Contextual/Thread-local Sessions¶

Recall from the section When do I construct a Session, when do I commit it, and when do I close it?, the concept of “session scopes” was introduced, with an emphasis on web applications and the practice of linking the scope of a Session with that of a web request. Most modern web frameworks include integration tools so that the scope of the Session can be managed automatically, and these tools should be used as they are available.

SQLAlchemy includes its own helper object, which helps with the establishment of user-defined Session scopes. It is also used by third-party integration systems to help construct their integration schemes.

The object is the scoped_session object, and it represents a registry of Session objects. If you’re not familiar with the registry pattern, a good introduction can be found in Patterns of Enterprise Architecture.

Warning

The scoped_session registry by default uses a Python threading.local() in order to track Session instances. This is not necessarily compatible with all application servers, particularly those which make use of greenlets or other alternative forms of concurrency control, which may lead to race conditions (e.g. randomly occurring failures) when used in moderate to high concurrency scenarios. Please read Thread-Local Scope and Using Thread-Local Scope with Web Applications below to more fully understand the implications of using threading.local() to track Session objects and consider more explicit means of scoping when using application servers which are not based on traditional threads.

Note

The scoped_session object is a very popular and useful object used by many SQLAlchemy applications. However, it is important to note that it presents only one approach to the issue of Session management. If you’re new to SQLAlchemy, and especially if the term “thread-local variable” seems strange to you, we recommend that if possible you familiarize first with an off-the-shelf integration system such as Flask-SQLAlchemy or zope.sqlalchemy.

A scoped_session is constructed by calling it, passing it a factory which can create new Session objects. A factory is just something that produces a new object when called, and in the case of Session, the most common factory is the sessionmaker, introduced earlier in this section. Below we illustrate this usage:

>>> from sqlalchemy.orm import scoped_session
>>> from sqlalchemy.orm import sessionmaker

>>> session_factory = sessionmaker(bind=some_engine)
>>> Session = scoped_session(session_factory)

The scoped_session object we’ve created will now call upon the sessionmaker when we “call” the registry:

>>> some_session = Session()

Above, some_session is an instance of Session, which we can now use to talk to the database. This same Session is also present within the scoped_session registry we’ve created. If we call upon the registry a second time, we get back the same Session:

>>> some_other_session = Session()
>>> some_session is some_other_session
True

This pattern allows disparate sections of the application to call upon a global scoped_session, so that all those areas may share the same session without the need to pass it explicitly. The Session we’ve established in our registry will remain, until we explicitly tell our registry to dispose of it, by calling scoped_session.remove():

>>> Session.remove()

The scoped_session.remove() method first calls Session.close() on the current Session, which has the effect of releasing any connection/transactional resources owned by the Session first, then discarding the Session itself. “Releasing” here means that connections are returned to their connection pool and any transactional state is rolled back, ultimately using the rollback() method of the underlying DBAPI connection.

At this point, the scoped_session object is “empty”, and will create a new Session when called again. As illustrated below, this is not the same Session we had before:

>>> new_session = Session()
>>> new_session is some_session
False

The above series of steps illustrates the idea of the “registry” pattern in a nutshell. With that basic idea in hand, we can discuss some of the details of how this pattern proceeds.

Implicit Method Access¶

The job of the scoped_session is simple; hold onto a Session for all who ask for it. As a means of producing more transparent access to this Session, the scoped_session also includes proxy behavior, meaning that the registry itself can be treated just like a Session directly; when methods are called on this object, they are proxied to the underlying Session being maintained by the registry:

Session = scoped_session(some_factory)

# equivalent to:
#
# session = Session()
# print(session.query(MyClass).all())
#
print(Session.query(MyClass).all())

The above code accomplishes the same task as that of acquiring the current Session by calling upon the registry, then using that Session.

Thread-Local Scope¶

Users who are familiar with multithreaded programming will note that representing anything as a global variable is usually a bad idea, as it implies that the global object will be accessed by many threads concurrently. The Session object is entirely designed to be used in a non-concurrent fashion, which in terms of multithreading means “only in one thread at a time”. So our above example of scoped_session usage, where the same Session object is maintained across multiple calls, suggests that some process needs to be in place such that multiple calls across many threads don’t actually get a handle to the same session. We call this notion thread local storage, which means, a special object is used that will maintain a distinct object per each application thread. Python provides this via the threading.local() construct. The scoped_session object by default uses this object as storage, so that a single Session is maintained for all who call upon the scoped_session registry, but only within the scope of a single thread. Callers who call upon the registry in a different thread get a Session instance that is local to that other thread.

Using this technique, the scoped_session provides a quick and relatively simple (if one is familiar with thread-local storage) way of providing a single, global object in an application that is safe to be called upon from multiple threads.

The scoped_session.remove() method, as always, removes the current Session associated with the thread, if any. However, one advantage of the threading.local() object is that if the application thread itself ends, the “storage” for that thread is also garbage collected. So it is in fact “safe” to use thread local scope with an application that spawns and tears down threads, without the need to call scoped_session.remove(). However, the scope of transactions themselves, i.e. ending them via Session.commit() or Session.rollback(), will usually still be something that must be explicitly arranged for at the appropriate time, unless the application actually ties the lifespan of a thread to the lifespan of a transaction.

Using Thread-Local Scope with Web Applications¶

As discussed in the section When do I construct a Session, when do I commit it, and when do I close it?, a web application is architected around the concept of a web request, and integrating such an application with the Session usually implies that the Session will be associated with that request. As it turns out, most Python web frameworks, with notable exceptions such as the asynchronous frameworks Twisted and Tornado, use threads in a simple way, such that a particular web request is received, processed, and completed within the scope of a single worker thread. When the request ends, the worker thread is released to a pool of workers where it is available to handle another request.

