Release: 2.0.35 | Release Date: September 16, 2024

# SQLAlchemy 2.0 Documentation

# Transactions and Connection Management

## Managing Transactions

Changed in version 1.4: Session transaction management has been revised to be clearer and easier to use. In particular, it now features “autobegin” operation, which means the point at which a transaction begins may be controlled, without using the legacy “autocommit” mode.

The `Session` tracks the state of a single “virtual” transaction at a time, using an object called `SessionTransaction`. This object then makes use of the underlying `Engine` or engines to which the `Session` object is bound in order to start real connection-level transactions using the `Connection` object as needed.

This “virtual” transaction is created automatically when needed, or can alternatively be started using the `Session.begin()` method. To as great a degree as possible, Python context manager use is supported both at the level of creating `Session` objects as well as to maintain the scope of the `SessionTransaction`.

Below, assume we start with a `Session`:

    from sqlalchemy.orm import Session

    session = Session(engine)

We can now run operations within a demarcated transaction using a context manager:

    with session.begin():
        session.add(some_object())
        session.add(some_other_object())
    # commits transaction at the end, or rolls back if there
    # was an exception raised

At the end of the above context, assuming no exceptions were raised, any pending objects will be flushed to the database and the database transaction will be committed. If an exception was raised within the above block, then the transaction would be rolled back. In both cases, the above `Session` subsequent to exiting the block is ready to be used in subsequent transactions.

The `Session.begin()` method is optional, and the `Session` may also be used in a commit-as-you-go approach, where it will begin transactions automatically as needed; these only need be committed or rolled back:

    session = Session(engine)

    session.add(some_object())
    session.add(some_other_object())

    session.commit()  # commits

    # will automatically begin again
    result = session.execute(text("< some select statement >"))
    session.add_all([more_objects, ...])
    session.commit()  # commits

    session.add(still_another_object)
    session.flush()  # flush still_another_object
    session.rollback()  # rolls back still_another_object

The `Session` itself features a `Session.close()` method. If the `Session` is begun within a transaction that has not yet been committed or rolled back, this method will cancel (i.e. rollback) that transaction, and also expunge all objects contained within the `Session` object’s state. If the `Session` is being used in such a way that a call to `Session.commit()` or `Session.rollback()` is not guaranteed (e.g. not within a context manager or similar), the `close` method may be used to ensure all resources are released:

    # expunges all objects, releases all transactions unconditionally
    # (with rollback), releases all database connections back to their
    # engines
    session.close()

Finally, the session construction / close process can itself be run via context manager. This is the best way to ensure that the scope of a `Session` object’s use is scoped within a fixed block. Illustrated via the `Session` constructor first:

    with Session(engine) as session:
        session.add(some_object())
        session.add(some_other_object())

        session.commit()  # commits

        session.add(still_another_object)
        session.flush()  # flush still_another_object

        session.commit()  # commits

        result = session.execute(text("<some SELECT statement>"))

    # remaining transactional state from the .execute() call is
    # discarded

Similarly, the `sessionmaker` can be used in the same way:

    Session = sessionmaker(engine)

    with Session() as session:
        with session.begin():
            session.add(some_object)
        # commits

    # closes the Session

`sessionmaker` itself includes a `sessionmaker.begin()` method to allow both operations to take place at once:

    with Session.begin() as session:
        session.add(some_object)

### Using SAVEPOINT

SAVEPOINT transactions, if supported by the underlying engine, may be delineated using the `Session.begin_nested()` method:

    Session = sessionmaker()

    with Session.begin() as session:
        session.add(u1)
        session.add(u2)

        nested = session.begin_nested()  # establish a savepoint
        session.add(u3)
        nested.rollback()  # rolls back u3, keeps u1 and u2

    # commits u1 and u2

Each time `Session.begin_nested()` is called, a new “BEGIN SAVEPOINT” command is emitted to the database within the scope of the current database transaction (starting one if not already in progress), and an object of type `SessionTransaction` is returned, which represents a handle to this SAVEPOINT. When the `.commit()` method on this object is called, “RELEASE SAVEPOINT” is emitted to the database, and if instead the `.rollback()` method is called, “ROLLBACK TO SAVEPOINT” is emitted. The enclosing database transaction remains in progress.

