SQLAlchemy 2.0 Future (Core)

This package includes a relatively small number of transitional elements to allow “2.0 mode” to take place within SQLAlchemy 1.4. The primary objects provided here are Engine and Connection, which are both subclasses of the existing Engine and Connection objects with essentially a smaller set of methods and the removal of “autocommit”.

Within the 1.4 series, the “2.0” style of engines and connections is enabled by passing the create_engine.future flag to create_engine():

from sqlalchemy import create_engine

engine = create_engine("postgresql://user:pass@host/dbname", future=True)

Similarly, with the ORM, to enable “future” behavior in the ORM Session, pass the Session.future parameter either to the Session constructor directly, or via the sessionmaker class:

from sqlalchemy.orm import sessionmaker

Session = sessionmaker(engine, future=True)

See also

Migrating to SQLAlchemy 2.0 - Introduction to the 2.0 series of SQLAlchemy

Object Name	Description
Connection	Provides high-level functionality for a wrapped DB-API connection.
create_engine(*arg, **kw)	Create a new Engine instance.
Engine	Connects a Pool and Dialect together to provide a source of database connectivity and behavior.
select(*entities)	Construct a new Select using the 2. x style API.

class sqlalchemy.future.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.

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.

This is the SQLAlchemy 2.0 version of the Connection class. The API and behavior of this object is largely the same, with the following differences in behavior:

• The result object returned for results is the CursorResult object, which is a subclass of the Result. This object has a slightly different API and behavior than the LegacyCursorResult returned for 1.x style usage.

• The object has Connection.commit() and Connection.rollback() methods which commit or roll back the current transaction in progress, if any.

• The object features “autobegin” behavior, such that any call to Connection.execute() will unconditionally start a transaction which can be controlled using the above mentioned Connection.commit() and Connection.rollback() methods.

• The object does not have any “autocommit” functionality. Any SQL statement or DDL statement will not be followed by any COMMIT until the transaction is explicitly committed, either via the Connection.commit() method, or if the connection is being used in a context manager that commits such as the one returned by Engine.begin().

• The SAVEPOINT method Connection.begin_nested() returns a NestedTransaction as was always the case, and the savepoint can be controlled by invoking NestedTransaction.commit() or NestedTransaction.rollback() as was the case before. However, this savepoint “transaction” is not associated with the transaction that is controlled by the connection itself; the overall transaction can be committed or rolled back directly which will not emit any special instructions for the SAVEPOINT (this will typically have the effect that one desires).

• The Connection object does not support “branching”, which was a pattern by which a sub “connection” would be used that refers to this connection as a parent.

Members

begin(), begin_nested(), close(), commit(), execute(), rollback(), scalar()

Class signature

class sqlalchemy.future.Connection (sqlalchemy.engine.Connection)

method sqlalchemy.future.Connection.begin()

Begin a transaction prior to autobegin occurring.

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

The Connection.begin() method in SQLAlchemy 2.0 begins a transaction that normally will be begun in any case when the connection is first used to execute a statement. The reason this method might be used would be to invoke the ConnectionEvents.begin() event at a specific time, or to organize code within the scope of a connection checkout in terms of context managed blocks, such as:

with engine.connect() as conn:
    with conn.begin():
        conn.execute(...)
        conn.execute(...)

    with conn.begin():
        conn.execute(...)
        conn.execute(...)

The above code is not fundamentally any different in its behavior than the following code which does not use Connection.begin(); the below style is referred towards as “commit as you go” style:

with engine.connect() as conn:
    conn.execute(...)
    conn.execute(...)
    conn.commit()

    conn.execute(...)
    conn.execute(...)
    conn.commit()

From a database point of view, the Connection.begin() method does not emit any SQL or change the state of the underlying DBAPI connection in any way; the Python DBAPI does not have any concept of explicit transaction begin.

See also

Working with Transactions and the DBAPI - in the SQLAlchemy 1.4 / 2.0 Tutorial

Connection.begin_nested() - use a SAVEPOINT

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

Engine.begin() - context manager available from Engine

method sqlalchemy.future.Connection.begin_nested()

Begin a nested transaction (i.e. SAVEPOINT) and return a transaction handle.

The returned object is an instance of NestedTransaction.

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.

If an outer RootTransaction is not present on this Connection, a new one is created using “autobegin”. This outer transaction may be completed using “commit-as-you-go” style usage, by calling upon Connection.commit() or Connection.rollback().

Tip

The “autobegin” behavior of Connection.begin_nested() is specific to 2.0 style use; for legacy behaviors, see Connection.begin_nested().

The NestedTransaction remains independent of the Connection object itself. Calling the Connection.commit() or Connection.rollback() will always affect the actual containing database transaction itself, and not the SAVEPOINT itself. When a database transaction is committed, any SAVEPOINTs that have been established are cleared and the data changes within their scope is also committed.

See also

Connection.begin()

method sqlalchemy.future.Connection.close()

Close this Connection.

This has the effect of also calling Connection.rollback() if any transaction is in place.

method sqlalchemy.future.Connection.commit()

Commit the transaction that is currently in progress.