This simple correspondence of web request and thread means that to associate a Session with a thread implies it is also associated with the web request running within that thread, and vice versa, provided that the Session is created only after the web request begins and torn down just before the web request ends. So it is a common practice to use scoped_session as a quick way to integrate the Session with a web application. The sequence diagram below illustrates this flow:

Web Server          Web Framework        SQLAlchemy ORM Code
--------------      --------------       ------------------------------
startup        ->   Web framework        # Session registry is established
                    initializes          Session = scoped_session(sessionmaker())

incoming
web request    ->   web request     ->   # The registry is *optionally*
                    starts               # called upon explicitly to create
                                         # a Session local to the thread and/or request
                                         Session()

                                         # the Session registry can otherwise
                                         # be used at any time, creating the
                                         # request-local Session() if not present,
                                         # or returning the existing one
                                         Session.query(MyClass) # ...

                                         Session.add(some_object) # ...

                                         # if data was modified, commit the
                                         # transaction
                                         Session.commit()

                    web request ends  -> # the registry is instructed to
                                         # remove the Session
                                         Session.remove()

                    sends output      <-
outgoing web    <-
response

Using the above flow, the process of integrating the Session with the web application has exactly two requirements:

Create a single scoped_session registry when the web application first starts, ensuring that this object is accessible by the rest of the application.
Ensure that scoped_session.remove() is called when the web request ends, usually by integrating with the web framework’s event system to establish an “on request end” event.

As noted earlier, the above pattern is just one potential way to integrate a Session with a web framework, one which in particular makes the significant assumption that the web framework associates web requests with application threads. It is however strongly recommended that the integration tools provided with the web framework itself be used, if available, instead of scoped_session.

In particular, while using a thread local can be convenient, it is preferable that the Session be associated directly with the request, rather than with the current thread. The next section on custom scopes details a more advanced configuration which can combine the usage of scoped_session with direct request based scope, or any kind of scope.

Using Custom Created Scopes¶

The scoped_session object’s default behavior of “thread local” scope is only one of many options on how to “scope” a Session. A custom scope can be defined based on any existing system of getting at “the current thing we are working with”.

Suppose a web framework defines a library function get_current_request(). An application built using this framework can call this function at any time, and the result will be some kind of Request object that represents the current request being processed. If the Request object is hashable, then this function can be easily integrated with scoped_session to associate the Session with the request. Below we illustrate this in conjunction with a hypothetical event marker provided by the web framework on_request_end, which allows code to be invoked whenever a request ends:

from my_web_framework import get_current_request, on_request_end
from sqlalchemy.orm import scoped_session, sessionmaker

Session = scoped_session(sessionmaker(bind=some_engine), scopefunc=get_current_request)


@on_request_end
def remove_session(req):
    Session.remove()

Above, we instantiate scoped_session in the usual way, except that we pass our request-returning function as the “scopefunc”. This instructs scoped_session to use this function to generate a dictionary key whenever the registry is called upon to return the current Session. In this case it is particularly important that we ensure a reliable “remove” system is implemented, as this dictionary is not otherwise self-managed.

Contextual Session API¶

Object Name	Description
scoped_session	Provides scoped management of `Session` objects.
ScopedRegistry	A Registry that can store one or multiple instances of a single class on the basis of a “scope” function.
ThreadLocalRegistry	A `ScopedRegistry` that uses a `threading.local()` variable for storage.

class sqlalchemy.orm.scoped_session(session_factory, scopefunc=None)¶

Provides scoped management of Session objects.

See Contextual/Thread-local Sessions for a tutorial.

Note

When using Asynchronous I/O (asyncio), the async-compatible async_scoped_session class should be used in place of scoped_session.

Class signature

class sqlalchemy.orm.scoped_session (sqlalchemy.orm.scoping.ScopedSessionMixin)

method sqlalchemy.orm.scoped_session.__call__(**kw)¶

inherited from the sqlalchemy.orm.scoping.ScopedSessionMixin.__call__ method of ScopedSessionMixin

Return the current Session, creating it using the scoped_session.session_factory if not present.

Parameters:: **kw – Keyword arguments will be passed to the scoped_session.session_factory callable, if an existing Session is not present. If the Session is present and keyword arguments have been passed, InvalidRequestError is raised.

method sqlalchemy.orm.scoped_session.__init__(session_factory, scopefunc=None)¶

Construct a new scoped_session.

Parameters:

session_factory – a factory to create new Session instances. This is usually, but not necessarily, an instance of sessionmaker.
scopefunc – optional function which defines the current scope. If not passed, the scoped_session object assumes “thread-local” scope, and will use a Python threading.local() in order to maintain the current Session. If passed, the function should return a hashable token; this token will be used as the key in a dictionary in order to store and retrieve the current Session.

method sqlalchemy.orm.scoped_session.add(instance, _warn=True)¶

Place an object into this Session.

Proxied for the Session class on behalf of the scoped_session class.

Objects that are in the transient state when passed to the Session.add() method will move to the pending state, until the next flush, at which point they will move to the persistent state.

Objects that are in the detached state when passed to the Session.add() method will move to the persistent state directly.

If the transaction used by the Session is rolled back, objects which were transient when they were passed to Session.add() will be moved back to the transient state, and will no longer be present within this Session.

See also

Session.add_all()

Adding New or Existing Items - at Basics of Using a Session

method sqlalchemy.orm.scoped_session.add_all(instances)¶

Add the given collection of instances to this Session.

Proxied for the Session class on behalf of the scoped_session class.

See the documentation for Session.add() for a general behavioral description.

See also

Session.add()

Adding New or Existing Items - at Basics of Using a Session

method sqlalchemy.orm.scoped_session.begin(subtransactions=False, nested=False, _subtrans=False)¶

Begin a transaction, or nested transaction, on this Session, if one is not already begun.

Proxied for the Session class on behalf of the scoped_session class.

The Session object features autobegin behavior, so that normally it is not necessary to call the Session.begin() method explicitly. However, it may be used in order to control the scope of when the transactional state is begun.

When used to begin the outermost transaction, an error is raised if this Session is already inside of a transaction.