`Session.begin_nested()` is typically used as a context manager where specific per-instance errors may be caught, in conjunction with a rollback emitted for that portion of the transaction’s state, without rolling back the whole transaction, as in the example below:

    for record in records:
        try:
            with session.begin_nested():
                session.merge(record)
        except:
            print("Skipped record %s" % record)
    session.commit()

When the context manager yielded by `Session.begin_nested()` completes, it “commits” the savepoint, which includes the usual behavior of flushing all pending state. When an error is raised, the savepoint is rolled back and the state of the `Session` local to the objects that were changed is expired.

This pattern is ideal for situations such as using PostgreSQL and catching `IntegrityError` to detect duplicate rows; PostgreSQL normally aborts the entire transaction when such an error is raised, however when using SAVEPOINT, the outer transaction is maintained. In the example below a list of data is persisted into the database, with the occasional “duplicate primary key” record skipped, without rolling back the entire operation:

    from sqlalchemy import exc

    with session.begin():
        for record in records:
            try:
                with session.begin_nested():
                    obj = SomeRecord(id=record["identifier"], name=record["name"])
                    session.add(obj)
            except exc.IntegrityError:
                print(f"Skipped record {record} - row already exists")

When `Session.begin_nested()` is called, the `Session` first flushes all currently pending state to the database; this occurs unconditionally, regardless of the value of the `Session.autoflush` parameter which normally may be used to disable automatic flush. The rationale for this behavior is so that when a rollback on this nested transaction occurs, the `Session` may expire any in-memory state that was created within the scope of the SAVEPOINT, while ensuring that when those expired objects are refreshed, the state of the object graph prior to the beginning of the SAVEPOINT will be available to re-load from the database.

In modern versions of SQLAlchemy, when a SAVEPOINT initiated by `Session.begin_nested()` is rolled back, in-memory object state that was modified since the SAVEPOINT was created is expired, however other object state that was not altered since the SAVEPOINT began is maintained. This is so that subsequent operations can continue to make use of the otherwise unaffected data without the need for refreshing it from the database.

See also

`Connection.begin_nested()` - Core SAVEPOINT API

### Session-level vs. Engine level transaction control

The `Connection` in Core and `_session.Session` in ORM feature equivalent transactional semantics, both at the level of the `sessionmaker` vs. the `Engine`, as well as the `Session` vs. the `Connection`. The following sections detail these scenarios based on the following scheme:

    ORM                                           Core
    -----------------------------------------     -----------------------------------
    sessionmaker                                  Engine
    Session                                       Connection
    sessionmaker.begin()                          Engine.begin()
    some_session.commit()                         some_connection.commit()
    with some_sessionmaker() as session:          with some_engine.connect() as conn:
    with some_sessionmaker.begin() as session:    with some_engine.begin() as conn:
    with some_session.begin_nested() as sp:       with some_connection.begin_nested() as sp:

#### Commit as you go

Both `Session` and `Connection` feature `Connection.commit()` and `Connection.rollback()` methods. Using SQLAlchemy 2.0-style operation, these methods affect the outermost transaction in all cases. For the `Session`, it is assumed that `Session.autobegin` is left at its default value of `True`.

`Engine`:

    engine = create_engine("postgresql+psycopg2://user:pass@host/dbname")

    with engine.connect() as conn:
        conn.execute(
            some_table.insert(),
            [
                {"data": "some data one"},
                {"data": "some data two"},
                {"data": "some data three"},
            ],
        )
        conn.commit()

`Session`:

    Session = sessionmaker(engine)

    with Session() as session:
        session.add_all(
            [
                SomeClass(data="some data one"),
                SomeClass(data="some data two"),
                SomeClass(data="some data three"),
            ]
        )
        session.commit()

#### Begin Once

Both `sessionmaker` and `Engine` feature a `Engine.begin()` method that will both procure a new object with which to execute SQL statements (the `Session` and `Connection`, respectively) and then return a context manager that will maintain a begin/commit/rollback context for that object.