This method commits the current transaction if one has been started. If no transaction was started, the method has no effect, assuming the connection is in a non-invalidated state.

A transaction is begun on a Connection automatically whenever a statement is first executed, or when the Connection.begin() method is called.

Note

The Connection.commit() method only acts upon the primary database transaction that is linked to the Connection object. It does not operate upon a SAVEPOINT that would have been invoked from the Connection.begin_nested() method; for control of a SAVEPOINT, call NestedTransaction.commit() on the NestedTransaction that is returned by the Connection.begin_nested() method itself.

method sqlalchemy.future.Connection.execute(statement, parameters=None, execution_options=None)

Executes a SQL statement construct and returns a Result.

Parameters:

• statement –

  The statement to be executed. This is always an object that is in both the ClauseElement and Executable hierarchies, including:

  • Select

  • Insert, Update, Delete

  • TextClause and TextualSelect

  • DDL and objects which inherit from DDLElement

• parameters – parameters which will be bound into the statement. This may be either a dictionary of parameter names to values, or a mutable sequence (e.g. a list) of dictionaries. When a list of dictionaries is passed, the underlying statement execution will make use of the DBAPI cursor.executemany() method. When a single dictionary is passed, the DBAPI cursor.execute() method will be used.

• execution_options – optional dictionary of execution options, which will be associated with the statement execution. This dictionary can provide a subset of the options that are accepted by Connection.execution_options().

Returns:

a Result object.

method sqlalchemy.future.Connection.rollback()

Roll back the transaction that is currently in progress.

This method rolls back the current transaction if one has been started. If no transaction was started, the method has no effect. If a transaction was started and the connection is in an invalidated state, the transaction is cleared using this method.

A transaction is begun on a Connection automatically whenever a statement is first executed, or when the Connection.begin() method is called.

Note

The Connection.rollback() method only acts upon the primary database transaction that is linked to the Connection object. It does not operate upon a SAVEPOINT that would have been invoked from the Connection.begin_nested() method; for control of a SAVEPOINT, call NestedTransaction.rollback() on the NestedTransaction that is returned by the Connection.begin_nested() method itself.

method sqlalchemy.future.Connection.scalar(statement, parameters=None, execution_options=None)

Executes a SQL statement construct and returns a scalar object.

This method is shorthand for invoking the Result.scalar() method after invoking the Connection.execute() method. Parameters are equivalent.

Returns:

a scalar Python value representing the first column of the first row returned.

function sqlalchemy.future.create_engine(*arg, **kw)

Create a new Engine instance.

Arguments passed to create_engine() are mostly identical to those passed to the 1.x create_engine() function. The difference is that the object returned is the Engine which has the 2.0 version of the API.

class sqlalchemy.future.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 2.0 version of the Engine.

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

See also

Engine Configuration

Working with Engines and Connections

Members

begin(), connect(), execute(), has_table(), run_callable(), scalar(), table_names(), transaction()

Class signature

class sqlalchemy.future.Engine (sqlalchemy.engine.Engine)

method sqlalchemy.future.Engine.begin()

Return a Connection object with a transaction begun.

Use of this method is similar to that of Engine.connect(), typically as a context manager, which will automatically maintain the state of the transaction when the block ends, either by calling Connection.commit() when the block succeeds normally, or Connection.rollback() when an exception is raised, before propagating the exception outwards:

with engine.begin() as connection:
    connection.execute(text("insert into table values ('foo')"))

See also

Engine.connect()

Connection.begin()

method sqlalchemy.future.Engine.connect()

Return a new Connection object.

The Connection acts as a Python context manager, so the typical use of this method looks like:

with engine.connect() as connection:
    connection.execute(text("insert into table values ('foo')"))
    connection.commit()

Where above, after the block is completed, the connection is “closed” and its underlying DBAPI resources are returned to the connection pool. This also has the effect of rolling back any transaction that was explicitly begun or was begun via autobegin, and will emit the ConnectionEvents.rollback() event if one was started and is still in progress.

See also

Engine.begin()

method sqlalchemy.future.Engine.execute(*arg, **kw)

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.future.Engine.has_table(*arg, **kw)

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.

method sqlalchemy.future.Engine.run_callable(*arg, **kw)

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.future.Engine.scalar(*arg, **kw)

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.future.Engine.table_names(*arg, **kw)

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.future.Engine.transaction(*arg, **kw)

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()

function sqlalchemy.future.select(*entities)

Construct a new Select using the 2. x style API.

New in version 1.4: - The select() function now accepts column arguments positionally. The top-level select() function will automatically use the 1.x or 2.x style API based on the incoming arguments; using select() from the sqlalchemy.future module will enforce that only the 2.x style constructor is used.

Similar functionality is also available via the FromClause.select() method on any FromClause.

See also

Selecting - Core Tutorial description of select().

Parameters:

*entities –

Entities to SELECT from. For Core usage, this is typically a series of ColumnElement and / or FromClause objects which will form the columns clause of the resulting statement. For those objects that are instances of FromClause (typically Table or Alias objects), the FromClause.c collection is extracted to form a collection of ColumnElement objects.

This parameter will also accept TextClause constructs as given, as well as ORM-mapped classes.