Parameters:

nested – if True, begins a SAVEPOINT transaction and is equivalent to calling Session.begin_nested(). For documentation on SAVEPOINT transactions, please see Using SAVEPOINT.
subtransactions –
if True, indicates that this Session.begin() can create a “subtransaction”.

Deprecated since version 1.4: The Session.begin.subtransactions flag is deprecated and will be removed in SQLAlchemy version 2.0. See the documentation at Migrating from the “subtransaction” pattern for background on a compatible alternative pattern.

Returns:

the SessionTransaction object. Note that SessionTransaction acts as a Python context manager, allowing Session.begin() to be used in a “with” block. See Explicit Begin for an example.

See also

Auto Begin

Managing Transactions

Session.begin_nested()

method sqlalchemy.orm.scoped_session.begin_nested()¶

Begin a “nested” transaction on this Session, e.g. SAVEPOINT.

Proxied for the Session class on behalf of the scoped_session class.

The target database(s) and associated drivers must support SQL SAVEPOINT for this method to function correctly.

For documentation on SAVEPOINT transactions, please see Using SAVEPOINT.

Returns:: the SessionTransaction object. Note that SessionTransaction acts as a context manager, allowing Session.begin_nested() to be used in a “with” block. See Using SAVEPOINT for a usage example.

See also

Using SAVEPOINT

Serializable isolation / Savepoints / Transactional DDL - special workarounds required with the SQLite driver in order for SAVEPOINT to work correctly.

method sqlalchemy.orm.scoped_session.bulk_insert_mappings(mapper, mappings, return_defaults=False, render_nulls=False)¶

Perform a bulk insert of the given list of mapping dictionaries.

Proxied for the Session class on behalf of the scoped_session class.

The bulk insert feature allows plain Python dictionaries to be used as the source of simple INSERT operations which can be more easily grouped together into higher performing “executemany” operations. Using dictionaries, there is no “history” or session state management features in use, reducing latency when inserting large numbers of simple rows.

The values within the dictionaries as given are typically passed without modification into Core sqlalchemy.sql.expression.Insert() constructs, after organizing the values within them across the tables to which the given mapper is mapped.

New in version 1.0.0.

Legacy Feature

The bulk insert feature allows for a lower-latency INSERT of rows at the expense of most other unit-of-work features. Features such as object management, relationship handling, and SQL clause support are silently omitted in favor of raw INSERT of records.

In SQLAlchemy 2.0, improved versions of the bulk insert/update methods are introduced, with clearer behavior and documentation, new capabilities, and much better performance.

For 1.4 use, please read the list of caveats at ORM Compatibility / Caveats before using this method, and fully test and confirm the functionality of all code developed using these systems.

Parameters:

mapper – a mapped class, or the actual Mapper object, representing the single kind of object represented within the mapping list.
mappings – a sequence of dictionaries, each one containing the state of the mapped row to be inserted, in terms of the attribute names on the mapped class. If the mapping refers to multiple tables, such as a joined-inheritance mapping, each dictionary must contain all keys to be populated into all tables.
return_defaults – when True, rows that are missing values which generate defaults, namely integer primary key defaults and sequences, will be inserted one at a time, so that the primary key value is available. In particular this will allow joined-inheritance and other multi-table mappings to insert correctly without the need to provide primary key values ahead of time; however, Session.bulk_insert_mappings.return_defaults greatly reduces the performance gains of the method overall. If the rows to be inserted only refer to a single table, then there is no reason this flag should be set as the returned default information is not used.
render_nulls –
When True, a value of None will result in a NULL value being included in the INSERT statement, rather than the column being omitted from the INSERT. This allows all the rows being INSERTed to have the identical set of columns which allows the full set of rows to be batched to the DBAPI. Normally, each column-set that contains a different combination of NULL values than the previous row must omit a different series of columns from the rendered INSERT statement, which means it must be emitted as a separate statement. By passing this flag, the full set of rows are guaranteed to be batchable into one batch; the cost however is that server-side defaults which are invoked by an omitted column will be skipped, so care must be taken to ensure that these are not necessary.

Warning

When this flag is set, server side default SQL values will not be invoked for those columns that are inserted as NULL; the NULL value will be sent explicitly. Care must be taken to ensure that no server-side default functions need to be invoked for the operation as a whole.

New in version 1.1.

See also

Bulk Operations

Session.bulk_save_objects()

Session.bulk_update_mappings()

method sqlalchemy.orm.scoped_session.bulk_save_objects(objects, return_defaults=False, update_changed_only=True, preserve_order=True)¶

Perform a bulk save of the given list of objects.

Proxied for the Session class on behalf of the scoped_session class.

The bulk save feature allows mapped objects to be used as the source of simple INSERT and UPDATE operations which can be more easily grouped together into higher performing “executemany” operations; the extraction of data from the objects is also performed using a lower-latency process that ignores whether or not attributes have actually been modified in the case of UPDATEs, and also ignores SQL expressions.

The objects as given are not added to the session and no additional state is established on them. If the Session.bulk_save_objects.return_defaults flag is set, then server-generated primary key values will be assigned to the returned objects, but not server side defaults; this is a limitation in the implementation. If stateful objects are desired, please use the standard Session.add_all() approach or as an alternative newer mass-insert features such as Using INSERT, UPDATE and ON CONFLICT (i.e. upsert) to return ORM Objects.

Legacy Feature

The bulk save feature allows for a lower-latency INSERT/UPDATE of rows at the expense of most other unit-of-work features. Features such as object management, relationship handling, and SQL clause support are silently omitted in favor of raw INSERT/UPDATES of records.

In SQLAlchemy 2.0, improved versions of the bulk insert/update methods are introduced, with clearer behavior and documentation, new capabilities, and much better performance.

For 1.4 use, please read the list of caveats at ORM Compatibility / Caveats before using this method, and fully test and confirm the functionality of all code developed using these systems.

Parameters:

objects –
a sequence of mapped object instances. The mapped objects are persisted as is, and are not associated with the Session afterwards.

