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.scalars(select(MyClass)).all())
#
print(Session.scalars(select(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.execute(select(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 |
---|---|
Describes the type applied to a class-level
|
|
Provides scoped management of |
|
A Registry that can store one or multiple instances of a single class on the basis of a “scope” function. |
|
A |
- class sqlalchemy.orm.scoped_session¶
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 ofscoped_session
.Members
__call__(), __init__(), add(), add_all(), autoflush, begin(), begin_nested(), bind, bulk_insert_mappings(), bulk_save_objects(), bulk_update_mappings(), close(), close_all(), commit(), configure(), connection(), delete(), deleted, dirty, execute(), expire(), expire_all(), expunge(), expunge_all(), flush(), get(), get_bind(), identity_key(), identity_map, info, is_active, is_modified(), merge(), new, no_autoflush, object_session(), query(), query_property(), refresh(), remove(), rollback(), scalar(), scalars(), session_factory
Class signature
class
sqlalchemy.orm.scoped_session
(typing.Generic
)-
method
sqlalchemy.orm.scoped_session.
__call__(**kw: Any) _S ¶ Return the current
Session
, creating it using thescoped_session.session_factory
if not present.- Parameters:
**kw – Keyword arguments will be passed to the
scoped_session.session_factory
callable, if an existingSession
is not present. If theSession
is present and keyword arguments have been passed,InvalidRequestError
is raised.
-
method
sqlalchemy.orm.scoped_session.
__init__(session_factory: sessionmaker[_S], scopefunc: Optional[Callable[[], Any]] = None)¶ Construct a new
scoped_session
.- Parameters:
session_factory – a factory to create new
Session
instances. This is usually, but not necessarily, an instance ofsessionmaker
.scopefunc – optional function which defines the current scope. If not passed, the
scoped_session
object assumes “thread-local” scope, and will use a Pythonthreading.local()
in order to maintain the currentSession
. 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 currentSession
.
-
method
sqlalchemy.orm.scoped_session.
add(instance: object, _warn: bool = True) None ¶ Place an object into this
Session
.Proxied for the
Session
class on behalf of thescoped_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 toSession.add()
will be moved back to the transient state, and will no longer be present within thisSession
.
-
method
sqlalchemy.orm.scoped_session.
add_all(instances: Iterable[object]) None ¶ Add the given collection of instances to this
Session
.Proxied for the
Session
class on behalf of thescoped_session
class.See the documentation for
Session.add()
for a general behavioral description.
-
attribute
sqlalchemy.orm.scoped_session.
autoflush¶ Proxy for the
Session.autoflush
attribute on behalf of thescoped_session
class.
-
method
sqlalchemy.orm.scoped_session.
begin(nested: bool = False) SessionTransaction ¶ Begin a transaction, or nested transaction, on this
Session
, if one is not already begun.Proxied for the
Session
class on behalf of thescoped_session
class.The
Session
object features autobegin behavior, so that normally it is not necessary to call theSession.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.- Returns:
the
SessionTransaction
object. Note thatSessionTransaction
acts as a Python context manager, allowingSession.begin()
to be used in a “with” block. See Explicit Begin for an example.
-
method
sqlalchemy.orm.scoped_session.
begin_nested() SessionTransaction ¶ Begin a “nested” transaction on this Session, e.g. SAVEPOINT.
Proxied for the
Session
class on behalf of thescoped_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 thatSessionTransaction
acts as a context manager, allowingSession.begin_nested()
to be used in a “with” block. See Using SAVEPOINT for a usage example.
See also
Serializable isolation / Savepoints / Transactional DDL - special workarounds required with the SQLite driver in order for SAVEPOINT to work correctly.
-
attribute
sqlalchemy.orm.scoped_session.
bind¶ Proxy for the
Session.bind
attribute on behalf of thescoped_session
class.
-
method
sqlalchemy.orm.scoped_session.
bulk_insert_mappings(mapper: Mapper[Any], mappings: Iterable[Dict[str, Any]], return_defaults: bool = False, render_nulls: bool = False) None ¶ Perform a bulk insert of the given list of mapping dictionaries.