Engine:

    engine = create_engine("postgresql+psycopg2://user:pass@host/dbname")

    with engine.begin() as conn:
        conn.execute(
            some_table.insert(),
            [
                {"data": "some data one"},
                {"data": "some data two"},
                {"data": "some data three"},
            ],
        )
    # commits and closes automatically

Session:

    Session = sessionmaker(engine)

    with Session.begin() as session:
        session.add_all(
            [
                SomeClass(data="some data one"),
                SomeClass(data="some data two"),
                SomeClass(data="some data three"),
            ]
        )
    # commits and closes automatically

#### Nested Transaction

When using a SAVEPOINT via the `Session.begin_nested()` or `Connection.begin_nested()` methods, the transaction object returned must be used to commit or rollback the SAVEPOINT. Calling the `Session.commit()` or `Connection.commit()` methods will always commit the outermost transaction; this is a SQLAlchemy 2.0 specific behavior that is reversed from the 1.x series.

Engine:

    engine = create_engine("postgresql+psycopg2://user:pass@host/dbname")

    with engine.begin() as conn:
        savepoint = conn.begin_nested()
        conn.execute(
            some_table.insert(),
            [
                {"data": "some data one"},
                {"data": "some data two"},
                {"data": "some data three"},
            ],
        )
        savepoint.commit()  # or rollback

    # commits automatically

Session:

    Session = sessionmaker(engine)

    with Session.begin() as session:
        savepoint = session.begin_nested()
        session.add_all(
            [
                SomeClass(data="some data one"),
                SomeClass(data="some data two"),
                SomeClass(data="some data three"),
            ]
        )
        savepoint.commit()  # or rollback
    # commits automatically

### Explicit Begin

The `Session` features “autobegin” behavior, meaning that as soon as operations begin to take place, it ensures a `SessionTransaction` is present to track ongoing operations. This transaction is completed when `Session.commit()` is called.

It is often desirable, particularly in framework integrations, to control the point at which the “begin” operation occurs. To suit this, the `Session` uses an “autobegin” strategy, such that the `Session.begin()` method may be called directly for a `Session` that has not already had a transaction begun:

    Session = sessionmaker(bind=engine)
    session = Session()
    session.begin()
    try:
        item1 = session.get(Item, 1)
        item2 = session.get(Item, 2)
        item1.foo = "bar"
        item2.bar = "foo"
        session.commit()
    except:
        session.rollback()
        raise

The above pattern is more idiomatically invoked using a context manager:

    Session = sessionmaker(bind=engine)
    session = Session()
    with session.begin():
        item1 = session.get(Item, 1)
        item2 = session.get(Item, 2)
        item1.foo = "bar"
        item2.bar = "foo"

The `Session.begin()` method and the session’s “autobegin” process use the same sequence of steps to begin the transaction. This includes that the `SessionEvents.after_transaction_create()` event is invoked when it occurs; this hook is used by frameworks in order to integrate their own transactional processes with that of the ORM `Session`.

### Enabling Two-Phase Commit

For backends which support two-phase operation (currently MySQL and PostgreSQL), the session can be instructed to use two-phase commit semantics. This will coordinate the committing of transactions across databases so that the transaction is either committed or rolled back in all databases. You can also `Session.prepare()` the session for interacting with transactions not managed by SQLAlchemy. To use two phase transactions set the flag `twophase=True` on the session:

    engine1 = create_engine("postgresql+psycopg2://db1")
    engine2 = create_engine("postgresql+psycopg2://db2")

    Session = sessionmaker(twophase=True)

    # bind User operations to engine 1, Account operations to engine 2
    Session.configure(binds={User: engine1, Account: engine2})

    session = Session()

    # .... work with accounts and users

    # commit.  session will issue a flush to all DBs, and a prepare step to all DBs,
    # before committing both transactions
    session.commit()

### Setting Transaction Isolation Levels / DBAPI AUTOCOMMIT

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

DBAPIs that support isolation levels also usually support the concept of true “autocommit”, which means that the DBAPI connection itself will be placed into a non-transactional autocommit mode. This usually means that the typical DBAPI behavior of emitting “BEGIN” to the database automatically no longer occurs, but it may also include other directives. When using this mode, the DBAPI does not use a transaction under any circumstances. SQLAlchemy methods like `.begin()`, `.commit()` and `.rollback()` pass silently.