For each object, whether the object is sent as an INSERT or an UPDATE is dependent on the same rules used by the Session in traditional operation; if the object has the InstanceState.key attribute set, then the object is assumed to be “detached” and will result in an UPDATE. Otherwise, an INSERT is used.

In the case of an UPDATE, statements are grouped based on which attributes have changed, and are thus to be the subject of each SET clause. If update_changed_only is False, then all attributes present within each object are applied to the UPDATE statement, which may help in allowing the statements to be grouped together into a larger executemany(), and will also reduce the overhead of checking history on attributes.
return_defaults – when True, rows that are missing values which generate defaults, namely integer primary key defaults and sequences, will be inserted one at a time, so that the primary key value is available. In particular this will allow joined-inheritance and other multi-table mappings to insert correctly without the need to provide primary key values ahead of time; however, Session.bulk_save_objects.return_defaults greatly reduces the performance gains of the method overall. It is strongly advised to please use the standard Session.add_all() approach.
update_changed_only – when True, UPDATE statements are rendered based on those attributes in each state that have logged changes. When False, all attributes present are rendered into the SET clause with the exception of primary key attributes.
preserve_order –
when True, the order of inserts and updates matches exactly the order in which the objects are given. When False, common types of objects are grouped into inserts and updates, to allow for more batching opportunities.

New in version 1.3.

See also

Bulk Operations

Session.bulk_insert_mappings()

Session.bulk_update_mappings()

method sqlalchemy.orm.scoped_session.bulk_update_mappings(mapper, mappings)¶

Perform a bulk update of the given list of mapping dictionaries.

Proxied for the Session class on behalf of the scoped_session class.

The bulk update feature allows plain Python dictionaries to be used as the source of simple UPDATE operations which can be more easily grouped together into higher performing “executemany” operations. Using dictionaries, there is no “history” or session state management features in use, reducing latency when updating large numbers of simple rows.

New in version 1.0.0.

Legacy Feature

The bulk update feature allows for a lower-latency UPDATE of rows at the expense of most other unit-of-work features. Features such as object management, relationship handling, and SQL clause support are silently omitted in favor of raw UPDATES of records.

In SQLAlchemy 2.0, improved versions of the bulk insert/update methods are introduced, with clearer behavior and documentation, new capabilities, and much better performance.

For 1.4 use, please read the list of caveats at ORM Compatibility / Caveats before using this method, and fully test and confirm the functionality of all code developed using these systems.

Parameters:

mapper – a mapped class, or the actual Mapper object, representing the single kind of object represented within the mapping list.
mappings – a sequence of dictionaries, each one containing the state of the mapped row to be updated, in terms of the attribute names on the mapped class. If the mapping refers to multiple tables, such as a joined-inheritance mapping, each dictionary may contain keys corresponding to all tables. All those keys which are present and are not part of the primary key are applied to the SET clause of the UPDATE statement; the primary key values, which are required, are applied to the WHERE clause.

See also

Bulk Operations

Session.bulk_insert_mappings()

Session.bulk_save_objects()

method sqlalchemy.orm.scoped_session.close()¶

Close out the transactional resources and ORM objects used by this Session.

Proxied for the Session class on behalf of the scoped_session class.

This expunges all ORM objects associated with this Session, ends any transaction in progress and releases any Connection objects which this Session itself has checked out from associated Engine objects. The operation then leaves the Session in a state which it may be used again.

Tip

The Session.close() method does not prevent the Session from being used again. The Session itself does not actually have a distinct “closed” state; it merely means the Session will release all database connections and ORM objects.

Changed in version 1.4: The Session.close() method does not immediately create a new SessionTransaction object; instead, the new SessionTransaction is created only if the Session is used again for a database operation.

See also

Closing - detail on the semantics of Session.close()

classmethod sqlalchemy.orm.scoped_session.close_all()¶: Close all sessions in memory.

Proxied for the Session class on behalf of the scoped_session class.

Deprecated since version 1.3: The Session.close_all() method is deprecated and will be removed in a future release. Please refer to close_all_sessions().

method sqlalchemy.orm.scoped_session.commit()¶

Flush pending changes and commit the current transaction.

Proxied for the Session class on behalf of the scoped_session class.

When the COMMIT operation is complete, all objects are fully expired, erasing their internal contents, which will be automatically re-loaded when the objects are next accessed. In the interim, these objects are in an expired state and will not function if they are detached from the Session. Additionally, this re-load operation is not supported when using asyncio-oriented APIs. The Session.expire_on_commit parameter may be used to disable this behavior.

When there is no transaction in place for the Session, indicating that no operations were invoked on this Session since the previous call to Session.commit(), the method will begin and commit an internal-only “logical” transaction, that does not normally affect the database unless pending flush changes were detected, but will still invoke event handlers and object expiration rules.

If 1.x-style use is in effect and there are currently SAVEPOINTs in progress via Session.begin_nested(), the operation will release the current SAVEPOINT but not commit the outermost database transaction.

If 2.0-style use is in effect via the Session.future flag, the outermost database transaction is committed unconditionally, automatically releasing any SAVEPOINTs in effect.

When using legacy “autocommit” mode, this method is only valid to call if a transaction is actually in progress, else an error is raised. Similarly, when using legacy “subtransactions”, the method will instead close out the current “subtransaction”, rather than the actual database transaction, if a transaction is in progress.

See also

Committing

Managing Transactions

Preventing Implicit IO when Using AsyncSession

method sqlalchemy.orm.scoped_session.configure(**kwargs)¶

inherited from the ScopedSessionMixin.configure() method of ScopedSessionMixin

reconfigure the sessionmaker used by this scoped_session.

See sessionmaker.configure().

method sqlalchemy.orm.scoped_session.connection(bind_arguments=None, close_with_result=False, execution_options=None, **kw)¶

Return a Connection object corresponding to this Session object’s transactional state.

Proxied for the Session class on behalf of the scoped_session class.