Proxied for the
Session
class on behalf of thescoped_session
class.Legacy Feature
This method is a legacy feature as of the 2.0 series of SQLAlchemy. For modern bulk INSERT and UPDATE, see the sections ORM Bulk INSERT Statements and ORM Bulk UPDATE by Primary Key. The 2.0 API shares implementation details with this method and adds new features as well.
- 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, the INSERT process will be altered to ensure that newly generated primary key values will be fetched. The rationale for this parameter is typically to enable Joined Table Inheritance mappings to be bulk inserted.
Note
for backends that don’t support RETURNING, the
Session.bulk_insert_mappings.return_defaults
parameter can significantly decrease performance as INSERT statements can no longer be batched. See “Insert Many Values” Behavior for INSERT statements for background on which backends are affected.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.
-
method
sqlalchemy.orm.scoped_session.
bulk_save_objects(objects: Iterable[object], return_defaults: bool = False, update_changed_only: bool = True, preserve_order: bool = True) None ¶ Perform a bulk save of the given list of objects.
Proxied for the
Session
class on behalf of thescoped_session
class.Legacy Feature
This method is a legacy feature as of the 2.0 series of SQLAlchemy. For modern bulk INSERT and UPDATE, see the sections ORM Bulk INSERT Statements and ORM Bulk UPDATE by Primary Key.
For general INSERT and UPDATE of existing ORM mapped objects, prefer standard unit of work data management patterns, introduced in the SQLAlchemy Unified Tutorial at Data Manipulation with the ORM. SQLAlchemy 2.0 now uses “Insert Many Values” Behavior for INSERT statements with modern dialects which solves previous issues of bulk INSERT slowness.
- 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 theInstanceState.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 standardSession.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.
-
method
sqlalchemy.orm.scoped_session.
bulk_update_mappings(mapper: Mapper[Any], mappings: Iterable[Dict[str, Any]]) None ¶ Perform a bulk update of the given list of mapping dictionaries.
Proxied for the
Session
class on behalf of thescoped_session
class.Legacy Feature
This method is a legacy feature as of the 2.0 series of SQLAlchemy. For modern bulk INSERT and UPDATE, see the sections ORM Bulk INSERT Statements and ORM Bulk UPDATE by Primary Key. The 2.0 API shares implementation details with this method and adds new features as well.
- 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.
-
method
sqlalchemy.orm.scoped_session.
close() None ¶ Close out the transactional resources and ORM objects used by this
Session
.Proxied for the
Session
class on behalf of thescoped_session
class.This expunges all ORM objects associated with this
Session
, ends any transaction in progress and releases anyConnection
objects which thisSession
itself has checked out from associatedEngine
objects. The operation then leaves theSession
in a state which it may be used again.Tip
The
Session.close()
method does not prevent the Session from being used again. TheSession
itself does not actually have a distinct “closed” state; it merely means theSession
will release all database connections and ORM objects.Changed in version 1.4: The
Session.close()
method does not immediately create a newSessionTransaction
object; instead, the newSessionTransaction
is created only if theSession
is used again for a database operation.See also
Closing - detail on the semantics of
Session.close()
-
classmethod
sqlalchemy.orm.scoped_session.
close_all() None ¶ Close all sessions in memory.
Proxied for the
Session
class on behalf of thescoped_session
class.Deprecated since version 1.3: The
Session.close_all()
method is deprecated and will be removed in a future release. Please refer toclose_all_sessions()
.
-
method
sqlalchemy.orm.scoped_session.
commit() None ¶ Flush pending changes and commit the current transaction.
Proxied for the
Session
class on behalf of thescoped_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. TheSession.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 thisSession
since the previous call toSession.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.The outermost database transaction is committed unconditionally, automatically releasing any SAVEPOINTs in effect.
-
method
sqlalchemy.orm.scoped_session.
configure(**kwargs: Any) None ¶ reconfigure the
sessionmaker
used by thisscoped_session
.
-
method
sqlalchemy.orm.scoped_session.
connection(bind_arguments: Optional[_BindArguments] = None, execution_options: Optional[_ExecuteOptions] = None) Connection ¶ Return a
Connection
object corresponding to thisSession
object’s transactional state.Proxied for the
Session
class on behalf of thescoped_session
class.Either the
Connection
corresponding to the current transaction is returned, or if no transaction is in progress, a new one is begun and theConnection
returned (note that no transactional state is established with the DBAPI until the first SQL statement is emitted).Ambiguity in multi-bind or unbound
Session
objects can be resolved through any of the optional keyword arguments. This ultimately makes usage of theget_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()
.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 theSession
, a warning is emitted and the arguments are ignored.
