by command line tools.

• stream_results –

  Available on: Connection, statement. Indicate to the dialect that results should be “streamed” and not pre-buffered, if possible. For backends such as PostgreSQL, MySQL and MariaDB, this indicates the use of a “server side cursor” as opposed to a client side cursor. Other backends such as that of Oracle may already use server side cursors by default.

  The usage of Connection.execution_options.stream_results is usually combined with setting a fixed number of rows to to be fetched in batches, to allow for efficient iteration of database rows while at the same time not loading all result rows into memory at once; this can be configured on a Result object using the Result.yield_per() method, after execution has returned a new Result. If Result.yield_per() is not used, the Connection.execution_options.stream_results mode of operation will instead use a dynamically sized buffer which buffers sets of rows at a time, growing on each batch based on a fixed growth size up until a limit which may be configured using the Connection.execution_options.max_row_buffer parameter.

  When using the ORM to fetch ORM mapped objects from a result, Result.yield_per() should always be used with Connection.execution_options.stream_results, so that the ORM does not fetch all rows into new ORM objects at once.

  For typical use, the Connection.execution_options.yield_per execution option should be preferred, which sets up both Connection.execution_options.stream_results and Result.yield_per() at once. This option is supported both at a core level by Connection as well as by the ORM Session; the latter is described at Fetching Large Result Sets with Yield Per.

  See also

  Using Server Side Cursors (a.k.a. stream results) - background on Connection.execution_options.stream_results

  Connection.execution_options.max_row_buffer

  Connection.execution_options.yield_per

  Fetching Large Result Sets with Yield Per - in the ORM Querying Guide describing the ORM version of yield_per

• max_row_buffer –

  Available on: Connection, Executable. Sets a maximum buffer size to use when the Connection.execution_options.stream_results execution option is used on a backend that supports server side cursors. The default value if not specified is 1000.

  See also

  Connection.execution_options.stream_results

  Using Server Side Cursors (a.k.a. stream results)

• yield_per –

  Available on: Connection, Executable. Integer value applied which will set the Connection.execution_options.stream_results execution option and invoke Result.yield_per() automatically at once. Allows equivalent functionality as is present when using this parameter with the ORM.

  New in version 1.4.40.

  See also

  Using Server Side Cursors (a.k.a. stream results) - background and examples on using server side cursors with Core.

  Fetching Large Result Sets with Yield Per - in the ORM Querying Guide describing the ORM version of yield_per

• schema_translate_map – Available on: Connection, Engine, Executable.

• schema_translate_map –

  Available on: Connection, Engine. A dictionary mapping schema names to schema names, that will be applied to the Table.schema element of each Table encountered when SQL or DDL expression elements are compiled into strings; the resulting schema name will be converted based on presence in the map of the original name.

  New in version 1.1.

  See also

  Translation of Schema Names

See also

Engine.execution_options()

Executable.execution_options()

Connection.get_execution_options()

method sqlalchemy.engine.Connection.get_execution_options()

Get the non-SQL options which will take effect during execution.

New in version 1.3.

See also

Connection.execution_options()

method sqlalchemy.engine.Connection.get_isolation_level()

Return the current actual isolation level that’s present on the database within the scope of this connection.

This attribute will perform a live SQL operation against the database in order to procure the current isolation level, so the value returned is the actual level on the underlying DBAPI connection regardless of how this state was set. This will be one of the four actual isolation modes READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, SERIALIZABLE. It will not include the AUTOCOMMIT isolation level setting. Third party dialects may also feature additional isolation level settings.

Note

This method will not report on the AUTOCOMMIT isolation level, which is a separate dbapi setting that’s independent of actual isolation level. When AUTOCOMMIT is in use, the database connection still has a “traditional” isolation mode in effect, that is typically one of the four values READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, SERIALIZABLE.

Compare to the Connection.default_isolation_level accessor which returns the isolation level that is present on the database at initial connection time.

New in version 0.9.9.

See also

Connection.default_isolation_level - view default level

create_engine.isolation_level - set per Engine isolation level

Connection.execution_options.isolation_level - set per Connection isolation level

method sqlalchemy.engine.Connection.get_nested_transaction()

Return the current nested transaction in progress, if any.

New in version 1.4.

method sqlalchemy.engine.Connection.get_transaction()

Return the current root transaction in progress, if any.

New in version 1.4.

method sqlalchemy.engine.Connection.in_nested_transaction()

Return True if a transaction is in progress.

method sqlalchemy.engine.Connection.in_transaction()

Return True if a transaction is in progress.

attribute sqlalchemy.engine.Connection.info

Info dictionary associated with the underlying DBAPI connection referred to by this Connection, allowing user-defined data to be associated with the connection.

The data here will follow along with the DBAPI connection including after it is returned to the connection pool and used again in subsequent instances of Connection.

method sqlalchemy.engine.Connection.invalidate(exception=None)

Invalidate the underlying DBAPI connection associated with this Connection.

An attempt will be made to close the underlying DBAPI connection immediately; however if this operation fails, the error is logged but not raised. The connection is then discarded whether or not close() succeeded.

Upon the next use (where “use” typically means using the Connection.execute() method or similar), this Connection will attempt to procure a new DBAPI connection using the services of the Pool as a source of connectivity (e.g. a “reconnection”).

If a transaction was in progress (e.g. the Connection.begin() method has been called) when Connection.invalidate() method is called, at the DBAPI level all state associated with this transaction is lost, as the DBAPI connection is closed. The Connection will not allow a reconnection to proceed until the Transaction object is ended, by calling the Transaction.rollback() method; until that point, any attempt at continuing to use the Connection will raise an InvalidRequestError. This is to prevent applications from accidentally continuing an ongoing transactional operations despite the fact that the transaction has been lost due to an invalidation.

The Connection.invalidate() method, just like auto-invalidation, will at the connection pool level invoke the PoolEvents.invalidate() event.

Parameters:

exception – an optional Exception instance that’s the reason for the invalidation. is passed along to event handlers and logging functions.

See also

More on Invalidation

attribute sqlalchemy.engine.Connection.invalidated

Return True if this connection was invalidated.

method sqlalchemy.engine.Connection.run_callable(callable_, *args, **kwargs)

Given a callable object or function, execute it, passing a Connection as the first argument.

Deprecated since version 1.4: The Connection.run_callable() method is deprecated and will be removed in a future release. Invoke the callable function directly, passing the Connection.

The given *args and **kwargs are passed subsequent to the Connection argument.

This function, along with Engine.run_callable(), allows a function to be run with a Connection or Engine object without the need to know which one is being dealt with.

method sqlalchemy.engine.Connection.scalar(object_, *multiparams, **params)

Executes and returns the first column of the first row.

The underlying result/cursor is closed after execution.

method sqlalchemy.engine.Connection.scalars(object_, *multiparams, **params)

Executes and returns a scalar result set, which yields scalar values from the first column of each row.

This method is equivalent to calling Connection.execute() to receive a Result object, then invoking the Result.scalars() method to produce a ScalarResult instance.

Returns:

a ScalarResult

New in version 1.4.24.

method sqlalchemy.engine.Connection.schema_for_object(obj)

Return the schema name for the given schema item taking into account current schema translate map.

method sqlalchemy.engine.Connection.transaction(callable_, *args, **kwargs)

Execute the given function within a transaction boundary.

Deprecated since version 1.4: The Connection.transaction() method is deprecated and will be removed in a future release. Use the Engine.begin() context manager instead.

The function is passed this Connection as the first argument, followed by the given *args and **kwargs, e.g.:

def do_something(conn, x, y):
    conn.execute(text("some statement"), {'x':x, 'y':y})

conn.transaction(do_something, 5, 10)

The operations inside the function are all invoked within the context of a single Transaction. Upon success, the transaction is committed. If an exception is raised, the transaction is rolled back before propagating the exception.

Note

The transaction() method is superseded by the usage of the Python with: statement, which can be used with Connection.begin():

with conn.begin():
    conn.execute(text("some statement"), {'x':5, 'y':10})

As well as with Engine.begin():

with engine.begin() as conn:
    conn.execute(text("some statement"), {'x':5, 'y':10})

See also

Engine.begin() - engine-level transactional context

Engine.transaction() - engine-level version of Connection.transaction()

class sqlalchemy.engine.CreateEnginePlugin(url, kwargs)

A set of hooks intended to augment the construction of an Engine object based on entrypoint names in a URL.

The purpose of CreateEnginePlugin is to allow third-party systems to apply engine, pool and dialect level event listeners without the need for the target application to be modified; instead, the plugin names can be added to the database URL. Target applications for CreateEnginePlugin include:

• connection and SQL performance tools, e.g. which use events to track number of checkouts and/or time spent with statements

• connectivity plugins such as proxies

A rudimentary CreateEnginePlugin that attaches a logger to an Engine object might look like:

import logging

from sqlalchemy.engine import CreateEnginePlugin
from sqlalchemy import event

class LogCursorEventsPlugin(CreateEnginePlugin):
    def __init__(self, url, kwargs):
        # consume the parameter "log_cursor_logging_name" from the
        # URL query
        logging_name = url.query.get("log_cursor_logging_name", "log_cursor")

        self.log = logging.getLogger(logging_name)

    def update_url(self, url):
        "update the URL to one that no longer includes our parameters"
        return url.difference_update_query(["log_cursor_logging_name"])

    def engine_created(self, engine):
        "attach an event listener after the new Engine is constructed"
        event.listen(engine, "before_cursor_execute", self._log_event)


    def _log_event(
        self,
        conn,
        cursor,
        statement,
        parameters,
        context,
        executemany):

        self.log.info("Plugin logged cursor event: %s", statement)

Plugins are registered using entry points in a similar way as that of dialects:

entry_points={
    'sqlalchemy.plugins': [
        'log_cursor_plugin = myapp.plugins:LogCursorEventsPlugin'
    ]

A plugin that uses the above names would be invoked from a database URL as in:

from sqlalchemy import create_engine

engine = create_engine(
    "mysql+pymysql://scott:tiger@localhost/test?"
    "plugin=log_cursor_plugin&log_cursor_logging_name=mylogger"
)

The plugin URL parameter supports multiple instances, so that a URL may specify multiple plugins; they are loaded in the order stated in the URL:

engine = create_engine(
  "mysql+pymysql://scott:tiger@localhost/test?"
  "plugin=plugin_one&plugin=plugin_twp&plugin=plugin_three")

The plugin names may also be passed directly to create_engine() using the create_engine.plugins argument:

engine = create_engine(
  "mysql+pymysql://scott:tiger@localhost/test",
  plugins=["myplugin"])

New in version 1.2.3: plugin names can also be specified to create_engine() as a list

A plugin may consume plugin-specific arguments from the URL object as well as the kwargs dictionary, which is the dictionary of arguments passed to the create_engine() call. “Consuming” these arguments includes that they must be removed when the plugin initializes, so that the arguments are not passed along to the Dialect constructor, where they will raise an ArgumentError because they are not known by the dialect.

As of version 1.4 of SQLAlchemy, arguments should continue to be consumed from the kwargs dictionary directly, by removing the values with a method such as dict.pop. Arguments from the URL object should be consumed by implementing the CreateEnginePlugin.update_url() method, returning a new copy of the URL with plugin-specific parameters removed:

class MyPlugin(CreateEnginePlugin):
    def __init__(self, url, kwargs):
        self.my_argument_one = url.query['my_argument_one']
        self.my_argument_two = url.query['my_argument_two']
        self.my_argument_three = kwargs.pop('my_argument_three', None)

    def update_url(self, url):
        return url.difference_update_query(
            ["my_argument_one", "my_argument_two"]
        )

Arguments like those illustrated above would be consumed from a create_engine() call such as:

from sqlalchemy import create_engine

engine = create_engine(
  "mysql+pymysql://scott:tiger@localhost/test?"
  "plugin=myplugin&my_argument_one=foo&my_argument_two=bar",
  my_argument_three='bat'
)

Changed in version 1.4: The URL object is now immutable; a CreateEnginePlugin that needs to alter the URL should implement the newly added CreateEnginePlugin.update_url() method, which is invoked after the plugin is constructed.