If this Session is configured with autocommit=False, either the Connection corresponding to the current transaction is returned, or if no transaction is in progress, a new one is begun and the Connection returned (note that no transactional state is established with the DBAPI until the first SQL statement is emitted).

Alternatively, if this Session is configured with autocommit=True, an ad-hoc Connection is returned using Engine.connect() on the underlying Engine.

Ambiguity in multi-bind or unbound Session objects can be resolved through any of the optional keyword arguments. This ultimately makes usage of the get_bind() method for resolution.

Parameters:

bind_arguments – dictionary of bind arguments. May include “mapper”, “bind”, “clause”, other custom arguments that are passed to Session.get_bind().
bind – deprecated; use bind_arguments
mapper – deprecated; use bind_arguments
clause – deprecated; use bind_arguments
close_with_result –
Passed to Engine.connect(), indicating the Connection should be considered “single use”, automatically closing when the first result set is closed. This flag only has an effect if this Session is configured with autocommit=True and does not already have a transaction in progress.

Deprecated since version 1.4: this parameter is deprecated and will be removed in SQLAlchemy 2.0
execution_options –
a dictionary of execution options that will be passed to Connection.execution_options(), when the connection is first procured only. If the connection is already present within the Session, a warning is emitted and the arguments are ignored.

See also

Setting Transaction Isolation Levels / DBAPI AUTOCOMMIT
**kw – deprecated; use bind_arguments

method sqlalchemy.orm.scoped_session.delete(instance)¶

Mark an instance as deleted.

Proxied for the Session class on behalf of the scoped_session class.

The object is assumed to be either persistent or detached when passed; after the method is called, the object will remain in the persistent state until the next flush proceeds. During this time, the object will also be a member of the Session.deleted collection.

When the next flush proceeds, the object will move to the deleted state, indicating a DELETE statement was emitted for its row within the current transaction. When the transaction is successfully committed, the deleted object is moved to the detached state and is no longer present within this Session.

See also

Deleting - at Basics of Using a Session

attribute sqlalchemy.orm.scoped_session.deleted¶: The set of all instances marked as ‘deleted’ within this Session

Proxied for the Session class on behalf of the scoped_session class.

attribute sqlalchemy.orm.scoped_session.dirty¶

The set of all persistent instances considered dirty.

Proxied for the Session class on behalf of the scoped_session class.

E.g.:

some_mapped_object in session.dirty

Instances are considered dirty when they were modified but not deleted.

Note that this ‘dirty’ calculation is ‘optimistic’; most attribute-setting or collection modification operations will mark an instance as ‘dirty’ and place it in this set, even if there is no net change to the attribute’s value. At flush time, the value of each attribute is compared to its previously saved value, and if there’s no net change, no SQL operation will occur (this is a more expensive operation so it’s only done at flush time).

To check if an instance has actionable net changes to its attributes, use the Session.is_modified() method.

method sqlalchemy.orm.scoped_session.execute(statement, params=None, execution_options={}, bind_arguments=None, _parent_execute_state=None, _add_event=None, **kw)¶

Execute a SQL expression construct.

Proxied for the Session class on behalf of the scoped_session class.

Returns a Result object representing results of the statement execution.

E.g.:

from sqlalchemy import select
result = session.execute(
    select(User).where(User.id == 5)
)

The API contract of Session.execute() is similar to that of Connection.execute(), the 2.0 style version of Connection.

Changed in version 1.4: the Session.execute() method is now the primary point of ORM statement execution when using 2.0 style ORM usage.

Parameters:

statement – An executable statement (i.e. an Executable expression such as select()).
params – Optional dictionary, or list of dictionaries, containing bound parameter values. If a single dictionary, single-row execution occurs; if a list of dictionaries, an “executemany” will be invoked. The keys in each dictionary must correspond to parameter names present in the statement.
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(), and may also provide additional options understood only in an ORM context.
bind_arguments – dictionary of additional arguments to determine the bind. May include “mapper”, “bind”, or other custom arguments. Contents of this dictionary are passed to the Session.get_bind() method.
mapper – deprecated; use the bind_arguments dictionary
bind – deprecated; use the bind_arguments dictionary
**kw – deprecated; use the bind_arguments dictionary

Returns:

a Result object.

method sqlalchemy.orm.scoped_session.expire(instance, attribute_names=None)¶

Expire the attributes on an instance.

Proxied for the Session class on behalf of the scoped_session class.

Marks the attributes of an instance as out of date. When an expired attribute is next accessed, a query will be issued to the Session object’s current transactional context in order to load all expired attributes for the given instance. Note that a highly isolated transaction will return the same values as were previously read in that same transaction, regardless of changes in database state outside of that transaction.

To expire all objects in the Session simultaneously, use Session.expire_all().

The Session object’s default behavior is to expire all state whenever the Session.rollback() or Session.commit() methods are called, so that new state can be loaded for the new transaction. For this reason, calling Session.expire() only makes sense for the specific case that a non-ORM SQL statement was emitted in the current transaction.

Parameters:

instance – The instance to be refreshed.
attribute_names – optional list of string attribute names indicating a subset of attributes to be expired.

See also

Refreshing / Expiring - introductory material

Session.expire()

Session.refresh()

Query.populate_existing()

method sqlalchemy.orm.scoped_session.expire_all()¶

Expires all persistent instances within this Session.

Proxied for the Session class on behalf of the scoped_session class.

When any attributes on a persistent instance is next accessed, a query will be issued using the Session object’s current transactional context in order to load all expired attributes for the given instance. Note that a highly isolated transaction will return the same values as were previously read in that same transaction, regardless of changes in database state outside of that transaction.

To expire individual objects and individual attributes on those objects, use Session.expire().

The Session object’s default behavior is to expire all state whenever the Session.rollback() or Session.commit() methods are called, so that new state can be loaded for the new transaction. For this reason, calling Session.expire_all() should not be needed when autocommit is False, assuming the transaction is isolated.