-
method
sqlalchemy.orm.scoped_session.
delete(instance: object) None ¶ Mark an instance as deleted.
Proxied for the
Session
class on behalf of thescoped_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 thisSession
.See also
-
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 thescoped_session
class.
-
attribute
sqlalchemy.orm.scoped_session.
dirty¶ The set of all persistent instances considered dirty.
Proxied for the
Session
class on behalf of thescoped_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: Executable, params: Optional[_CoreAnyExecuteParams] = None, *, execution_options: OrmExecuteOptionsParameter = {}, bind_arguments: Optional[_BindArguments] = None, _parent_execute_state: Optional[Any] = None, _add_event: Optional[Any] = None) Result[Any] ¶ Execute a SQL expression construct.
Proxied for the
Session
class on behalf of thescoped_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 ofConnection.execute()
, the 2.0 style version ofConnection
.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 asselect()
).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.See also
ORM Execution Options - ORM-specific execution options
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.
- Returns:
a
Result
object.
-
method
sqlalchemy.orm.scoped_session.
expire(instance: object, attribute_names: Optional[Iterable[str]] = None) None ¶ Expire the attributes on an instance.
Proxied for the
Session
class on behalf of thescoped_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, useSession.expire_all()
.The
Session
object’s default behavior is to expire all state whenever theSession.rollback()
orSession.commit()
methods are called, so that new state can be loaded for the new transaction. For this reason, callingSession.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
-
method
sqlalchemy.orm.scoped_session.
expire_all() None ¶ Expires all persistent instances within this Session.
Proxied for the
Session
class on behalf of thescoped_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 theSession.rollback()
orSession.commit()
methods are called, so that new state can be loaded for the new transaction. For this reason, callingSession.expire_all()
is not usually needed, assuming the transaction is isolated.See also
Refreshing / Expiring - introductory material
-
method
sqlalchemy.orm.scoped_session.
expunge(instance: object) None ¶ Remove the instance from this
Session
.Proxied for the
Session
class on behalf of thescoped_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() None ¶ Remove all object instances from this
Session
.Proxied for the
Session
class on behalf of thescoped_session
class.This is equivalent to calling
expunge(obj)
on all objects in thisSession
.
-
method
sqlalchemy.orm.scoped_session.
flush(objects: Optional[Sequence[Any]] = None) None ¶ Flush all the object changes to the database.
Proxied for the
Session
class on behalf of thescoped_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.
- 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: _EntityBindKey[_O], ident: _PKIdentityArgument, *, options: Optional[Sequence[ORMOption]] = None, populate_existing: bool = False, with_for_update: ForUpdateParameter = None, identity_token: Optional[Any] = None, execution_options: OrmExecuteOptionsParameter = {}, bind_arguments: Optional[_BindArguments] = None) Optional[_O] ¶ Return an instance based on the given primary key identifier, or
None
if not found.Proxied for the
Session
class on behalf of thescoped_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 legacyQuery.get()
method.Session.get()
is special in that it provides direct access to the identity map of theSession
. 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 theMapper.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 ofQuery.with_for_update()
. Supersedes theSession.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
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.
- Returns:
The object instance, or
None
.
-
method
sqlalchemy.orm.scoped_session.
get_bind(mapper: Optional[_EntityBindKey[_O]] = None, *, clause: Optional[ClauseElement] = None, bind: Optional[_SessionBind] = None, _sa_skip_events: Optional[bool] = None, _sa_skip_for_implicit_returning: bool = False, **kw: Any) Union[Engine, Connection] ¶ Return a “bind” to which this
Session
is bound.Proxied for the
Session
class on behalf of thescoped_session
class.The “bind” is usually an instance of
Engine
, except in the case where theSession
has been explicitly bound directly to aConnection
.For a multiply-bound or unbound
Session
, themapper
orclause
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 aSession.query()
, each individual INSERT/UPDATE/DELETE operation within aSession.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 onTable
objects found in the given clause present inSession.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 theTable
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 ofSession
to provide any kind of bind resolution scheme. See the example at Custom Vertical Partitioning.- Parameters:
mapper – Optional mapped class or corresponding
Mapper
instance. The bind can be derived from aMapper
first by consulting the “binds” map associated with thisSession
, and secondly by consulting theMetaData
associated with theTable
to which theMapper
is mapped for a bind.clause – A
ClauseElement
(i.e.select()
,text()
, etc.). If themapper
argument is not present or could not produce a bind, the given expression construct will be searched for a bound element, typically aTable
associated with boundMetaData
.