For migration, construct the plugin in the following way, checking for the existence of the CreateEnginePlugin.update_url() method to detect which version is running:

class MyPlugin(CreateEnginePlugin):
    def __init__(self, url, kwargs):
        if hasattr(CreateEnginePlugin, "update_url"):
            # detect the 1.4 API
            self.my_argument_one = url.query['my_argument_one']
            self.my_argument_two = url.query['my_argument_two']
        else:
            # detect the 1.3 and earlier API - mutate the
            # URL directly
            self.my_argument_one = url.query.pop('my_argument_one')
            self.my_argument_two = url.query.pop('my_argument_two')

        self.my_argument_three = kwargs.pop('my_argument_three', None)

    def update_url(self, url):
        # this method is only called in the 1.4 version
        return url.difference_update_query(
            ["my_argument_one", "my_argument_two"]
        )

See also

The URL object is now immutable - overview of the URL change which also includes notes regarding CreateEnginePlugin.

When the engine creation process completes and produces the Engine object, it is again passed to the plugin via the CreateEnginePlugin.engine_created() hook. In this hook, additional changes can be made to the engine, most typically involving setup of events (e.g. those defined in Core Events).

Members

__init__(), engine_created(), handle_dialect_kwargs(), handle_pool_kwargs(), update_url()

New in version 1.1.

method sqlalchemy.engine.CreateEnginePlugin.__init__(url, kwargs)

Construct a new CreateEnginePlugin.

The plugin object is instantiated individually for each call to create_engine(). A single Engine will be passed to the CreateEnginePlugin.engine_created() method corresponding to this URL.

Parameters:

• url –

  the URL object. The plugin may inspect the URL for arguments. Arguments used by the plugin should be removed, by returning an updated URL from the CreateEnginePlugin.update_url() method.

  Changed in version 1.4: The URL object is now immutable, so a CreateEnginePlugin that needs to alter the URL object should implement the CreateEnginePlugin.update_url() method.

• kwargs – The keyword arguments passed to create_engine().

method sqlalchemy.engine.CreateEnginePlugin.engine_created(engine)

Receive the Engine object when it is fully constructed.

The plugin may make additional changes to the engine, such as registering engine or connection pool events.

method sqlalchemy.engine.CreateEnginePlugin.handle_dialect_kwargs(dialect_cls, dialect_args)

parse and modify dialect kwargs

method sqlalchemy.engine.CreateEnginePlugin.handle_pool_kwargs(pool_cls, pool_args)

parse and modify pool kwargs

method sqlalchemy.engine.CreateEnginePlugin.update_url(url)

Update the URL.

A new URL should be returned. This method is typically used to consume configuration arguments from the URL which must be removed, as they will not be recognized by the dialect. The URL.difference_update_query() method is available to remove these arguments. See the docstring at CreateEnginePlugin for an example.

New in version 1.4.

class sqlalchemy.engine.Engine(pool, dialect, url, logging_name=None, echo=None, query_cache_size=500, execution_options=None, hide_parameters=False)

Connects a Pool and Dialect together to provide a source of database connectivity and behavior.

This is the SQLAlchemy 1.x version of Engine. For the 2.0 style version, which includes some API differences, see Engine.

An Engine object is instantiated publicly using the create_engine() function.

See also

Engine Configuration

Working with Engines and Connections

Members

begin(), clear_compiled_cache(), connect(), dispose(), driver, engine, execute(), execution_options(), get_execution_options(), has_table(), name, raw_connection(), run_callable(), scalar(), table_names(), transaction(), update_execution_options()

Class signature

class sqlalchemy.engine.Engine (sqlalchemy.engine.Connectable, sqlalchemy.log.Identified)

method sqlalchemy.engine.Engine.begin(close_with_result=False)

Return a context manager delivering a Connection with a Transaction established.

E.g.:

with engine.begin() as conn:
    conn.execute(
        text("insert into table (x, y, z) values (1, 2, 3)")
    )
    conn.execute(text("my_special_procedure(5)"))

Upon successful operation, the Transaction is committed. If an error is raised, the Transaction is rolled back.

Legacy use only: the close_with_result flag is normally False, and indicates that the Connection will be closed when the operation is complete. When set to True, it indicates the Connection is in “single use” mode, where the CursorResult returned by the first call to Connection.execute() will close the Connection when that CursorResult has exhausted all result rows.

See also

Engine.connect() - procure a Connection from an Engine.

Connection.begin() - start a Transaction for a particular Connection.

method sqlalchemy.engine.Engine.clear_compiled_cache()

Clear the compiled cache associated with the dialect.

This applies only to the built-in cache that is established via the create_engine.query_cache_size parameter. It will not impact any dictionary caches that were passed via the Connection.execution_options.query_cache parameter.

New in version 1.4.

method sqlalchemy.engine.Engine.connect(close_with_result=False)

Return a new Connection object.

The Connection object is a facade that uses a DBAPI connection internally in order to communicate with the database. This connection is procured from the connection-holding Pool referenced by this Engine. When the Connection.close() method of the Connection object is called, the underlying DBAPI connection is then returned to the connection pool, where it may be used again in a subsequent call to Engine.connect().

method sqlalchemy.engine.Engine.dispose(close=True)

Dispose of the connection pool used by this Engine.

A new connection pool is created immediately after the old one has been disposed. The previous connection pool is disposed either actively, by closing out all currently checked-in connections in that pool, or passively, by losing references to it but otherwise not closing any connections. The latter strategy is more appropriate for an initializer in a forked Python process.

Parameters:

close –

if left at its default of True, has the effect of fully closing all currently checked in database connections. Connections that are still checked out will not be closed, however they will no longer be associated with this Engine, so when they are closed individually, eventually the Pool which they are associated with will be garbage collected and they will be closed out fully, if not already closed on checkin.

If set to False, the previous connection pool is de-referenced, and otherwise not touched in any way.

New in version 1.4.33: Added the Engine.dispose.close parameter to allow the replacement of a connection pool in a child process without interfering with the connections used by the parent process.

See also

Engine Disposal

Using Connection Pools with Multiprocessing or os.fork()

attribute sqlalchemy.engine.Engine.driver

Driver name of the Dialect in use by this Engine.

attribute sqlalchemy.engine.Engine.engine

The Engine instance referred to by this Connectable.

May be self if this is already an Engine.

method sqlalchemy.engine.Engine.execute(statement, *multiparams, **params)

Executes the given construct and returns a CursorResult.

Deprecated since version 1.4: The Engine.execute() method is considered legacy as of the 1.x series of SQLAlchemy and will be removed in 2.0. All statement execution in SQLAlchemy 2.0 is performed by the Connection.execute() method of Connection, or in the ORM by the Session.execute() method of Session. (Background on SQLAlchemy 2.0 at: Migrating to SQLAlchemy 2.0)

The arguments are the same as those used by Connection.execute().

Here, a Connection is acquired using the Engine.connect() method, and the statement executed with that connection. The returned CursorResult is flagged such that when the CursorResult is exhausted and its underlying cursor is closed, the Connection created here will also be closed, which allows its associated DBAPI connection resource to be returned to the connection pool.

method sqlalchemy.engine.Engine.execution_options(**opt)

Return a new Engine that will provide Connection objects with the given execution options.

The returned Engine remains related to the original Engine in that it shares the same connection pool and other state:

• The Pool used by the new Engine is the same instance. The Engine.dispose() method will replace the connection pool instance for the parent engine as well as this one.

• Event listeners are “cascaded” - meaning, the new Engine inherits the events of the parent, and new events can be associated with the new Engine individually.

• The logging configuration and logging_name is copied from the parent Engine.

The intent of the Engine.execution_options() method is to implement “sharding” schemes where multiple Engine objects refer to the same connection pool, but are differentiated by options that would be consumed by a custom event:

primary_engine = create_engine("mysql://")
shard1 = primary_engine.execution_options(shard_id="shard1")
shard2 = primary_engine.execution_options(shard_id="shard2")

Above, the shard1 engine serves as a factory for Connection objects that will contain the execution option shard_id=shard1, and shard2 will produce Connection objects that contain the execution option shard_id=shard2.

An event handler can consume the above execution option to perform a schema switch or other operation, given a connection. Below we emit a MySQL use statement to switch databases, at the same time keeping track of which database we’ve established using the Connection.info dictionary, which gives us a persistent storage space that follows the DBAPI connection:

from sqlalchemy import event
from sqlalchemy.engine import Engine

shards = {"default": "base", shard_1: "db1", "shard_2": "db2"}

@event.listens_for(Engine, "before_cursor_execute")
def _switch_shard(conn, cursor, stmt,
        params, context, executemany):
    shard_id = conn._execution_options.get('shard_id', "default")
    current_shard = conn.info.get("current_shard", None)

    if current_shard != shard_id:
        cursor.execute("use %s" % shards[shard_id])
        conn.info["current_shard"] = shard_id

See also

Connection.execution_options() - update execution options on a Connection object.

Engine.update_execution_options() - update the execution options for a given Engine in place.

Engine.get_execution_options()

method sqlalchemy.engine.Engine.get_execution_options()

Get the non-SQL options which will take effect during execution.

See also

Engine.execution_options()

method sqlalchemy.engine.Engine.has_table(table_name, schema=None)

Return True if the given backend has a table of the given name.

Deprecated since version 1.4: The Engine.has_table() method is deprecated and will be removed in a future release. Please refer to Inspector.has_table().

See also

Fine Grained Reflection with Inspector - detailed schema inspection using the Inspector interface.

quoted_name - used to pass quoting information along with a schema identifier.

attribute sqlalchemy.engine.Engine.name

String name of the Dialect in use by this Engine.

method sqlalchemy.engine.Engine.raw_connection(_connection=None)

Return a “raw” DBAPI connection from the connection pool.

The returned object is a proxied version of the DBAPI connection object used by the underlying driver in use. The object will have all the same behavior as the real DBAPI connection, except that its close() method will result in the connection being returned to the pool, rather than being closed for real.

This method provides direct DBAPI connection access for special situations when the API provided by Connection is not needed. When a Connection object is already present, the DBAPI connection is available using the Connection.connection accessor.

See also

Working with Driver SQL and Raw DBAPI Connections

method sqlalchemy.engine.Engine.run_callable(callable_, *args, **kwargs)

Given a callable object or function, execute it, passing a Connection as the first argument.

Deprecated since version 1.4: The Engine.run_callable() method is deprecated and will be removed in a future release. Use the Engine.begin() context manager instead.

The given *args and **kwargs are passed subsequent to the Connection argument.

This function, along with Connection.run_callable(), allows a function to be run with a Connection or Engine object without the need to know which one is being dealt with.

method sqlalchemy.engine.Engine.scalar(statement, *multiparams, **params)

Executes and returns the first column of the first row.

Deprecated since version 1.4: The Engine.scalar() method is considered legacy as of the 1.x series of SQLAlchemy and will be removed in 2.0. All statement execution in SQLAlchemy 2.0 is performed by the Connection.execute() method of Connection, or in the ORM by the Session.execute() method of Session; the Result.scalar() method can then be used to return a scalar result. (Background on SQLAlchemy 2.0 at: Migrating to SQLAlchemy 2.0)

The underlying result/cursor is closed after execution.

method sqlalchemy.engine.Engine.table_names(schema=None, connection=None)

Return a list of all table names available in the database.

Deprecated since version 1.4: The Engine.table_names() method is deprecated and will be removed in a future release. Please refer to Inspector.get_table_names().

Parameters:

• schema – Optional, retrieve names from a non-default schema.

• connection – Optional, use a specified connection.

method sqlalchemy.engine.Engine.transaction(callable_, *args, **kwargs)

Execute the given function within a transaction boundary.

Deprecated since version 1.4: The Engine.transaction() method is deprecated and will be removed in a future release. Use the Engine.begin() context manager instead.

The function is passed a Connection newly procured from Engine.connect() as the first argument, followed by the given *args and **kwargs.

e.g.:

def do_something(conn, x, y):
    conn.execute(text("some statement"), {'x':x, 'y':y})

engine.transaction(do_something, 5, 10)

The operations inside the function are all invoked within the context of a single Transaction. Upon success, the transaction is committed. If an exception is raised, the transaction is rolled back before propagating the exception.