See also

Refreshing / Expiring - introductory material

Session.expire()

Session.refresh()

Query.populate_existing()

method sqlalchemy.orm.scoped_session.expunge(instance)¶

Remove the instance from this Session.

Proxied for the Session class on behalf of the scoped_session class.

This will free all internal references to the instance. Cascading will be applied according to the expunge cascade rule.

method sqlalchemy.orm.scoped_session.expunge_all()¶

Remove all object instances from this Session.

Proxied for the Session class on behalf of the scoped_session class.

This is equivalent to calling expunge(obj) on all objects in this Session.

method sqlalchemy.orm.scoped_session.flush(objects=None)¶

Flush all the object changes to the database.

Proxied for the Session class on behalf of the scoped_session class.

Writes out all pending object creations, deletions and modifications to the database as INSERTs, DELETEs, UPDATEs, etc. Operations are automatically ordered by the Session’s unit of work dependency solver.

Database operations will be issued in the current transactional context and do not affect the state of the transaction, unless an error occurs, in which case the entire transaction is rolled back. You may flush() as often as you like within a transaction to move changes from Python to the database’s transaction buffer.

For autocommit Sessions with no active manual transaction, flush() will create a transaction on the fly that surrounds the entire set of operations into the flush.

Parameters:

objects –

Optional; restricts the flush operation to operate only on elements that are in the given collection.

This feature is for an extremely narrow set of use cases where particular objects may need to be operated upon before the full flush() occurs. It is not intended for general use.

method sqlalchemy.orm.scoped_session.get(entity, ident, options=None, populate_existing=False, with_for_update=None, identity_token=None, execution_options=None)¶

Return an instance based on the given primary key identifier, or None if not found.

Proxied for the Session class on behalf of the scoped_session class.

E.g.:

my_user = session.get(User, 5)

some_object = session.get(VersionedFoo, (5, 10))

some_object = session.get(
    VersionedFoo,
    {"id": 5, "version_id": 10}
)

New in version 1.4: Added Session.get(), which is moved from the now deprecated Query.get() method.

Session.get() is special in that it provides direct access to the identity map of the Session. If the given primary key identifier is present in the local identity map, the object is returned directly from this collection and no SQL is emitted, unless the object has been marked fully expired. If not present, a SELECT is performed in order to locate the object.

Session.get() also will perform a check if the object is present in the identity map and marked as expired - a SELECT is emitted to refresh the object as well as to ensure that the row is still present. If not, ObjectDeletedError is raised.

Parameters:

entity – a mapped class or Mapper indicating the type of entity to be loaded.
ident –
A scalar, tuple, or dictionary representing the primary key. For a composite (e.g. multiple column) primary key, a tuple or dictionary should be passed.

For a single-column primary key, the scalar calling form is typically the most expedient. If the primary key of a row is the value “5”, the call looks like:
```
my_object = session.get(SomeClass, 5)
```
The tuple form contains primary key values typically in the order in which they correspond to the mapped Table object’s primary key columns, or if the Mapper.primary_key configuration parameter were used, in the order used for that parameter. For example, if the primary key of a row is represented by the integer digits “5, 10” the call would look like:
```
my_object = session.get(SomeClass, (5, 10))
```
The dictionary form should include as keys the mapped attribute names corresponding to each element of the primary key. If the mapped class has the attributes id, version_id as the attributes which store the object’s primary key value, the call would look like:
```
my_object = session.get(SomeClass, {"id": 5, "version_id": 10})
```
options – optional sequence of loader options which will be applied to the query, if one is emitted.
populate_existing – causes the method to unconditionally emit a SQL query and refresh the object with the newly loaded data, regardless of whether or not the object is already present.
with_for_update – optional boolean True indicating FOR UPDATE should be used, or may be a dictionary containing flags to indicate a more specific set of FOR UPDATE flags for the SELECT; flags should match the parameters of Query.with_for_update(). Supersedes the Session.refresh.lockmode parameter.
execution_options –
optional dictionary of execution options, which will be associated with the query execution if one is emitted. This dictionary can provide a subset of the options that are accepted by Connection.execution_options(), and may also provide additional options understood only in an ORM context.

New in version 1.4.29.

See also

ORM Execution Options - ORM-specific execution options

Returns:

The object instance, or None.

method sqlalchemy.orm.scoped_session.get_bind(mapper=None, clause=None, bind=None, _sa_skip_events=None, _sa_skip_for_implicit_returning=False)¶

Return a “bind” to which this Session is bound.

Proxied for the Session class on behalf of the scoped_session class.

The “bind” is usually an instance of Engine, except in the case where the Session has been explicitly bound directly to a Connection.

For a multiply-bound or unbound Session, the mapper or clause arguments are used to determine the appropriate bind to return.

Note that the “mapper” argument is usually present when Session.get_bind() is called via an ORM operation such as a Session.query(), each individual INSERT/UPDATE/DELETE operation within a Session.flush(), call, etc.

The order of resolution is:

if mapper given and Session.binds is present, locate a bind based first on the mapper in use, then on the mapped class in use, then on any base classes that are present in the __mro__ of the mapped class, from more specific superclasses to more general.
if clause given and Session.binds is present, locate a bind based on Table objects found in the given clause present in Session.binds.
if Session.binds is present, return that.
if clause given, attempt to return a bind linked to the MetaData ultimately associated with the clause.
if mapper given, attempt to return a bind linked to the MetaData ultimately associated with the Table or other selectable to which the mapper is mapped.
No bind can be found, UnboundExecutionError is raised.

Note that the Session.get_bind() method can be overridden on a user-defined subclass of Session to provide any kind of bind resolution scheme. See the example at Custom Vertical Partitioning.

Parameters:

mapper – Optional mapper() mapped class or instance of Mapper. The bind can be derived from a Mapper first by consulting the “binds” map associated with this Session, and secondly by consulting the MetaData associated with the Table to which the Mapper is mapped for a bind.
clause – A ClauseElement (i.e. select(), text(), etc.). If the mapper argument is not present or could not produce a bind, the given expression construct will be searched for a bound element, typically a Table associated with bound MetaData.