-
classmethod
sqlalchemy.orm.scoped_session.
identity_key(class_: Optional[Type[Any]] = None, ident: Union[Any, Tuple[Any, ...]] = None, *, instance: Optional[Any] = None, row: Optional[Union[Row[Any], RowMapping]] = None, identity_token: Optional[Any] = None) _IdentityKeyType[Any] ¶ Return an identity key.
Proxied for the
Session
class on behalf of thescoped_session
class.This is an alias of
identity_key()
.
-
attribute
sqlalchemy.orm.scoped_session.
identity_map¶ Proxy for the
Session.identity_map
attribute on behalf of thescoped_session
class.
-
attribute
sqlalchemy.orm.scoped_session.
info¶ A user-modifiable dictionary.
Proxied for the
Session
class on behalf of thescoped_session
class.The initial value of this dictionary can be populated using the
info
argument to theSession
constructor orsessionmaker
constructor or factory methods. The dictionary here is always local to thisSession
and can be modified independently of all otherSession
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 thescoped_session
class.Changed in version 1.4: The
Session
no longer begins a new transaction immediately, so this attribute will be False when theSession
is first instantiated.“partial rollback” state typically indicates that the flush process of the
Session
has failed, and that theSession.rollback()
method must be emitted in order to fully roll back the transaction.If this
Session
is not in a transaction at all, theSession
will autobegin when it is first used, so in this caseSession.is_active
will return True.Otherwise, if this
Session
is within a transaction, and that transaction has not been rolled back internally, theSession.is_active
will also return True.
-
method
sqlalchemy.orm.scoped_session.
is_modified(instance: object, include_collections: bool = True) bool ¶ Return
True
if the given instance has locally modified attributes.Proxied for the
Session
class on behalf of thescoped_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 reportFalse
when tested with this method. This is because the object may have received change events via attribute mutation, thus placing it inSession.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 toTrue
. 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 theactive_history
argument withcolumn_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: _O, *, load: bool = True, options: Optional[Sequence[ORMOption]] = None) _O ¶ Copy the state of a given instance into a corresponding instance within this
Session
.Proxied for the
Session
class on behalf of thescoped_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 theSession
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.
- 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 aSession
from a second level cache, or to transfer just-loaded objects into theSession
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 anySession
. 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 fromload=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 theSession
-
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 thescoped_session
class.
-
attribute
sqlalchemy.orm.scoped_session.
no_autoflush¶ Return a context manager that disables autoflush.
Proxied for the
Session
class on behalf of thescoped_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: object) Optional[Session] ¶ Return the
Session
to which an object belongs.Proxied for the
Session
class on behalf of thescoped_session
class.This is an alias of
object_session()
.
-
method
sqlalchemy.orm.scoped_session.
query(*entities: _ColumnsClauseArgument[Any], **kwargs: Any) Query[Any] ¶ Return a new
Query
object corresponding to thisSession
.Proxied for the
Session
class on behalf of thescoped_session
class.Note that the
Query
object is legacy as of SQLAlchemy 2.0; theselect()
construct is now used to construct ORM queries.
-
method
sqlalchemy.orm.scoped_session.
query_property(query_cls: Optional[Type[Query[_T]]] = None) QueryPropertyDescriptor ¶ return a class property which produces a legacy
Query
object against the class and the currentSession
when called.Legacy Feature
The
scoped_session.query_property()
accessor is specific to the legacyQuery
object and is not considered to be part of 2.0-style ORM use.e.g.:
from sqlalchemy.orm import QueryPropertyDescriptor from sqlalchemy.orm import scoped_session from sqlalchemy.orm import sessionmaker Session = scoped_session(sessionmaker()) class MyClass: query: QueryPropertyDescriptor = 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: object, attribute_names: Optional[Iterable[str]] = None, with_for_update: ForUpdateParameter = None) None ¶ Expire and refresh attributes on the given instance.