SQLAlchemy’s dialects support settable isolation modes on a per-`Engine` or per-`Connection` basis, using flags at both the `create_engine()` level as well as at the `Connection.execution_options()` level.

When using the ORM `Session`, it acts as a facade for engines and connections, but does not expose transaction isolation directly. So in order to affect transaction isolation level, we need to act upon the `Engine` or `Connection` as appropriate.

See also

Setting Transaction Isolation Levels including DBAPI Autocommit - be sure to review how isolation levels work at the level of the SQLAlchemy `Connection` object as well.

#### Setting Isolation For A Sessionmaker / Engine Wide

To set up a `Session` or `sessionmaker` with a specific isolation level globally, the first technique is that an `Engine` can be constructed against a specific isolation level in all cases, which is then used as the source of connectivity for a `Session` and/or `sessionmaker`:

    from sqlalchemy import create_engine
    from sqlalchemy.orm import sessionmaker

    eng = create_engine(
        "postgresql+psycopg2://scott:tiger@localhost/test",
        isolation_level="REPEATABLE READ",
    )

    Session = sessionmaker(eng)

Another option, useful if there are to be two engines with different isolation levels at once, is to use the `Engine.execution_options()` method, which will produce a shallow copy of the original `Engine` which shares the same connection pool as the parent engine. This is often preferable when operations will be separated into “transactional” and “autocommit” operations:

    from sqlalchemy import create_engine
    from sqlalchemy.orm import sessionmaker

    eng = create_engine("postgresql+psycopg2://scott:tiger@localhost/test")

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

    transactional_session = sessionmaker(eng)
    autocommit_session = sessionmaker(autocommit_engine)

Above, both “`eng`” and `"autocommit_engine"` share the same dialect and connection pool. However the “AUTOCOMMIT” mode will be set upon connections when they are acquired from the `autocommit_engine`. The two `sessionmaker` objects “`transactional_session`” and “`autocommit_session"` then inherit these characteristics when they work with database connections.

The “`autocommit_session`” continues to have transactional semantics, including that `Session.commit()` and `Session.rollback()` still consider themselves to be “committing” and “rolling back” objects, however the transaction will be silently absent. For this reason, it is typical, though not strictly required, that a Session with AUTOCOMMIT isolation be used in a read-only fashion, that is:

    with autocommit_session() as session:
        some_objects = session.execute(text("<statement>"))
        some_other_objects = session.execute(text("<statement>"))

    # closes connection

#### Setting Isolation for Individual Sessions

When we make a new `Session`, either using the constructor directly or when we call upon the callable produced by a `sessionmaker`, we can pass the `bind` argument directly, overriding the pre-existing bind. We can for example create our `Session` from a default `sessionmaker` and pass an engine set for autocommit:

    plain_engine = create_engine("postgresql+psycopg2://scott:tiger@localhost/test")

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

    # will normally use plain_engine
    Session = sessionmaker(plain_engine)

    # make a specific Session that will use the "autocommit" engine
    with Session(bind=autocommit_engine) as session:
        # work with session
        ...

For the case where the `Session` or `sessionmaker` is configured with multiple “binds”, we can either re-specify the `binds` argument fully, or if we want to only replace specific binds, we can use the `Session.bind_mapper()` or `Session.bind_table()` methods:

    with Session() as session:
        session.bind_mapper(User, autocommit_engine)

#### Setting Isolation for Individual Transactions

A key caveat regarding isolation level is that the setting cannot be safely modified on a `Connection` where a transaction has already started. Databases cannot change the isolation level of a transaction in progress, and some DBAPIs and SQLAlchemy dialects have inconsistent behaviors in this area.