Note

The transaction() method is superseded by the usage of the Python with: statement, which can be used with Engine.begin():

with engine.begin() as conn:
    conn.execute(text("some statement"), {'x':5, 'y':10})

See also

Engine.begin() - engine-level transactional context

Connection.transaction() - connection-level version of Engine.transaction()

method sqlalchemy.engine.Engine.update_execution_options(**opt)

Update the default execution_options dictionary of this Engine.

The given keys/values in **opt are added to the default execution options that will be used for all connections. The initial contents of this dictionary can be sent via the execution_options parameter to create_engine().

See also

Connection.execution_options()

Engine.execution_options()

class sqlalchemy.engine.ExceptionContext

Encapsulate information about an error condition in progress.

This object exists solely to be passed to the ConnectionEvents.handle_error() event, supporting an interface that can be extended without backwards-incompatibility.

Members

chained_exception, connection, cursor, engine, execution_context, invalidate_pool_on_disconnect, is_disconnect, original_exception, parameters, sqlalchemy_exception, statement

New in version 0.9.7.

attribute sqlalchemy.engine.ExceptionContext.chained_exception = None

The exception that was returned by the previous handler in the exception chain, if any.

If present, this exception will be the one ultimately raised by SQLAlchemy unless a subsequent handler replaces it.

May be None.

attribute sqlalchemy.engine.ExceptionContext.connection = None

The Connection in use during the exception.

This member is present, except in the case of a failure when first connecting.

See also

ExceptionContext.engine

attribute sqlalchemy.engine.ExceptionContext.cursor = None

The DBAPI cursor object.

May be None.

attribute sqlalchemy.engine.ExceptionContext.engine = None

The Engine in use during the exception.

This member should always be present, even in the case of a failure when first connecting.

New in version 1.0.0.

attribute sqlalchemy.engine.ExceptionContext.execution_context = None

The ExecutionContext corresponding to the execution operation in progress.

This is present for statement execution operations, but not for operations such as transaction begin/end. It also is not present when the exception was raised before the ExecutionContext could be constructed.

Note that the ExceptionContext.statement and ExceptionContext.parameters members may represent a different value than that of the ExecutionContext, potentially in the case where a ConnectionEvents.before_cursor_execute() event or similar modified the statement/parameters to be sent.

May be None.

attribute sqlalchemy.engine.ExceptionContext.invalidate_pool_on_disconnect = True

Represent whether all connections in the pool should be invalidated when a “disconnect” condition is in effect.

Setting this flag to False within the scope of the ConnectionEvents.handle_error() event will have the effect such that the full collection of connections in the pool will not be invalidated during a disconnect; only the current connection that is the subject of the error will actually be invalidated.

The purpose of this flag is for custom disconnect-handling schemes where the invalidation of other connections in the pool is to be performed based on other conditions, or even on a per-connection basis.

New in version 1.0.3.

attribute sqlalchemy.engine.ExceptionContext.is_disconnect = None

Represent whether the exception as occurred represents a “disconnect” condition.

This flag will always be True or False within the scope of the ConnectionEvents.handle_error() handler.

SQLAlchemy will defer to this flag in order to determine whether or not the connection should be invalidated subsequently. That is, by assigning to this flag, a “disconnect” event which then results in a connection and pool invalidation can be invoked or prevented by changing this flag.

Note

The pool “pre_ping” handler enabled using the create_engine.pool_pre_ping parameter does not consult this event before deciding if the “ping” returned false, as opposed to receiving an unhandled error. For this use case, the legacy recipe based on engine_connect() may be used. A future API allow more comprehensive customization of the “disconnect” detection mechanism across all functions.

attribute sqlalchemy.engine.ExceptionContext.original_exception = None

The exception object which was caught.

This member is always present.

attribute sqlalchemy.engine.ExceptionContext.parameters = None

Parameter collection that was emitted directly to the DBAPI.

May be None.

attribute sqlalchemy.engine.ExceptionContext.sqlalchemy_exception = None

The sqlalchemy.exc.StatementError which wraps the original, and will be raised if exception handling is not circumvented by the event.

May be None, as not all exception types are wrapped by SQLAlchemy. For DBAPI-level exceptions that subclass the dbapi’s Error class, this field will always be present.

attribute sqlalchemy.engine.ExceptionContext.statement = None

String SQL statement that was emitted directly to the DBAPI.

May be None.

class sqlalchemy.engine.NestedTransaction(connection)

Represent a ‘nested’, or SAVEPOINT transaction.

The NestedTransaction object is created by calling the Connection.begin_nested() method of Connection.

When using NestedTransaction, the semantics of “begin” / “commit” / “rollback” are as follows:

• the “begin” operation corresponds to the “BEGIN SAVEPOINT” command, where the savepoint is given an explicit name that is part of the state of this object.

• The NestedTransaction.commit() method corresponds to a “RELEASE SAVEPOINT” operation, using the savepoint identifier associated with this NestedTransaction.

• The NestedTransaction.rollback() method corresponds to a “ROLLBACK TO SAVEPOINT” operation, using the savepoint identifier associated with this NestedTransaction.

The rationale for mimicking the semantics of an outer transaction in terms of savepoints so that code may deal with a “savepoint” transaction and an “outer” transaction in an agnostic way.

See also

Using SAVEPOINT - ORM version of the SAVEPOINT API.

Members

close(), commit(), rollback()

Class signature

class sqlalchemy.engine.NestedTransaction (sqlalchemy.engine.Transaction)

method sqlalchemy.engine.NestedTransaction.close()

inherited from the Transaction.close() method of Transaction

Close this Transaction.

If this transaction is the base transaction in a begin/commit nesting, the transaction will rollback(). Otherwise, the method returns.

This is used to cancel a Transaction without affecting the scope of an enclosing transaction.

method sqlalchemy.engine.NestedTransaction.commit()

inherited from the Transaction.commit() method of Transaction

Commit this Transaction.

The implementation of this may vary based on the type of transaction in use:

• For a simple database transaction (e.g. RootTransaction), it corresponds to a COMMIT.

• For a NestedTransaction, it corresponds to a “RELEASE SAVEPOINT” operation.

• For a TwoPhaseTransaction, DBAPI-specific methods for two phase transactions may be used.

method sqlalchemy.engine.NestedTransaction.rollback()

inherited from the Transaction.rollback() method of Transaction

Roll back this Transaction.

The implementation of this may vary based on the type of transaction in use:

• For a simple database transaction (e.g. RootTransaction), it corresponds to a ROLLBACK.

• For a NestedTransaction, it corresponds to a “ROLLBACK TO SAVEPOINT” operation.

• For a TwoPhaseTransaction, DBAPI-specific methods for two phase transactions may be used.

class sqlalchemy.engine.RootTransaction(connection)

Represent the “root” transaction on a Connection.

This corresponds to the current “BEGIN/COMMIT/ROLLBACK” that’s occurring for the Connection. The RootTransaction is created by calling upon the Connection.begin() method, and remains associated with the Connection throughout its active span. The current RootTransaction in use is accessible via the Connection.get_transaction method of Connection.

In 2.0 style use, the Connection also employs “autobegin” behavior that will create a new RootTransaction whenever a connection in a non-transactional state is used to emit commands on the DBAPI connection. The scope of the RootTransaction in 2.0 style use can be controlled using the Connection.commit() and Connection.rollback() methods.

Members

close(), commit(), rollback()

Class signature

class sqlalchemy.engine.RootTransaction (sqlalchemy.engine.Transaction)

method sqlalchemy.engine.RootTransaction.close()

inherited from the Transaction.close() method of Transaction

Close this Transaction.

If this transaction is the base transaction in a begin/commit nesting, the transaction will rollback(). Otherwise, the method returns.

This is used to cancel a Transaction without affecting the scope of an enclosing transaction.

method sqlalchemy.engine.RootTransaction.commit()

inherited from the Transaction.commit() method of Transaction

Commit this Transaction.

The implementation of this may vary based on the type of transaction in use:

• For a simple database transaction (e.g. RootTransaction), it corresponds to a COMMIT.

• For a NestedTransaction, it corresponds to a “RELEASE SAVEPOINT” operation.

• For a TwoPhaseTransaction, DBAPI-specific methods for two phase transactions may be used.

method sqlalchemy.engine.RootTransaction.rollback()

inherited from the Transaction.rollback() method of Transaction

Roll back this Transaction.

The implementation of this may vary based on the type of transaction in use:

• For a simple database transaction (e.g. RootTransaction), it corresponds to a ROLLBACK.

• For a NestedTransaction, it corresponds to a “ROLLBACK TO SAVEPOINT” operation.

• For a TwoPhaseTransaction, DBAPI-specific methods for two phase transactions may be used.

class sqlalchemy.engine.Transaction(connection)

Represent a database transaction in progress.

The Transaction object is procured by calling the Connection.begin() method of Connection:

from sqlalchemy import create_engine
engine = create_engine("postgresql://scott:tiger@localhost/test")
connection = engine.connect()
trans = connection.begin()
connection.execute(text("insert into x (a, b) values (1, 2)"))
trans.commit()

The object provides rollback() and commit() methods in order to control transaction boundaries. It also implements a context manager interface so that the Python with statement can be used with the Connection.begin() method:

with connection.begin():
    connection.execute(text("insert into x (a, b) values (1, 2)"))

The Transaction object is not threadsafe.

Members

close(), commit(), rollback()

See also

Connection.begin()

Connection.begin_twophase()

Connection.begin_nested()

Class signature

class sqlalchemy.engine.Transaction (sqlalchemy.engine.util.TransactionalContext)

method sqlalchemy.engine.Transaction.close()

Close this Transaction.

If this transaction is the base transaction in a begin/commit nesting, the transaction will rollback(). Otherwise, the method returns.

This is used to cancel a Transaction without affecting the scope of an enclosing transaction.

method sqlalchemy.engine.Transaction.commit()

Commit this Transaction.

The implementation of this may vary based on the type of transaction in use:

• For a simple database transaction (e.g. RootTransaction), it corresponds to a COMMIT.

• For a NestedTransaction, it corresponds to a “RELEASE SAVEPOINT” operation.

• For a TwoPhaseTransaction, DBAPI-specific methods for two phase transactions may be used.

method sqlalchemy.engine.Transaction.rollback()

Roll back this Transaction.

The implementation of this may vary based on the type of transaction in use:

• For a simple database transaction (e.g. RootTransaction), it corresponds to a ROLLBACK.

• For a NestedTransaction, it corresponds to a “ROLLBACK TO SAVEPOINT” operation.

• For a TwoPhaseTransaction, DBAPI-specific methods for two phase transactions may be used.

class sqlalchemy.engine.TwoPhaseTransaction(connection, xid)

Represent a two-phase transaction.

A new TwoPhaseTransaction object may be procured using the Connection.begin_twophase() method.

The interface is the same as that of Transaction with the addition of the prepare() method.

Members

close(), commit(), prepare(), rollback()

Class signature

class sqlalchemy.engine.TwoPhaseTransaction (sqlalchemy.engine.RootTransaction)

method sqlalchemy.engine.TwoPhaseTransaction.close()

inherited from the Transaction.close() method of Transaction

Close this Transaction.

If this transaction is the base transaction in a begin/commit nesting, the transaction will rollback(). Otherwise, the method returns.

This is used to cancel a Transaction without affecting the scope of an enclosing transaction.

method sqlalchemy.engine.TwoPhaseTransaction.commit()

inherited from the Transaction.commit() method of Transaction

Commit this Transaction.

The implementation of this may vary based on the type of transaction in use:

• For a simple database transaction (e.g. RootTransaction), it corresponds to a COMMIT.

• For a NestedTransaction, it corresponds to a “RELEASE SAVEPOINT” operation.

• For a TwoPhaseTransaction, DBAPI-specific methods for two phase transactions may be used.

method sqlalchemy.engine.TwoPhaseTransaction.prepare()

Prepare this TwoPhaseTransaction.

After a PREPARE, the transaction can be committed.

method sqlalchemy.engine.TwoPhaseTransaction.rollback()

inherited from the Transaction.rollback() method of Transaction

Roll back this Transaction.