Session.binds

Session.bind_mapper()

Session.bind_table()

classmethod sqlalchemy.orm.scoped_session.identity_key(*args, **kwargs)¶

Return an identity key.

Proxied for the Session class on behalf of the scoped_session class.

This is an alias of identity_key().

attribute sqlalchemy.orm.scoped_session.info¶

A user-modifiable dictionary.

Proxied for the Session class on behalf of the scoped_session class.

The initial value of this dictionary can be populated using the info argument to the Session constructor or sessionmaker constructor or factory methods. The dictionary here is always local to this Session and can be modified independently of all other Session objects.

attribute sqlalchemy.orm.scoped_session.is_active¶

True if this Session not in “partial rollback” state.

Proxied for the Session class on behalf of the scoped_session class.

Changed in version 1.4: The Session no longer begins a new transaction immediately, so this attribute will be False when the Session is first instantiated.

“partial rollback” state typically indicates that the flush process of the Session has failed, and that the Session.rollback() method must be emitted in order to fully roll back the transaction.

If this Session is not in a transaction at all, the Session will autobegin when it is first used, so in this case Session.is_active will return True.

Otherwise, if this Session is within a transaction, and that transaction has not been rolled back internally, the Session.is_active will also return True.

Session.in_transaction()

method sqlalchemy.orm.scoped_session.is_modified(instance, include_collections=True)¶

Return True if the given instance has locally modified attributes.

Proxied for the Session class on behalf of the scoped_session class.

This method retrieves the history for each instrumented attribute on the instance and performs a comparison of the current value to its previously committed value, if any.

It is in effect a more expensive and accurate version of checking for the given instance in the Session.dirty collection; a full test for each attribute’s net “dirty” status is performed.

E.g.:

return session.is_modified(someobject)

A few caveats to this method apply:

Instances present in the Session.dirty collection may report False when tested with this method. This is because the object may have received change events via attribute mutation, thus placing it in Session.dirty, but ultimately the state is the same as that loaded from the database, resulting in no net change here.
Scalar attributes may not have recorded the previously set value when a new value was applied, if the attribute was not loaded, or was expired, at the time the new value was received - in these cases, the attribute is assumed to have a change, even if there is ultimately no net change against its database value. SQLAlchemy in most cases does not need the “old” value when a set event occurs, so it skips the expense of a SQL call if the old value isn’t present, based on the assumption that an UPDATE of the scalar value is usually needed, and in those few cases where it isn’t, is less expensive on average than issuing a defensive SELECT.

The “old” value is fetched unconditionally upon set only if the attribute container has the active_history flag set to True. This flag is set typically for primary key attributes and scalar object references that are not a simple many-to-one. To set this flag for any arbitrary mapped column, use the active_history argument with column_property().

Parameters:

instance – mapped instance to be tested for pending changes.
include_collections – Indicates if multivalued collections should be included in the operation. Setting this to False is a way to detect only local-column based properties (i.e. scalar columns or many-to-one foreign keys) that would result in an UPDATE for this instance upon flush.

method sqlalchemy.orm.scoped_session.merge(instance, load=True, options=None)¶

Copy the state of a given instance into a corresponding instance within this Session.

Proxied for the Session class on behalf of the scoped_session class.

Session.merge() examines the primary key attributes of the source instance, and attempts to reconcile it with an instance of the same primary key in the session. If not found locally, it attempts to load the object from the database based on primary key, and if none can be located, creates a new instance. The state of each attribute on the source instance is then copied to the target instance. The resulting target instance is then returned by the method; the original source instance is left unmodified, and un-associated with the Session if not already.

This operation cascades to associated instances if the association is mapped with cascade="merge".

See Merging for a detailed discussion of merging.

Changed in version 1.1: - Session.merge() will now reconcile pending objects with overlapping primary keys in the same way as persistent. See Session.merge resolves pending conflicts the same as persistent for discussion.

Parameters:

instance – Instance to be merged.
load –
Boolean, when False, merge() switches into a “high performance” mode which causes it to forego emitting history events as well as all database access. This flag is used for cases such as transferring graphs of objects into a Session from a second level cache, or to transfer just-loaded objects into the Session owned by a worker thread or process without re-querying the database.

The load=False use case adds the caveat that the given object has to be in a “clean” state, that is, has no pending changes to be flushed - even if the incoming object is detached from any Session. This is so that when the merge operation populates local attributes and cascades to related objects and collections, the values can be “stamped” onto the target object as is, without generating any history or attribute events, and without the need to reconcile the incoming data with any existing related objects or collections that might not be loaded. The resulting objects from load=False are always produced as “clean”, so it is only appropriate that the given objects should be “clean” as well, else this suggests a mis-use of the method.
options –
optional sequence of loader options which will be applied to the Session.get() method when the merge operation loads the existing version of the object from the database.

New in version 1.4.24.

See also

make_transient_to_detached() - provides for an alternative means of “merging” a single object into the Session

attribute sqlalchemy.orm.scoped_session.new¶: The set of all instances marked as ‘new’ within this Session.

Proxied for the Session class on behalf of the scoped_session class.

attribute sqlalchemy.orm.scoped_session.no_autoflush¶

Return a context manager that disables autoflush.

Proxied for the Session class on behalf of the scoped_session class.

e.g.:

with session.no_autoflush:

    some_object = SomeClass()
    session.add(some_object)
    # won't autoflush
    some_object.related_thing = session.query(SomeRelated).first()

Operations that proceed within the with: block will not be subject to flushes occurring upon query access. This is useful when initializing a series of objects which involve existing database queries, where the uncompleted object should not yet be flushed.

classmethod sqlalchemy.orm.scoped_session.object_session(instance)¶

Return the Session to which an object belongs.

Proxied for the Session class on behalf of the scoped_session class.