Therefore it is preferable to use a `Session` that is up front bound to an engine with the desired isolation level. However, the isolation level on a per-connection basis can be affected by using the `Session.connection()` method at the start of a transaction:

    from sqlalchemy.orm import Session

    # assume session just constructed
    sess = Session(bind=engine)

    # call connection() with options before any other operations proceed.
    # this will procure a new connection from the bound engine and begin a real
    # database transaction.
    sess.connection(execution_options={"isolation_level": "SERIALIZABLE"})

    # ... work with session in SERIALIZABLE isolation level...

    # commit transaction.  the connection is released
    # and reverted to its previous isolation level.
    sess.commit()

    # subsequent to commit() above, a new transaction may be begun if desired,
    # which will proceed with the previous default isolation level unless
    # it is set again.

Above, we first produce a `Session` using either the constructor or a `sessionmaker`. Then we explicitly set up the start of a database-level transaction by calling upon `Session.connection()`, which provides for execution options that will be passed to the connection before the database-level transaction is begun. The transaction proceeds with this selected isolation level. When the transaction completes, the isolation level is reset on the connection to its default before the connection is returned to the connection pool.

The `Session.begin()` method may also be used to begin the `Session` level transaction; calling upon `Session.connection()` subsequent to that call may be used to set up the per-connection-transaction isolation level:

    sess = Session(bind=engine)

    with sess.begin():
        # call connection() with options before any other operations proceed.
        # this will procure a new connection from the bound engine and begin a
        # real database transaction.
        sess.connection(execution_options={"isolation_level": "SERIALIZABLE"})

        # ... work with session in SERIALIZABLE isolation level...

    # outside the block, the transaction has been committed.  the connection is
    # released and reverted to its previous isolation level.

### Tracking Transaction State with Events

See the section Transaction Events for an overview of the available event hooks for session transaction state changes.

## Joining a Session into an External Transaction (such as for test suites)

If a `Connection` is being used which is already in a transactional state (i.e. has a `Transaction` established), a `Session` can be made to participate within that transaction by just binding the `Session` to that `Connection`. The usual rationale for this is a test suite that allows ORM code to work freely with a `Session`, including the ability to call `Session.commit()`, where afterwards the entire database interaction is rolled back.

Changed in version 2.0: The “join into an external transaction” recipe is newly improved again in 2.0; event handlers to “reset” the nested transaction are no longer required.

The recipe works by establishing a `Connection` within a transaction and optionally a SAVEPOINT, then passing it to a `Session` as the “bind”; the `Session.join_transaction_mode` parameter is passed with the setting `"create_savepoint"`, which indicates that new SAVEPOINTs should be created in order to implement BEGIN/COMMIT/ROLLBACK for the `Session`, which will leave the external transaction in the same state in which it was passed.

When the test tears down, the external transaction is rolled back so that any data changes throughout the test are reverted:

    from sqlalchemy.orm import sessionmaker
    from sqlalchemy import create_engine
    from unittest import TestCase

    # global application scope.  create Session class, engine
    Session = sessionmaker()

    engine = create_engine("postgresql+psycopg2://...")

    class SomeTest(TestCase):
        def setUp(self):
            # connect to the database
            self.connection = engine.connect()

            # begin a non-ORM transaction
            self.trans = self.connection.begin()

            # bind an individual Session to the connection, selecting
            # "create_savepoint" join_transaction_mode
            self.session = Session(
                bind=self.connection, join_transaction_mode="create_savepoint"
            )

        def test_something(self):
            # use the session in tests.

            self.session.add(Foo())
            self.session.commit()

        def test_something_with_rollbacks(self):
            self.session.add(Bar())
            self.session.flush()
            self.session.rollback()

            self.session.add(Foo())
            self.session.commit()

        def tearDown(self):
            self.session.close()

            # rollback - everything that happened with the
            # Session above (including calls to commit())
            # is rolled back.
            self.trans.rollback()

            # return connection to the Engine
            self.connection.close()

The above recipe is part of SQLAlchemy’s own CI to ensure that it remains working as expected.