The implementation of this may vary based on the type of transaction in use:

• For a simple database transaction (e.g. RootTransaction), it corresponds to a ROLLBACK.

• For a NestedTransaction, it corresponds to a “ROLLBACK TO SAVEPOINT” operation.

• For a TwoPhaseTransaction, DBAPI-specific methods for two phase transactions may be used.

Result Set API

Object Name	Description
BaseCursorResult	Base class for database result objects.
ChunkedIteratorResult	An IteratorResult that works from an iterator-producing callable.
CursorResult	A Result that is representing state from a DBAPI cursor.
FilterResult	A wrapper for a Result that returns objects other than Row objects, such as dictionaries or scalar objects.
FrozenResult	Represents a Result object in a “frozen” state suitable for caching.
IteratorResult	A Result that gets data from a Python iterator of Row objects or similar row-like data.
LegacyCursorResult	Legacy version of CursorResult.
LegacyRow	A subclass of Row that delivers 1.x SQLAlchemy behaviors for Core.
MappingResult	A wrapper for a Result that returns dictionary values rather than Row values.
MergedResult	A Result that is merged from any number of Result objects.
Result	Represent a set of database results.
Row	Represent a single result row.
RowMapping	A Mapping that maps column names and objects to Row values.
ScalarResult	A wrapper for a Result that returns scalar values rather than Row values.

class sqlalchemy.engine.BaseCursorResult(context, cursor_strategy, cursor_description)

Members

inserted_primary_key, inserted_primary_key_rows, is_insert, last_inserted_params(), last_updated_params(), lastrow_has_defaults(), lastrowid, postfetch_cols(), prefetch_cols(), returned_defaults, returned_defaults_rows, returns_rows, rowcount, supports_sane_multi_rowcount(), supports_sane_rowcount()

Base class for database result objects.

attribute sqlalchemy.engine.BaseCursorResult.inserted_primary_key

Return the primary key for the row just inserted.

The return value is a Row object representing a named tuple of primary key values in the order in which the primary key columns are configured in the source Table.

Changed in version 1.4.8: - the CursorResult.inserted_primary_key value is now a named tuple via the Row class, rather than a plain tuple.

This accessor only applies to single row insert() constructs which did not explicitly specify Insert.returning(). Support for multirow inserts, while not yet available for most backends, would be accessed using the CursorResult.inserted_primary_key_rows accessor.

Note that primary key columns which specify a server_default clause, or otherwise do not qualify as “autoincrement” columns (see the notes at Column), and were generated using the database-side default, will appear in this list as None unless the backend supports “returning” and the insert statement executed with the “implicit returning” enabled.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an insert() construct.

attribute sqlalchemy.engine.BaseCursorResult.inserted_primary_key_rows

Return the value of CursorResult.inserted_primary_key as a row contained within a list; some dialects may support a multiple row form as well.

Note

As indicated below, in current SQLAlchemy versions this accessor is only useful beyond what’s already supplied by CursorResult.inserted_primary_key when using the psycopg2 dialect. Future versions hope to generalize this feature to more dialects.

This accessor is added to support dialects that offer the feature that is currently implemented by the Psycopg2 Fast Execution Helpers feature, currently only the psycopg2 dialect, which provides for many rows to be INSERTed at once while still retaining the behavior of being able to return server-generated primary key values.

• When using the psycopg2 dialect, or other dialects that may support “fast executemany” style inserts in upcoming releases : When invoking an INSERT statement while passing a list of rows as the second argument to Connection.execute(), this accessor will then provide a list of rows, where each row contains the primary key value for each row that was INSERTed.

• When using all other dialects / backends that don’t yet support this feature: This accessor is only useful for single row INSERT statements, and returns the same information as that of the CursorResult.inserted_primary_key within a single-element list. When an INSERT statement is executed in conjunction with a list of rows to be INSERTed, the list will contain one row per row inserted in the statement, however it will contain None for any server-generated values.

Future releases of SQLAlchemy will further generalize the “fast execution helper” feature of psycopg2 to suit other dialects, thus allowing this accessor to be of more general use.

New in version 1.4.

See also

CursorResult.inserted_primary_key

attribute sqlalchemy.engine.BaseCursorResult.is_insert

True if this CursorResult is the result of a executing an expression language compiled insert() construct.

When True, this implies that the inserted_primary_key attribute is accessible, assuming the statement did not include a user defined “returning” construct.

method sqlalchemy.engine.BaseCursorResult.last_inserted_params()

Return the collection of inserted parameters from this execution.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an insert() construct.

method sqlalchemy.engine.BaseCursorResult.last_updated_params()

Return the collection of updated parameters from this execution.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an update() construct.

method sqlalchemy.engine.BaseCursorResult.lastrow_has_defaults()

Return lastrow_has_defaults() from the underlying ExecutionContext.

See ExecutionContext for details.

attribute sqlalchemy.engine.BaseCursorResult.lastrowid

Return the ‘lastrowid’ accessor on the DBAPI cursor.

This is a DBAPI specific method and is only functional for those backends which support it, for statements where it is appropriate. It’s behavior is not consistent across backends.

Usage of this method is normally unnecessary when using insert() expression constructs; the CursorResult.inserted_primary_key attribute provides a tuple of primary key values for a newly inserted row, regardless of database backend.

method sqlalchemy.engine.BaseCursorResult.postfetch_cols()

Return postfetch_cols() from the underlying ExecutionContext.

See ExecutionContext for details.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an insert() or update() construct.

method sqlalchemy.engine.BaseCursorResult.prefetch_cols()

Return prefetch_cols() from the underlying ExecutionContext.

See ExecutionContext for details.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an insert() or update() construct.

attribute sqlalchemy.engine.BaseCursorResult.returned_defaults

Return the values of default columns that were fetched using the ValuesBase.return_defaults() feature.

The value is an instance of Row, or None if ValuesBase.return_defaults() was not used or if the backend does not support RETURNING.

New in version 0.9.0.

See also

ValuesBase.return_defaults()

attribute sqlalchemy.engine.BaseCursorResult.returned_defaults_rows

Return a list of rows each containing the values of default columns that were fetched using the ValuesBase.return_defaults() feature.

The return value is a list of Row objects.

New in version 1.4.

attribute sqlalchemy.engine.BaseCursorResult.returns_rows

True if this CursorResult returns zero or more rows.

I.e. if it is legal to call the methods CursorResult.fetchone(), CursorResult.fetchmany() CursorResult.fetchall().

Overall, the value of CursorResult.returns_rows should always be synonymous with whether or not the DBAPI cursor had a .description attribute, indicating the presence of result columns, noting that a cursor that returns zero rows still has a .description if a row-returning statement was emitted.

This attribute should be True for all results that are against SELECT statements, as well as for DML statements INSERT/UPDATE/DELETE that use RETURNING. For INSERT/UPDATE/DELETE statements that were not using RETURNING, the value will usually be False, however there are some dialect-specific exceptions to this, such as when using the MSSQL / pyodbc dialect a SELECT is emitted inline in order to retrieve an inserted primary key value.

attribute sqlalchemy.engine.BaseCursorResult.rowcount

Return the ‘rowcount’ for this result.

The ‘rowcount’ reports the number of rows matched by the WHERE criterion of an UPDATE or DELETE statement.

Note

Notes regarding CursorResult.rowcount:

• This attribute returns the number of rows matched, which is not necessarily the same as the number of rows that were actually modified - an UPDATE statement, for example, may have no net change on a given row if the SET values given are the same as those present in the row already. Such a row would be matched but not modified. On backends that feature both styles, such as MySQL, rowcount is configured by default to return the match count in all cases.

• CursorResult.rowcount is only useful in conjunction with an UPDATE or DELETE statement. Contrary to what the Python DBAPI says, it does not return the number of rows available from the results of a SELECT statement as DBAPIs cannot support this functionality when rows are unbuffered.

• CursorResult.rowcount may not be fully implemented by all dialects. In particular, most DBAPIs do not support an aggregate rowcount result from an executemany call. The CursorResult.supports_sane_rowcount() and CursorResult.supports_sane_multi_rowcount() methods will report from the dialect if each usage is known to be supported.

• Statements that use RETURNING may not return a correct rowcount.

See also

Getting Affected Row Count from UPDATE, DELETE - in the SQLAlchemy 1.4 / 2.0 Tutorial

method sqlalchemy.engine.BaseCursorResult.supports_sane_multi_rowcount()

Return supports_sane_multi_rowcount from the dialect.

See CursorResult.rowcount for background.

method sqlalchemy.engine.BaseCursorResult.supports_sane_rowcount()

Return supports_sane_rowcount from the dialect.

See CursorResult.rowcount for background.

class sqlalchemy.engine.ChunkedIteratorResult(cursor_metadata, chunks, source_supports_scalars=False, raw=None, dynamic_yield_per=False)

An IteratorResult that works from an iterator-producing callable.

The given chunks argument is a function that is given a number of rows to return in each chunk, or None for all rows. The function should then return an un-consumed iterator of lists, each list of the requested size.

The function can be called at any time again, in which case it should continue from the same result set but adjust the chunk size as given.

New in version 1.4.

Members

yield_per()

Class signature

class sqlalchemy.engine.ChunkedIteratorResult (sqlalchemy.engine.IteratorResult)

method sqlalchemy.engine.ChunkedIteratorResult.yield_per(num)

Configure the row-fetching strategy to fetch num rows at a time.

This impacts the underlying behavior of the result when iterating over the result object, or otherwise making use of methods such as Result.fetchone() that return one row at a time. Data from the underlying cursor or other data source will be buffered up to this many rows in memory, and the buffered collection will then be yielded out one row at at time or as many rows are requested. Each time the buffer clears, it will be refreshed to this many rows or as many rows remain if fewer remain.

The Result.yield_per() method is generally used in conjunction with the Connection.execution_options.stream_results execution option, which will allow the database dialect in use to make use of a server side cursor, if the DBAPI supports a specific “server side cursor” mode separate from its default mode of operation.

Tip

Consider using the Connection.execution_options.yield_per execution option, which will simultaneously set Connection.execution_options.stream_results to ensure the use of server side cursors, as well as automatically invoke the Result.yield_per() method to establish a fixed row buffer size at once.

The Connection.execution_options.yield_per execution option is available for ORM operations, with Session-oriented use described at Fetching Large Result Sets with Yield Per. The Core-only version which works with Connection is new as of SQLAlchemy 1.4.40.

New in version 1.4.

Parameters:

num – number of rows to fetch each time the buffer is refilled. If set to a value below 1, fetches all rows for the next buffer.

See also

Using Server Side Cursors (a.k.a. stream results) - describes Core behavior for Result.yield_per()

Fetching Large Result Sets with Yield Per - in the ORM Querying Guide

class sqlalchemy.engine.FilterResult

A wrapper for a Result that returns objects other than Row objects, such as dictionaries or scalar objects.

FilterResult is the common base for additional result APIs including MappingResult, ScalarResult and AsyncResult.

Members

close(), closed, yield_per()

Class signature

class sqlalchemy.engine.FilterResult (sqlalchemy.engine.ResultInternal)

method sqlalchemy.engine.FilterResult.close()

Close this FilterResult.

New in version 1.4.43.

attribute sqlalchemy.engine.FilterResult.closed

Return True if the underlying Result reports closed

New in version 1.4.43.

method sqlalchemy.engine.FilterResult.yield_per(num)

Configure the row-fetching strategy to fetch num rows at a time.

The FilterResult.yield_per() method is a pass through to the Result.yield_per() method. See that method’s documentation for usage notes.

New in version 1.4.40: - added FilterResult.yield_per() so that the method is available on all result set implementations

See also

Using Server Side Cursors (a.k.a. stream results) - describes Core behavior for Result.yield_per()

Fetching Large Result Sets with Yield Per - in the ORM Querying Guide

class sqlalchemy.engine.FrozenResult(result)

Represents a Result object in a “frozen” state suitable for caching.

The FrozenResult object is returned from the Result.freeze() method of any Result object.