This is an alias of object_session().

method sqlalchemy.orm.scoped_session.query(*entities, **kwargs)¶: Return a new Query object corresponding to this Session.

Proxied for the Session class on behalf of the scoped_session class.

method sqlalchemy.orm.scoped_session.query_property(query_cls=None)¶

return a class property which produces a Query object against the class and the current Session when called.

e.g.:

Session = scoped_session(sessionmaker())

class MyClass(object):
    query = Session.query_property()

# after mappers are defined
result = MyClass.query.filter(MyClass.name=='foo').all()

Produces instances of the session’s configured query class by default. To override and use a custom implementation, provide a query_cls callable. The callable will be invoked with the class’s mapper as a positional argument and a session keyword argument.

There is no limit to the number of query properties placed on a class.

method sqlalchemy.orm.scoped_session.refresh(instance, attribute_names=None, with_for_update=None)¶

Expire and refresh attributes on the given instance.

Proxied for the Session class on behalf of the scoped_session class.

The selected attributes will first be expired as they would when using Session.expire(); then a SELECT statement will be issued to the database to refresh column-oriented attributes with the current value available in the current transaction.

relationship() oriented attributes will also be immediately loaded if they were already eagerly loaded on the object, using the same eager loading strategy that they were loaded with originally. Unloaded relationship attributes will remain unloaded, as will relationship attributes that were originally lazy loaded.

New in version 1.4: - the Session.refresh() method can also refresh eagerly loaded attributes.

Tip

While the Session.refresh() method is capable of refreshing both column and relationship oriented attributes, its primary focus is on refreshing of local column-oriented attributes on a single instance. For more open ended “refresh” functionality, including the ability to refresh the attributes on many objects at once while having explicit control over relationship loader strategies, use the populate existing feature instead.

Note that a highly isolated transaction will return the same values as were previously read in that same transaction, regardless of changes in database state outside of that transaction. Refreshing attributes usually only makes sense at the start of a transaction where database rows have not yet been accessed.

Parameters:

attribute_names – optional. An iterable collection of string attribute names indicating a subset of attributes to be refreshed.
with_for_update – optional boolean True indicating FOR UPDATE should be used, or may be a dictionary containing flags to indicate a more specific set of FOR UPDATE flags for the SELECT; flags should match the parameters of Query.with_for_update(). Supersedes the Session.refresh.lockmode parameter.

See also

Refreshing / Expiring - introductory material

Session.expire()

Session.expire_all()

Populate Existing - allows any ORM query to refresh objects as they would be loaded normally.

method sqlalchemy.orm.scoped_session.remove()¶

Dispose of the current Session, if present.

This will first call Session.close() method on the current Session, which releases any existing transactional/connection resources still being held; transactions specifically are rolled back. The Session is then discarded. Upon next usage within the same scope, the scoped_session will produce a new Session object.

method sqlalchemy.orm.scoped_session.rollback()¶

Rollback the current transaction in progress.

Proxied for the Session class on behalf of the scoped_session class.

If no transaction is in progress, this method is a pass-through.

In 1.x-style use, this method rolls back the topmost database transaction if no nested transactions are in effect, or to the current nested transaction if one is in effect.

When 2.0-style use is in effect via the Session.future flag, the method always rolls back the topmost database transaction, discarding any nested transactions that may be in progress.

See also

Rolling Back

Managing Transactions

method sqlalchemy.orm.scoped_session.scalar(statement, params=None, execution_options={}, bind_arguments=None, **kw)¶

Execute a statement and return a scalar result.

Proxied for the Session class on behalf of the scoped_session class.

Usage and parameters are the same as that of Session.execute(); the return result is a scalar Python value.

method sqlalchemy.orm.scoped_session.scalars(statement, params=None, execution_options={}, bind_arguments=None, **kw)¶

Execute a statement and return the results as scalars.

Proxied for the Session class on behalf of the scoped_session class.

Usage and parameters are the same as that of Session.execute(); the return result is a ScalarResult filtering object which will return single elements rather than Row objects.

Returns:: a ScalarResult object

New in version 1.4.24: Added Session.scalars()

New in version 1.4.26: Added scoped_session.scalars()

attribute sqlalchemy.orm.scoped_session.session_factory = None¶: The session_factory provided to __init__ is stored in this attribute and may be accessed at a later time. This can be useful when a new non-scoped Session or Connection to the database is needed.

class sqlalchemy.util.ScopedRegistry(createfunc, scopefunc)¶

A Registry that can store one or multiple instances of a single class on the basis of a “scope” function.

The object implements __call__ as the “getter”, so by calling myregistry() the contained object is returned for the current scope.

Members

__init__(), clear(), has(), set()

Parameters:

createfunc – a callable that returns a new object to be placed in the registry
scopefunc – a callable that will return a key to store/retrieve an object.

method sqlalchemy.util.ScopedRegistry.__init__(createfunc, scopefunc)¶

Construct a new ScopedRegistry.

Parameters:

createfunc – A creation function that will generate a new value for the current scope, if none is present.
scopefunc – A function that returns a hashable token representing the current scope (such as, current thread identifier).

method sqlalchemy.util.ScopedRegistry.clear()¶: Clear the current scope, if any.

method sqlalchemy.util.ScopedRegistry.has()¶: Return True if an object is present in the current scope.

method sqlalchemy.util.ScopedRegistry.set(obj)¶: Set the value for the current scope.

class sqlalchemy.util.ThreadLocalRegistry(createfunc)¶: A ScopedRegistry that uses a threading.local() variable for storage.

Class signature

class sqlalchemy.util.ThreadLocalRegistry (sqlalchemy.util.ScopedRegistry)

SQLAlchemy 1.4 Documentation

SQLAlchemy 1.4 Documentation

SQLAlchemy ORM

Contextual/Thread-local Sessions¶

Implicit Method Access¶

Thread-Local Scope¶

Using Thread-Local Scope with Web Applications¶

Using Custom Created Scopes¶

Contextual Session API¶