Proxied for the
Session
class on behalf of thescoped_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.New in version 1.4: - the
Session.refresh()
method can also refresh eagerly loaded attributes.relationship()
oriented attributes that would normally load using theselect
(or “lazy”) loader strategy will also load if they are named explicitly in the attribute_names collection, emitting a SELECT statement for the attribute using theimmediate
loader strategy. If lazy-loaded relationships are not named inSession.refresh.attribute_names
, then they remain as “lazy loaded” attributes and are not implicitly refreshed.Changed in version 2.0.4: The
Session.refresh()
method will now refresh lazy-loadedrelationship()
oriented attributes for those which are named explicitly in theSession.refresh.attribute_names
collection.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 ofQuery.with_for_update()
. Supersedes theSession.refresh.lockmode
parameter.
See also
Refreshing / Expiring - introductory material
Populate Existing - allows any ORM query to refresh objects as they would be loaded normally.
-
method
sqlalchemy.orm.scoped_session.
remove() None ¶ Dispose of the current
Session
, if present.This will first call
Session.close()
method on the currentSession
, which releases any existing transactional/connection resources still being held; transactions specifically are rolled back. TheSession
is then discarded. Upon next usage within the same scope, thescoped_session
will produce a newSession
object.
-
method
sqlalchemy.orm.scoped_session.
rollback() None ¶ Rollback the current transaction in progress.
Proxied for the
Session
class on behalf of thescoped_session
class.If no transaction is in progress, this method is a pass-through.
The method always rolls back the topmost database transaction, discarding any nested transactions that may be in progress.
-
method
sqlalchemy.orm.scoped_session.
scalar(statement: Executable, params: Optional[_CoreSingleExecuteParams] = None, *, execution_options: OrmExecuteOptionsParameter = {}, bind_arguments: Optional[_BindArguments] = None, **kw: Any) Any ¶ Execute a statement and return a scalar result.
Proxied for the
Session
class on behalf of thescoped_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: Executable, params: Optional[_CoreAnyExecuteParams] = None, *, execution_options: OrmExecuteOptionsParameter = {}, bind_arguments: Optional[_BindArguments] = None, **kw: Any) ScalarResult[Any] ¶ Execute a statement and return the results as scalars.
Proxied for the
Session
class on behalf of thescoped_session
class.Usage and parameters are the same as that of
Session.execute()
; the return result is aScalarResult
filtering object which will return single elements rather thanRow
objects.- Returns:
a
ScalarResult
object
New in version 1.4.24: Added
Session.scalars()
New in version 1.4.26: Added
scoped_session.scalars()
See also
Selecting ORM Entities - contrasts the behavior of
Session.execute()
toSession.scalars()
-
attribute
sqlalchemy.orm.scoped_session.
session_factory: sessionmaker[_S]¶ 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
is needed.
-
method
- class sqlalchemy.util.ScopedRegistry¶
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 callingmyregistry()
the contained object is returned for the current scope.- 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.
Members
Class signature
class
sqlalchemy.util.ScopedRegistry
(typing.Generic
)-
method
sqlalchemy.util.ScopedRegistry.
__init__(createfunc: Callable[[], _T], scopefunc: Callable[[], Any])¶ 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() None ¶ Clear the current scope, if any.
-
method
sqlalchemy.util.ScopedRegistry.
has() bool ¶ Return True if an object is present in the current scope.
-
method
sqlalchemy.util.ScopedRegistry.
set(obj: _T) None ¶ Set the value for the current scope.
- class sqlalchemy.util.ThreadLocalRegistry¶
A
ScopedRegistry
that uses athreading.local()
variable for storage.Class signature
class
sqlalchemy.util.ThreadLocalRegistry
(sqlalchemy.util.ScopedRegistry
)
- class sqlalchemy.orm.QueryPropertyDescriptor¶
Describes the type applied to a class-level
scoped_session.query_property()
attribute.New in version 2.0.5.
Class signature
class
sqlalchemy.orm.QueryPropertyDescriptor
(typing.Protocol
)