A new iterable Result object is generated from a fixed set of data each time the FrozenResult is invoked as a callable:

result = connection.execute(query)

frozen = result.freeze()

unfrozen_result_one = frozen()

for row in unfrozen_result_one:
    print(row)

unfrozen_result_two = frozen()
rows = unfrozen_result_two.all()

# ... etc

New in version 1.4.

See also

Re-Executing Statements - example usage within the ORM to implement a result-set cache.

merge_frozen_result() - ORM function to merge a frozen result back into a Session.

class sqlalchemy.engine.IteratorResult(cursor_metadata, iterator, raw=None, _source_supports_scalars=False)

A Result that gets data from a Python iterator of Row objects or similar row-like data.

New in version 1.4.

Members

closed

Class signature

class sqlalchemy.engine.IteratorResult (sqlalchemy.engine.Result)

attribute sqlalchemy.engine.IteratorResult.closed

Return True if this IteratorResult has been closed

New in version 1.4.43.

class sqlalchemy.engine.LegacyRow(parent, processors, keymap, key_style, data)

A subclass of Row that delivers 1.x SQLAlchemy behaviors for Core.

The LegacyRow class is where most of the Python mapping (i.e. dictionary-like) behaviors are implemented for the row object. The mapping behavior of Row going forward is accessible via the _mapping attribute.

New in version 1.4: - added LegacyRow which encapsulates most of the deprecated behaviors of Row.

Members

has_key(), items(), iterkeys(), itervalues(), values()

Class signature

class sqlalchemy.engine.LegacyRow (sqlalchemy.engine.Row)

method sqlalchemy.engine.LegacyRow.has_key(key)

Return True if this LegacyRow contains the given key.

Deprecated since version 1.4: The LegacyRow.has_key() method is deprecated and will be removed in a future release. To test for key membership, use the Row._mapping attribute, i.e. ‘key in row._mapping`.

Through the SQLAlchemy 1.x series, the __contains__() method of Row (or LegacyRow as of SQLAlchemy 1.4) also links to Row.has_key(), in that an expression such as

"some_col" in row

Will return True if the row contains a column named "some_col", in the way that a Python mapping works.

However, it is planned that the 2.0 series of SQLAlchemy will reverse this behavior so that __contains__() will refer to a value being present in the row, in the way that a Python tuple works.

See also

RowProxy is no longer a “proxy”; is now called Row and behaves like an enhanced named tuple

method sqlalchemy.engine.LegacyRow.items()

Return a list of tuples, each tuple containing a key/value pair.

Deprecated since version 1.4: The LegacyRow.items() method is deprecated and will be removed in a future release. Use the Row._mapping attribute, i.e., ‘row._mapping.items()’.

This method is analogous to the Python dictionary .items() method, except that it returns a list, not an iterator.

method sqlalchemy.engine.LegacyRow.iterkeys()

Return a an iterator against the Row.keys() method.

Deprecated since version 1.4: The LegacyRow.iterkeys() method is deprecated and will be removed in a future release. Use the Row._mapping attribute, i.e., ‘row._mapping.keys()’.

This method is analogous to the Python-2-only dictionary .iterkeys() method.

method sqlalchemy.engine.LegacyRow.itervalues()

Return a an iterator against the Row.values() method.

Deprecated since version 1.4: The LegacyRow.itervalues() method is deprecated and will be removed in a future release. Use the Row._mapping attribute, i.e., ‘row._mapping.values()’.

This method is analogous to the Python-2-only dictionary .itervalues() method.

method sqlalchemy.engine.LegacyRow.values()

Return the values represented by this Row as a list.

Deprecated since version 1.4: The LegacyRow.values() method is deprecated and will be removed in a future release. Use the Row._mapping attribute, i.e., ‘row._mapping.values()’.

This method is analogous to the Python dictionary .values() method, except that it returns a list, not an iterator.

class sqlalchemy.engine.MergedResult(cursor_metadata, results)

A Result that is merged from any number of Result objects.

Returned by the Result.merge() method.

New in version 1.4.

Class signature

class sqlalchemy.engine.MergedResult (sqlalchemy.engine.IteratorResult)

class sqlalchemy.engine.Result(cursor_metadata)

Represent a set of database results.

New in version 1.4: The Result object provides a completely updated usage model and calling facade for SQLAlchemy Core and SQLAlchemy ORM. In Core, it forms the basis of the CursorResult object which replaces the previous ResultProxy interface. When using the ORM, a higher level object called ChunkedIteratorResult is normally used.

Note

In SQLAlchemy 1.4 and above, this object is used for ORM results returned by Session.execute(), which can yield instances of ORM mapped objects either individually or within tuple-like rows. Note that the Result object does not deduplicate instances or rows automatically as is the case with the legacy Query object. For in-Python de-duplication of instances or rows, use the Result.unique() modifier method.

See also

Fetching Rows - in the SQLAlchemy 1.4 / 2.0 Tutorial

Members

all(), close(), closed, columns(), fetchall(), fetchmany(), fetchone(), first(), freeze(), keys(), mappings(), merge(), one(), one_or_none(), partitions(), scalar(), scalar_one(), scalar_one_or_none(), scalars(), unique(), yield_per()

Class signature

class sqlalchemy.engine.Result (sqlalchemy.engine._WithKeys, sqlalchemy.engine.ResultInternal)

method sqlalchemy.engine.Result.all()

Return all rows in a list.

Closes the result set after invocation. Subsequent invocations will return an empty list.

New in version 1.4.

Returns:

a list of Row objects.

method sqlalchemy.engine.Result.close()

close this Result.

The behavior of this method is implementation specific, and is not implemented by default. The method should generally end the resources in use by the result object and also cause any subsequent iteration or row fetching to raise ResourceClosedError.

New in version 1.4.27: - .close() was previously not generally available for all Result classes, instead only being available on the CursorResult returned for Core statement executions. As most other result objects, namely the ones used by the ORM, are proxying a CursorResult in any case, this allows the underlying cursor result to be closed from the outside facade for the case when the ORM query is using the yield_per execution option where it does not immediately exhaust and autoclose the database cursor.

attribute sqlalchemy.engine.Result.closed

return True if this Result reports .closed

New in version 1.4.43.

method sqlalchemy.engine.Result.columns(*col_expressions)

Establish the columns that should be returned in each row.

This method may be used to limit the columns returned as well as to reorder them. The given list of expressions are normally a series of integers or string key names. They may also be appropriate ColumnElement objects which correspond to a given statement construct.

E.g.:

statement = select(table.c.x, table.c.y, table.c.z)
result = connection.execute(statement)

for z, y in result.columns('z', 'y'):
    # ...

Example of using the column objects from the statement itself:

for z, y in result.columns(
        statement.selected_columns.c.z,
        statement.selected_columns.c.y
):
    # ...

New in version 1.4.

Parameters:

*col_expressions – indicates columns to be returned. Elements may be integer row indexes, string column names, or appropriate ColumnElement objects corresponding to a select construct.

Returns:

this Result object with the modifications given.

method sqlalchemy.engine.Result.fetchall()

A synonym for the Result.all() method.

method sqlalchemy.engine.Result.fetchmany(size=None)

Fetch many rows.

When all rows are exhausted, returns an empty list.

This method is provided for backwards compatibility with SQLAlchemy 1.x.x.

To fetch rows in groups, use the Result.partitions() method.

Returns:

a list of Row objects.

See also

Result.partitions()

method sqlalchemy.engine.Result.fetchone()

Fetch one row.

When all rows are exhausted, returns None.

This method is provided for backwards compatibility with SQLAlchemy 1.x.x.

To fetch the first row of a result only, use the Result.first() method. To iterate through all rows, iterate the Result object directly.

Returns:

a Row object if no filters are applied, or None if no rows remain.

method sqlalchemy.engine.Result.first()

Fetch the first row or None if no row is present.

Closes the result set and discards remaining rows.

Note

This method returns one row, e.g. tuple, by default. To return exactly one single scalar value, that is, the first column of the first row, use the Result.scalar() method, or combine Result.scalars() and Result.first().

Additionally, in contrast to the behavior of the legacy ORM Query.first() method, no limit is applied to the SQL query which was invoked to produce this Result; for a DBAPI driver that buffers results in memory before yielding rows, all rows will be sent to the Python process and all but the first row will be discarded.

See also

ORM Query Unified with Core Select

Returns:

a Row object, or None if no rows remain.

See also

Result.scalar()

Result.one()

method sqlalchemy.engine.Result.freeze()

Return a callable object that will produce copies of this Result when invoked.

The callable object returned is an instance of FrozenResult.

This is used for result set caching. The method must be called on the result when it has been unconsumed, and calling the method will consume the result fully. When the FrozenResult is retrieved from a cache, it can be called any number of times where it will produce a new Result object each time against its stored set of rows.

See also

Re-Executing Statements - example usage within the ORM to implement a result-set cache.

method sqlalchemy.engine.Result.keys()

inherited from the sqlalchemy.engine._WithKeys.keys method of sqlalchemy.engine._WithKeys

Return an iterable view which yields the string keys that would be represented by each Row.

The keys can represent the labels of the columns returned by a core statement or the names of the orm classes returned by an orm execution.

The view also can be tested for key containment using the Python in operator, which will test both for the string keys represented in the view, as well as for alternate keys such as column objects.

Changed in version 1.4: a key view object is returned rather than a plain list.

method sqlalchemy.engine.Result.mappings()

Apply a mappings filter to returned rows, returning an instance of MappingResult.

When this filter is applied, fetching rows will return RowMapping objects instead of Row objects.

New in version 1.4.

Returns:

a new MappingResult filtering object referring to this Result object.

method sqlalchemy.engine.Result.merge(*others)

Merge this Result with other compatible result objects.

The object returned is an instance of MergedResult, which will be composed of iterators from the given result objects.

The new result will use the metadata from this result object. The subsequent result objects must be against an identical set of result / cursor metadata, otherwise the behavior is undefined.

method sqlalchemy.engine.Result.one()

Return exactly one row or raise an exception.

Raises NoResultFound if the result returns no rows, or MultipleResultsFound if multiple rows would be returned.

Note

This method returns one row, e.g. tuple, by default. To return exactly one single scalar value, that is, the first column of the first row, use the Result.scalar_one() method, or combine Result.scalars() and Result.one().

New in version 1.4.

Returns:

The first Row.

Raises:

MultipleResultsFound, NoResultFound

See also

Result.first()

Result.one_or_none()

Result.scalar_one()

method sqlalchemy.engine.Result.one_or_none()

Return at most one result or raise an exception.

Returns None if the result has no rows. Raises MultipleResultsFound if multiple rows are returned.

New in version 1.4.

Returns:

The first Row or None if no row is available.

Raises:

MultipleResultsFound

See also

Result.first()

Result.one()

method sqlalchemy.engine.Result.partitions(size=None)

Iterate through sub-lists of rows of the size given.

Each list will be of the size given, excluding the last list to be yielded, which may have a small number of rows. No empty lists will be yielded.

The result object is automatically closed when the iterator is fully consumed.

Note that the backend driver will usually buffer the entire result ahead of time unless the Connection.execution_options.stream_results execution option is used indicating that the driver should not pre-buffer results, if possible. Not all drivers support this option and the option is silently ignored for those who do not.

When using the ORM, the Result.partitions() method is typically more effective from a memory perspective when it is combined with use of the yield_per execution option, which instructs both the DBAPI driver to use server side cursors, if available, as well as instructs the ORM loading internals to only build a certain amount of ORM objects from a result at a time before yielding them out.

New in version 1.4.

Parameters:

size – indicate the maximum number of rows to be present in each list yielded. If None, makes use of the value set by the Result.yield_per(), method, if it were called, or the Connection.execution_options.yield_per execution option, which is equivalent in this regard. If yield_per weren’t set, it makes use of the Result.fetchmany() default, which may be backend specific and not well defined.

Returns:

iterator of lists

See also

Using Server Side Cursors (a.k.a. stream results)

Fetching Large Result Sets with Yield Per - in the ORM Querying Guide

method sqlalchemy.engine.Result.scalar()

Fetch the first column of the first row, and close the result set.

Returns None if there are no rows to fetch.

No validation is performed to test if additional rows remain.

After calling this method, the object is fully closed, e.g. the CursorResult.close() method will have been called.

Returns:

a Python scalar value, or None if no rows remain.

method sqlalchemy.engine.Result.scalar_one()

Return exactly one scalar result or raise an exception.

This is equivalent to calling Result.scalars() and then Result.one().

See also

Result.one()

Result.scalars()

method sqlalchemy.engine.Result.scalar_one_or_none()

Return exactly one scalar result or None.

This is equivalent to calling Result.scalars() and then Result.one_or_none().

See also

Result.one_or_none()

Result.scalars()

method sqlalchemy.engine.Result.scalars(index=0)

Return a ScalarResult filtering object which will return single elements rather than Row objects.

E.g.:

>>> result = conn.execute(text("select int_id from table"))
>>> result.scalars().all()
[1, 2, 3]

When results are fetched from the ScalarResult filtering object, the single column-row that would be returned by the Result is instead returned as the column’s value.

New in version 1.4.

Parameters:

index – integer or row key indicating the column to be fetched from each row, defaults to 0 indicating the first column.

Returns:

a new ScalarResult filtering object referring to this Result object.

method sqlalchemy.engine.Result.unique(strategy=None)

Apply unique filtering to the objects returned by this Result.

When this filter is applied with no arguments, the rows or objects returned will filtered such that each row is returned uniquely. The algorithm used to determine this uniqueness is by default the Python hashing identity of the whole tuple. In some cases a specialized per-entity hashing scheme may be used, such as when using the ORM, a scheme is applied which works against the primary key identity of returned objects.

The unique filter is applied after all other filters, which means if the columns returned have been refined using a method such as the Result.columns() or Result.scalars() method, the uniquing is applied to only the column or columns returned. This occurs regardless of the order in which these methods have been called upon the Result object.

The unique filter also changes the calculus used for methods like Result.fetchmany() and Result.partitions(). When using Result.unique(), these methods will continue to yield the number of rows or objects requested, after uniquing has been applied. However, this necessarily impacts the buffering behavior of the underlying cursor or datasource, such that multiple underlying calls to cursor.fetchmany() may be necessary in order to accumulate enough objects in order to provide a unique collection of the requested size.

Parameters:

strategy – a callable that will be applied to rows or objects being iterated, which should return an object that represents the unique value of the row. A Python set() is used to store these identities. If not passed, a default uniqueness strategy is used which may have been assembled by the source of this Result object.

method sqlalchemy.engine.Result.yield_per(num)

Configure the row-fetching strategy to fetch num rows at a time.

This impacts the underlying behavior of the result when iterating over the result object, or otherwise making use of methods such as Result.fetchone() that return one row at a time. Data from the underlying cursor or other data source will be buffered up to this many rows in memory, and the buffered collection will then be yielded out one row at at time or as many rows are requested. Each time the buffer clears, it will be refreshed to this many rows or as many rows remain if fewer remain.

The Result.yield_per() method is generally used in conjunction with the Connection.execution_options.stream_results execution option, which will allow the database dialect in use to make use of a server side cursor, if the DBAPI supports a specific “server side cursor” mode separate from its default mode of operation.

Tip

Consider using the Connection.execution_options.yield_per execution option, which will simultaneously set Connection.execution_options.stream_results to ensure the use of server side cursors, as well as automatically invoke the Result.yield_per() method to establish a fixed row buffer size at once.

The Connection.execution_options.yield_per execution option is available for ORM operations, with Session-oriented use described at Fetching Large Result Sets with Yield Per. The Core-only version which works with Connection is new as of SQLAlchemy 1.4.40.

New in version 1.4.

Parameters:

num – number of rows to fetch each time the buffer is refilled. If set to a value below 1, fetches all rows for the next buffer.

See also

Using Server Side Cursors (a.k.a. stream results) - describes Core behavior for Result.yield_per()

Fetching Large Result Sets with Yield Per - in the ORM Querying Guide

class sqlalchemy.engine.ScalarResult(real_result, index)

A wrapper for a Result that returns scalar values rather than Row values.

The ScalarResult object is acquired by calling the Result.scalars() method.

A special limitation of ScalarResult is that it has no fetchone() method; since the semantics of fetchone() are that the None value indicates no more results, this is not compatible with ScalarResult since there is no way to distinguish between None as a row value versus None as an indicator. Use next(result) to receive values individually.

Members

all(), close(), closed, fetchall(), fetchmany(), first(), one(), one_or_none(), partitions(), unique(), yield_per()

Class signature

class sqlalchemy.engine.ScalarResult (sqlalchemy.engine.FilterResult)

method sqlalchemy.engine.ScalarResult.all()

Return all scalar values in a list.

Equivalent to Result.all() except that scalar values, rather than Row objects, are returned.

method sqlalchemy.engine.ScalarResult.close()

inherited from the FilterResult.close() method of FilterResult

Close this FilterResult.

New in version 1.4.43.

attribute sqlalchemy.engine.ScalarResult.closed

inherited from the FilterResult.closed attribute of FilterResult

Return True if the underlying Result reports closed

New in version 1.4.43.

method sqlalchemy.engine.ScalarResult.fetchall()

A synonym for the ScalarResult.all() method.

method sqlalchemy.engine.ScalarResult.fetchmany(size=None)

Fetch many objects.

Equivalent to Result.fetchmany() except that scalar values, rather than Row objects, are returned.

method sqlalchemy.engine.ScalarResult.first()

Fetch the first object or None if no object is present.

Equivalent to Result.first() except that scalar values, rather than Row objects, are returned.

method sqlalchemy.engine.ScalarResult.one()

Return exactly one object or raise an exception.

Equivalent to Result.one() except that scalar values, rather than Row objects, are returned.

method sqlalchemy.engine.ScalarResult.one_or_none()

Return at most one object or raise an exception.

Equivalent to Result.one_or_none() except that scalar values, rather than Row objects, are returned.

method sqlalchemy.engine.ScalarResult.partitions(size=None)

Iterate through sub-lists of elements of the size given.

Equivalent to Result.partitions() except that scalar values, rather than Row objects, are returned.

method sqlalchemy.engine.ScalarResult.unique(strategy=None)

Apply unique filtering to the objects returned by this ScalarResult.

See Result.unique() for usage details.

method sqlalchemy.engine.ScalarResult.yield_per(num)

inherited from the FilterResult.yield_per() method of FilterResult

Configure the row-fetching strategy to fetch num rows at a time.

The FilterResult.yield_per() method is a pass through to the Result.yield_per() method. See that method’s documentation for usage notes.

New in version 1.4.40: - added FilterResult.yield_per() so that the method is available on all result set implementations

See also

Using Server Side Cursors (a.k.a. stream results) - describes Core behavior for Result.yield_per()

Fetching Large Result Sets with Yield Per - in the ORM Querying Guide

class sqlalchemy.engine.MappingResult(result)

A wrapper for a Result that returns dictionary values rather than Row values.

The MappingResult object is acquired by calling the Result.mappings() method.

Members

all(), close(), closed, columns(), fetchall(), fetchmany(), fetchone(), first(), keys(), one(), one_or_none(), partitions(), unique(), yield_per()

Class signature

class sqlalchemy.engine.MappingResult (sqlalchemy.engine._WithKeys, sqlalchemy.engine.FilterResult)

method sqlalchemy.engine.MappingResult.all()

Return all scalar values in a list.

Equivalent to Result.all() except that RowMapping values, rather than Row objects, are returned.

method sqlalchemy.engine.MappingResult.close()

inherited from the FilterResult.close() method of FilterResult

Close this FilterResult.

New in version 1.4.43.

attribute sqlalchemy.engine.MappingResult.closed

inherited from the FilterResult.closed attribute of FilterResult

Return True if the underlying Result reports closed

New in version 1.4.43.

method sqlalchemy.engine.MappingResult.columns(*col_expressions)

Establish the columns that should be returned in each row.

method sqlalchemy.engine.MappingResult.fetchall()

A synonym for the MappingResult.all() method.

method sqlalchemy.engine.MappingResult.fetchmany(size=None)

Fetch many objects.

Equivalent to Result.fetchmany() except that RowMapping values, rather than Row objects, are returned.

method sqlalchemy.engine.MappingResult.fetchone()

Fetch one object.

Equivalent to Result.fetchone() except that RowMapping values, rather than Row objects, are returned.

method sqlalchemy.engine.MappingResult.first()

Fetch the first object or None if no object is present.

Equivalent to Result.first() except that RowMapping values, rather than Row objects, are returned.

method sqlalchemy.engine.MappingResult.keys()

inherited from the sqlalchemy.engine._WithKeys.keys method of sqlalchemy.engine._WithKeys

Return an iterable view which yields the string keys that would be represented by each Row.

The keys can represent the labels of the columns returned by a core statement or the names of the orm classes returned by an orm execution.

The view also can be tested for key containment using the Python in operator, which will test both for the string keys represented in the view, as well as for alternate keys such as column objects.

Changed in version 1.4: a key view object is returned rather than a plain list.

method sqlalchemy.engine.MappingResult.one()

Return exactly one object or raise an exception.

Equivalent to Result.one() except that RowMapping values, rather than Row objects, are returned.

method sqlalchemy.engine.MappingResult.one_or_none()

Return at most one object or raise an exception.

Equivalent to Result.one_or_none() except that RowMapping values, rather than Row objects, are returned.

method sqlalchemy.engine.MappingResult.partitions(size=None)

Iterate through sub-lists of elements of the size given.

Equivalent to Result.partitions() except that RowMapping values, rather than Row objects, are returned.

method sqlalchemy.engine.MappingResult.unique(strategy=None)

Apply unique filtering to the objects returned by this MappingResult.

See Result.unique() for usage details.

method sqlalchemy.engine.MappingResult.yield_per(num)

inherited from the FilterResult.yield_per() method of FilterResult

Configure the row-fetching strategy to fetch num rows at a time.

The FilterResult.yield_per() method is a pass through to the Result.yield_per() method. See that method’s documentation for usage notes.

New in version 1.4.40: - added FilterResult.yield_per() so that the method is available on all result set implementations

See also

Using Server Side Cursors (a.k.a. stream results) - describes Core behavior for Result.yield_per()

Fetching Large Result Sets with Yield Per - in the ORM Querying Guide

class sqlalchemy.engine.CursorResult(context, cursor_strategy, cursor_description)

A Result that is representing state from a DBAPI cursor.

Changed in version 1.4: The CursorResult and LegacyCursorResult classes replace the previous ResultProxy interface. These classes are based on the Result calling API which provides an updated usage model and calling facade for SQLAlchemy Core and SQLAlchemy ORM.

Returns database rows via the Row class, which provides additional API features and behaviors on top of the raw data returned by the DBAPI. Through the use of filters such as the Result.scalars() method, other kinds of objects may also be returned.

Within the scope of the 1.x series of SQLAlchemy, Core SQL results in version 1.4 return an instance of LegacyCursorResult which takes the place of the CursorResult class used for the 1.3 series and previously. This object returns rows as LegacyRow objects, which maintains Python mapping (i.e. dictionary) like behaviors upon the object itself. Going forward, the Row._mapping attribute should be used for dictionary behaviors.

See also

Selecting - introductory material for accessing CursorResult and Row objects.

Members

all(), close(), columns(), fetchall(), fetchmany(), fetchone(), first(), freeze(), inserted_primary_key, inserted_primary_key_rows, is_insert, keys(), last_inserted_params(), last_updated_params(), lastrow_has_defaults(), lastrowid, mappings(), merge(), one(), one_or_none(), partitions(), postfetch_cols(), prefetch_cols(), returned_defaults, returned_defaults_rows, returns_rows, rowcount, scalar(), scalar_one(), scalar_one_or_none(), scalars(), supports_sane_multi_rowcount(), supports_sane_rowcount(), unique(), yield_per()

Class signature

class sqlalchemy.engine.CursorResult (sqlalchemy.engine.BaseCursorResult, sqlalchemy.engine.Result)

method sqlalchemy.engine.CursorResult.all()

inherited from the Result.all() method of Result

Return all rows in a list.

Closes the result set after invocation. Subsequent invocations will return an empty list.

New in version 1.4.

Returns:

a list of Row objects.

method sqlalchemy.engine.CursorResult.close()

Close this CursorResult.

This closes out the underlying DBAPI cursor corresponding to the statement execution, if one is still present. Note that the DBAPI cursor is automatically released when the CursorResult exhausts all available rows. CursorResult.close() is generally an optional method except in the case when discarding a CursorResult that still has additional rows pending for fetch.

After this method is called, it is no longer valid to call upon the fetch methods, which will raise a ResourceClosedError on subsequent use.

See also

Working with Engines and Connections

method sqlalchemy.engine.CursorResult.columns(*col_expressions)

inherited from the Result.columns() method of Result

Establish the columns that should be returned in each row.

This method may be used to limit the columns returned as well as to reorder them. The given list of expressions are normally a series of integers or string key names. They may also be appropriate ColumnElement objects which correspond to a given statement construct.

E.g.:

statement = select(table.c.x, table.c.y, table.c.z)
result = connection.execute(statement)

for z, y in result.columns('z', 'y'):
    # ...

Example of using the column objects from the statement itself:

for z, y in result.columns(
        statement.selected_columns.c.z,
        statement.selected_columns.c.y
):
    # ...

New in version 1.4.

Parameters:

*col_expressions – indicates columns to be returned. Elements may be integer row indexes, string column names, or appropriate ColumnElement objects corresponding to a select construct.

Returns:

this Result object with the modifications given.

method sqlalchemy.engine.CursorResult.fetchall()

inherited from the Result.fetchall() method of Result

A synonym for the Result.all() method.

method sqlalchemy.engine.CursorResult.fetchmany(size=None)

inherited from the Result.fetchmany() method of Result

Fetch many rows.

When all rows are exhausted, returns an empty list.

This method is provided for backwards compatibility with SQLAlchemy 1.x.x.

To fetch rows in groups, use the Result.partitions() method.

Returns:

a list of Row objects.

See also

Result.partitions()

method sqlalchemy.engine.CursorResult.fetchone()

inherited from the Result.fetchone() method of Result

Fetch one row.

When all rows are exhausted, returns None.

This method is provided for backwards compatibility with SQLAlchemy 1.x.x.

To fetch the first row of a result only, use the Result.first() method. To iterate through all rows, iterate the Result object directly.

Returns:

a Row object if no filters are applied, or None if no rows remain.

method sqlalchemy.engine.CursorResult.first()

inherited from the Result.first() method of Result

Fetch the first row or None if no row is present.

Closes the result set and discards remaining rows.

Note

This method returns one row, e.g. tuple, by default. To return exactly one single scalar value, that is, the first column of the first row, use the Result.scalar() method, or combine Result.scalars() and Result.first().

Additionally, in contrast to the behavior of the legacy ORM Query.first() method, no limit is applied to the SQL query which was invoked to produce this Result; for a DBAPI driver that buffers results in memory before yielding rows, all rows will be sent to the Python process and all but the first row will be discarded.

See also

ORM Query Unified with Core Select

Returns:

a Row object, or None if no rows remain.

See also

Result.scalar()

Result.one()

method sqlalchemy.engine.CursorResult.freeze()

inherited from the Result.freeze() method of Result

Return a callable object that will produce copies of this Result when invoked.

The callable object returned is an instance of FrozenResult.

This is used for result set caching. The method must be called on the result when it has been unconsumed, and calling the method will consume the result fully. When the FrozenResult is retrieved from a cache, it can be called any number of times where it will produce a new Result object each time against its stored set of rows.

See also

Re-Executing Statements - example usage within the ORM to implement a result-set cache.

attribute sqlalchemy.engine.CursorResult.inserted_primary_key

inherited from the BaseCursorResult.inserted_primary_key attribute of BaseCursorResult

Return the primary key for the row just inserted.

The return value is a Row object representing a named tuple of primary key values in the order in which the primary key columns are configured in the source Table.

Changed in version 1.4.8: - the CursorResult.inserted_primary_key value is now a named tuple via the Row class, rather than a plain tuple.

This accessor only applies to single row insert() constructs which did not explicitly specify Insert.returning(). Support for multirow inserts, while not yet available for most backends, would be accessed using the CursorResult.inserted_primary_key_rows accessor.

Note that primary key columns which specify a server_default clause, or otherwise do not qualify as “autoincrement” columns (see the notes at Column), and were generated using the database-side default, will appear in this list as None unless the backend supports “returning” and the insert statement executed with the “implicit returning” enabled.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an insert() construct.

attribute sqlalchemy.engine.CursorResult.inserted_primary_key_rows

inherited from the BaseCursorResult.inserted_primary_key_rows attribute of BaseCursorResult

Return the value of CursorResult.inserted_primary_key as a row contained within a list; some dialects may support a multiple row form as well.

Note

As indicated below, in current SQLAlchemy versions this accessor is only useful beyond what’s already supplied by CursorResult.inserted_primary_key when using the psycopg2 dialect. Future versions hope to generalize this feature to more dialects.

This accessor is added to support dialects that offer the feature that is currently implemented by the Psycopg2 Fast Execution Helpers feature, currently only the psycopg2 dialect, which provides for many rows to be INSERTed at once while still retaining the behavior of being able to return server-generated primary key values.

• When using the psycopg2 dialect, or other dialects that may support “fast executemany” style inserts in upcoming releases : When invoking an INSERT statement while passing a list of rows as the second argument to Connection.execute(), this accessor will then provide a list of rows, where each row contains the primary key value for each row that was INSERTed.

• When using all other dialects / backends that don’t yet support this feature: This accessor is only useful for single row INSERT statements, and returns the same information as that of the CursorResult.inserted_primary_key within a single-element list. When an INSERT statement is executed in conjunction with a list of rows to be INSERTed, the list will contain one row per row inserted in the statement, however it will contain None for any server-generated values.

Future releases of SQLAlchemy will further generalize the “fast execution helper” feature of psycopg2 to suit other dialects, thus allowing this accessor to be of more general use.

New in version 1.4.

See also

CursorResult.inserted_primary_key

attribute sqlalchemy.engine.CursorResult.is_insert

inherited from the BaseCursorResult.is_insert attribute of BaseCursorResult

True if this CursorResult is the result of a executing an expression language compiled insert() construct.

When True, this implies that the inserted_primary_key attribute is accessible, assuming the statement did not include a user defined “returning” construct.

method sqlalchemy.engine.CursorResult.keys()

inherited from the sqlalchemy.engine._WithKeys.keys method of sqlalchemy.engine._WithKeys

Return an iterable view which yields the string keys that would be represented by each Row.

The keys can represent the labels of the columns returned by a core statement or the names of the orm classes returned by an orm execution.

The view also can be tested for key containment using the Python in operator, which will test both for the string keys represented in the view, as well as for alternate keys such as column objects.

Changed in version 1.4: a key view object is returned rather than a plain list.

method sqlalchemy.engine.CursorResult.last_inserted_params()

inherited from the BaseCursorResult.last_inserted_params() method of BaseCursorResult

Return the collection of inserted parameters from this execution.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an insert() construct.

method sqlalchemy.engine.CursorResult.last_updated_params()

inherited from the BaseCursorResult.last_updated_params() method of BaseCursorResult

Return the collection of updated parameters from this execution.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an update() construct.

method sqlalchemy.engine.CursorResult.lastrow_has_defaults()

inherited from the BaseCursorResult.lastrow_has_defaults() method of BaseCursorResult

Return lastrow_has_defaults() from the underlying ExecutionContext.

See ExecutionContext for details.

attribute sqlalchemy.engine.CursorResult.lastrowid

inherited from the BaseCursorResult.lastrowid attribute of BaseCursorResult

Return the ‘lastrowid’ accessor on the DBAPI cursor.

This is a DBAPI specific method and is only functional for those backends which support it, for statements where it is appropriate. It’s behavior is not consistent across backends.

Usage of this method is normally unnecessary when using insert() expression constructs; the CursorResult.inserted_primary_key attribute provides a tuple of primary key values for a newly inserted row, regardless of database backend.

method sqlalchemy.engine.CursorResult.mappings()

inherited from the Result.mappings() method of Result

Apply a mappings filter to returned rows, returning an instance of MappingResult.

When this filter is applied, fetching rows will return RowMapping objects instead of Row objects.

New in version 1.4.

Returns:

a new MappingResult filtering object referring to this Result object.

method sqlalchemy.engine.CursorResult.merge(*others)

Merge this Result with other compatible result objects.

The object returned is an instance of MergedResult, which will be composed of iterators from the given result objects.

The new result will use the metadata from this result object. The subsequent result objects must be against an identical set of result / cursor metadata, otherwise the behavior is undefined.

method sqlalchemy.engine.CursorResult.one()

inherited from the Result.one() method of Result

Return exactly one row or raise an exception.

Raises NoResultFound if the result returns no rows, or MultipleResultsFound if multiple rows would be returned.

Note

This method returns one row, e.g. tuple, by default. To return exactly one single scalar value, that is, the first column of the first row, use the Result.scalar_one() method, or combine Result.scalars() and Result.one().

New in version 1.4.

Returns:

The first Row.

Raises:

MultipleResultsFound, NoResultFound

See also

Result.first()

Result.one_or_none()

Result.scalar_one()

method sqlalchemy.engine.CursorResult.one_or_none()

inherited from the Result.one_or_none() method of Result

Return at most one result or raise an exception.

Returns None if the result has no rows. Raises MultipleResultsFound if multiple rows are returned.

New in version 1.4.

Returns:

The first Row or None if no row is available.

Raises:

MultipleResultsFound

See also

Result.first()

Result.one()

method sqlalchemy.engine.CursorResult.partitions(size=None)

inherited from the Result.partitions() method of Result

Iterate through sub-lists of rows of the size given.

Each list will be of the size given, excluding the last list to be yielded, which may have a small number of rows. No empty lists will be yielded.

The result object is automatically closed when the iterator is fully consumed.

Note that the backend driver will usually buffer the entire result ahead of time unless the Connection.execution_options.stream_results execution option is used indicating that the driver should not pre-buffer results, if possible. Not all drivers support this option and the option is silently ignored for those who do not.

When using the ORM, the Result.partitions() method is typically more effective from a memory perspective when it is combined with use of the yield_per execution option, which instructs both the DBAPI driver to use server side cursors, if available, as well as instructs the ORM loading internals to only build a certain amount of ORM objects from a result at a time before yielding them out.

New in version 1.4.

Parameters:

size – indicate the maximum number of rows to be present in each list yielded. If None, makes use of the value set by the Result.yield_per(), method, if it were called, or the Connection.execution_options.yield_per execution option, which is equivalent in this regard. If yield_per weren’t set, it makes use of the Result.fetchmany() default, which may be backend specific and not well defined.

Returns:

iterator of lists

See also

Using Server Side Cursors (a.k.a. stream results)

Fetching Large Result Sets with Yield Per - in the ORM Querying Guide

method sqlalchemy.engine.CursorResult.postfetch_cols()

inherited from the BaseCursorResult.postfetch_cols() method of BaseCursorResult

Return postfetch_cols() from the underlying ExecutionContext.

See ExecutionContext for details.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an insert() or update() construct.

method sqlalchemy.engine.CursorResult.prefetch_cols()

inherited from the BaseCursorResult.prefetch_cols() method of BaseCursorResult

Return prefetch_cols() from the underlying ExecutionContext.

See ExecutionContext for details.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an insert() or update() construct.

attribute sqlalchemy.engine.CursorResult.returned_defaults

inherited from the BaseCursorResult.returned_defaults attribute of BaseCursorResult

Return the values of default columns that were fetched using the ValuesBase.return_defaults() feature.

The value is an instance of Row, or None if ValuesBase.return_defaults() was not used or if the backend does not support RETURNING.

New in version 0.9.0.

See also

ValuesBase.return_defaults()

attribute sqlalchemy.engine.CursorResult.returned_defaults_rows

inherited from the BaseCursorResult.returned_defaults_rows attribute of BaseCursorResult

Return a list of rows each containing the values of default columns that were fetched using the ValuesBase.return_defaults() feature.

The return value is a list of Row objects.

New in version 1.4.

attribute sqlalchemy.engine.CursorResult.returns_rows

inherited from the BaseCursorResult.returns_rows attribute of BaseCursorResult

True if this CursorResult returns zero or more rows.

I.e. if it is legal to call the methods CursorResult.fetchone(), CursorResult.fetchmany() CursorResult.fetchall().

Overall, the value of CursorResult.returns_rows should always be synonymous with whether or not the DBAPI cursor had a .description attribute, indicating the presence of result columns, noting that a cursor that returns zero rows still has a .description if a row-returning statement was emitted.

This attribute should be True for all results that are against SELECT statements, as well as for DML statements INSERT/UPDATE/DELETE that use RETURNING. For INSERT/UPDATE/DELETE statements that were not using RETURNING, the value will usually be False, however there are some dialect-specific exceptions to this, such as when using the MSSQL / pyodbc dialect a SELECT is emitted inline in order to retrieve an inserted primary key value.

attribute sqlalchemy.engine.CursorResult.rowcount

inherited from the BaseCursorResult.rowcount attribute of BaseCursorResult

Return the ‘rowcount’ for this result.

The ‘rowcount’ reports the number of rows matched by the WHERE criterion of an UPDATE or DELETE statement.

Note

Notes regarding CursorResult.rowcount:

• This attribute returns the number of rows matched, which is not necessarily the same as the number of rows that were actually modified - an UPDATE statement, for example, may have no net change on a given row if the SET values given are the same as those present in the row already. Such a row would be matched but not modified. On backends that feature both styles, such as MySQL, rowcount is configured by default to return the match count in all cases.

• CursorResult.rowcount is only useful in conjunction with an UPDATE or DELETE statement. Contrary to what the Python DBAPI says, it does not return the number of rows available from the results of a SELECT statement as DBAPIs cannot support this functionality when rows are unbuffered.

• CursorResult.rowcount may not be fully implemented by all dialects. In particular, most DBAPIs do not support an aggregate rowcount result from an executemany call. The CursorResult.supports_sane_rowcount() and CursorResult.supports_sane_multi_rowcount() methods will report from the dialect if each usage is known to be supported.

• Statements that use RETURNING may not return a correct rowcount.

See also

Getting Affected Row Count from UPDATE, DELETE - in the SQLAlchemy 1.4 / 2.0 Tutorial

method sqlalchemy.engine.CursorResult.scalar()

inherited from the Result.scalar() method of Result

Fetch the first column of the first row, and close the result set.

Returns None if there are no rows to fetch.

No validation is performed to test if additional rows remain.

After calling this method, the object is fully closed, e.g. the CursorResult.close() method will have been called.

Returns:

a Python scalar value, or None if no rows remain.

method sqlalchemy.engine.CursorResult.scalar_one()

inherited from the Result.scalar_one() method of Result

Return exactly one scalar result or raise an exception.

This is equivalent to calling Result.scalars() and then Result.one().

See also

Result.one()

Result.scalars()

method sqlalchemy.engine.CursorResult.scalar_one_or_none()

inherited from the Result.scalar_one_or_none() method of Result

Return exactly one scalar result or None.

This is equivalent to calling Result.scalars() and then Result.one_or_none().

See also

Result.one_or_none()

Result.scalars()

method sqlalchemy.engine.CursorResult.scalars(index=0)

inherited from the Result.scalars() method of Result

Return a ScalarResult filtering object which will return single elements rather than Row objects.

E.g.:

>>> result = conn.execute(text("select int_id from table"))
>>> result.scalars().all()
[1, 2, 3]

When results are fetched from the ScalarResult filtering object, the single column-row that would be returned by the Result is instead returned as the column’s value.

New in version 1.4.

Parameters:

index – integer or row key indicating the column to be fetched from each row, defaults to 0 indicating the first column.

Returns:

a new ScalarResult filtering object referring to this Result object.

method sqlalchemy.engine.CursorResult.supports_sane_multi_rowcount()

inherited from the BaseCursorResult.supports_sane_multi_rowcount() method of BaseCursorResult

Return supports_sane_multi_rowcount from the dialect.

See CursorResult.rowcount for background.

method sqlalchemy.engine.CursorResult.supports_sane_rowcount()

inherited from the BaseCursorResult.supports_sane_rowcount() method of BaseCursorResult

Return supports_sane_rowcount from the dialect.

See CursorResult.rowcount for background.

method sqlalchemy.engine.CursorResult.unique(strategy=None)

inherited from the Result.unique() method of Result

Apply unique filtering to the objects returned by this Result.

When this filter is applied with no arguments, the rows or objects returned will filtered such that each row is returned uniquely. The algorithm used to determine this uniqueness is by default the Python hashing identity of the whole tuple. In some cases a specialized per-entity hashing scheme may be used, such as when using the ORM, a scheme is applied which works against the primary key identity of returned objects.

The unique filter is applied after all other filters, which means if the columns returned have been refined using a method such as the Result.columns() or Result.scalars() method, the uniquing is applied to only the column or columns returned. This occurs regardless of the order in which these methods have been called upon the Result object.

The unique filter also changes the calculus used for methods like Result.fetchmany() and Result.partitions(). When using Result.unique(), these methods will continue to yield the number of rows or objects requested, after uniquing has been applied. However, this necessarily impacts the buffering behavior of the underlying cursor or datasource, such that multiple underlying calls to cursor.fetchmany() may be necessary in order to accumulate enough objects in order to provide a unique collection of the requested size.

Parameters:

strategy – a callable that will be applied to rows or objects being iterated, which should return an object that represents the unique value of the row. A Python set() is used to store these identities. If not passed, a default uniqueness strategy is used which may have been assembled by the source of this Result object.

method sqlalchemy.engine.CursorResult.yield_per(num)

Configure the row-fetching strategy to fetch num rows at a time.

This impacts the underlying behavior of the result when iterating over the result object, or otherwise making use of methods such as Result.fetchone() that return one row at a time. Data from the underlying cursor or other data source will be buffered up to this many rows in memory, and the buffered collection will then be yielded out one row at at time or as many rows are requested. Each time the buffer clears, it will be refreshed to this many rows or as many rows remain if fewer remain.

The Result.yield_per() method is generally used in conjunction with the Connection.execution_options.stream_results execution option, which will allow the database dialect in use to make use of a server side cursor, if the DBAPI supports a specific “server side cursor” mode separate from its default mode of operation.

Tip

Consider using the Connection.execution_options.yield_per execution option, which will simultaneously set Connection.execution_options.stream_results to ensure the use of server side cursors, as well as automatically invoke the Result.yield_per() method to establish a fixed row buffer size at once.

The Connection.execution_options.yield_per execution option is available for ORM operations, with Session-oriented use described at Fetching Large Result Sets with Yield Per. The Core-only version which works with Connection is new as of SQLAlchemy 1.4.40.

New in version 1.4.

Parameters:

num – number of rows to fetch each time the buffer is refilled. If set to a value below 1, fetches all rows for the next buffer.

See also

Using Server Side Cursors (a.k.a. stream results) - describes Core behavior for Result.yield_per()

Fetching Large Result Sets with Yield Per - in the ORM Querying Guide

class sqlalchemy.engine.LegacyCursorResult(context, cursor_strategy, cursor_description)

Legacy version of CursorResult.

This class includes connection “connection autoclose” behavior for use with “connectionless” execution, as well as delivers rows using the LegacyRow row implementation.

New in version 1.4.

Members

close()

Class signature

class sqlalchemy.engine.LegacyCursorResult (sqlalchemy.engine.CursorResult)

method sqlalchemy.engine.LegacyCursorResult.close()

Close this LegacyCursorResult.

This method has the same behavior as that of sqlalchemy.engine.CursorResult(), but it also may close the underlying Connection for the case of “connectionless” execution.

Deprecated since version 2.0: “connectionless” execution is deprecated and will be removed in version 2.0. Version 2.0 will feature the Result object that will no longer affect the status of the originating connection in any case.

After this method is called, it is no longer valid to call upon the fetch methods, which will raise a ResourceClosedError on subsequent use.

See also

Working with Engines and Connections

Connectionless Execution, Implicit Execution

class sqlalchemy.engine.Row(parent, processors, keymap, key_style, data)

Represent a single result row.

The Row object represents a row of a database result. It is typically associated in the 1.x series of SQLAlchemy with the CursorResult object, however is also used by the ORM for tuple-like results as of SQLAlchemy 1.4.

The Row object seeks to act as much like a Python named tuple as possible. For mapping (i.e. dictionary) behavior on a row, such as testing for containment of keys, refer to the Row._mapping attribute.

See also

Selecting Rows with Core or ORM - includes examples of selecting rows from SELECT statements.

LegacyRow - Compatibility interface introduced in SQLAlchemy 1.4.

Changed in version 1.4: Renamed RowProxy to Row. Row is no longer a “proxy” object in that it contains the final form of data within it, and now acts mostly like a named tuple. Mapping-like functionality is moved to the Row._mapping attribute, but will remain available in SQLAlchemy 1.x series via the LegacyRow class that is used by LegacyCursorResult. See RowProxy is no longer a “proxy”; is now called Row and behaves like an enhanced named tuple for background on this change.

Members

_asdict(), _fields, _mapping, count, index, keys()

Class signature

class sqlalchemy.engine.Row (sqlalchemy.engine.BaseRow, collections.abc.Sequence)

method sqlalchemy.engine.Row._asdict()

Return a new dict which maps field names to their corresponding values.

This method is analogous to the Python named tuple ._asdict() method, and works by applying the dict() constructor to the Row._mapping attribute.

New in version 1.4.

See also

Row._mapping

attribute sqlalchemy.engine.Row._fields

Return a tuple of string keys as represented by this Row.

The keys can represent the labels of the columns returned by a core statement or the names of the orm classes returned by an orm execution.

This attribute is analogous to the Python named tuple ._fields attribute.

New in version 1.4.

See also

Row._mapping

attribute sqlalchemy.engine.Row._mapping

Return a RowMapping for this Row.

This object provides a consistent Python mapping (i.e. dictionary) interface for the data contained within the row. The Row by itself behaves like a named tuple, however in the 1.4 series of SQLAlchemy, the LegacyRow class is still used by Core which continues to have mapping-like behaviors against the row object itself.

See also

Row._fields

New in version 1.4.

attribute sqlalchemy.engine.Row.count

attribute sqlalchemy.engine.Row.index

method sqlalchemy.engine.Row.keys()

Return the list of keys as strings represented by this Row.

Deprecated since version 1.4: The Row.keys() method is considered legacy as of the 1.x series of SQLAlchemy and will be removed in 2.0. Use the namedtuple standard accessor Row._fields, or for full mapping behavior use row._mapping.keys() (Background on SQLAlchemy 2.0 at: Migrating to SQLAlchemy 2.0)

The keys can represent the labels of the columns returned by a core statement or the names of the orm classes returned by an orm execution.

This method is analogous to the Python dictionary .keys() method, except that it returns a list, not an iterator.

See also

Row._fields

Row._mapping

class sqlalchemy.engine.RowMapping(parent, processors, keymap, key_style, data)

A Mapping that maps column names and objects to Row values.

The RowMapping is available from a Row via the Row._mapping attribute, as well as from the iterable interface provided by the MappingResult object returned by the Result.mappings() method.

RowMapping supplies Python mapping (i.e. dictionary) access to the contents of the row. This includes support for testing of containment of specific keys (string column names or objects), as well as iteration of keys, values, and items:

for row in result:
    if 'a' in row._mapping:
        print("Column 'a': %s" % row._mapping['a'])

    print("Column b: %s" % row._mapping[table.c.b])

New in version 1.4: The RowMapping object replaces the mapping-like access previously provided by a database result row, which now seeks to behave mostly like a named tuple.

Members

items(), keys(), values()

Class signature

class sqlalchemy.engine.RowMapping (sqlalchemy.engine.BaseRow, collections.abc.Mapping)

method sqlalchemy.engine.RowMapping.items()

Return a view of key/value tuples for the elements in the underlying Row.

method sqlalchemy.engine.RowMapping.keys()

Return a view of ‘keys’ for string column names represented by the underlying Row.

method sqlalchemy.engine.RowMapping.values()

Return a view of values for the values represented in the underlying Row.