Release: 1.3.2 | Release Date: April 2, 2019

# SQLAlchemy 1.3 Documentation

# Events

SQLAlchemy includes an event API which publishes a wide variety of hooks into the internals of both SQLAlchemy Core and ORM.

## Event Registration

Subscribing to an event occurs through a single API point, the `listen()` function, or alternatively the `listens_for()` decorator. These functions accept a target, a string identifier which identifies the event to be intercepted, and a user-defined listening function. Additional positional and keyword arguments to these two functions may be supported by specific types of events, which may specify alternate interfaces for the given event function, or provide instructions regarding secondary event targets based on the given target.

The name of an event and the argument signature of a corresponding listener function is derived from a class bound specification method, which exists bound to a marker class that’s described in the documentation. For example, the documentation for `PoolEvents.connect()` indicates that the event name is `"connect"` and that a user-defined listener function should receive two positional arguments:

    from sqlalchemy.event import listen
    from sqlalchemy.pool import Pool

    def my_on_connect(dbapi_con, connection_record):
        print("New DBAPI connection:", dbapi_con)

    listen(Pool, 'connect', my_on_connect)

To listen with the `listens_for()` decorator looks like:

    from sqlalchemy.event import listens_for
    from sqlalchemy.pool import Pool

    @listens_for(Pool, "connect")
    def my_on_connect(dbapi_con, connection_record):
        print("New DBAPI connection:", dbapi_con)

## Named Argument Styles

There are some varieties of argument styles which can be accepted by listener functions. Taking the example of `PoolEvents.connect()`, this function is documented as receiving `dbapi_connection` and `connection_record` arguments. We can opt to receive these arguments by name, by establishing a listener function that accepts `**keyword` arguments, by passing `named=True` to either `listen()` or `listens_for()`:

    from sqlalchemy.event import listens_for
    from sqlalchemy.pool import Pool

    @listens_for(Pool, "connect", named=True)
    def my_on_connect(**kw):
        print("New DBAPI connection:", kw['dbapi_connection'])

When using named argument passing, the names listed in the function argument specification will be used as keys in the dictionary.

Named style passes all arguments by name regardless of the function signature, so specific arguments may be listed as well, in any order, as long as the names match up:

    from sqlalchemy.event import listens_for
    from sqlalchemy.pool import Pool

    @listens_for(Pool, "connect", named=True)
    def my_on_connect(dbapi_connection, **kw):
        print("New DBAPI connection:", dbapi_connection)
        print("Connection record:", kw['connection_record'])

Above, the presence of `**kw` tells `listens_for()` that arguments should be passed to the function by name, rather than positionally.

New in version 0.9.0: Added optional `named` argument dispatch to event calling.

## Targets

The `listen()` function is very flexible regarding targets. It generally accepts classes, instances of those classes, and related classes or objects from which the appropriate target can be derived. For example, the above mentioned `"connect"` event accepts `Engine` classes and objects as well as `Pool` classes and objects:

    from sqlalchemy.event import listen
    from sqlalchemy.pool import Pool, QueuePool
    from sqlalchemy import create_engine
    from sqlalchemy.engine import Engine
    import psycopg2

    def connect():
        return psycopg2.connect(username='ed', host='127.0.0.1', dbname='test')

    my_pool = QueuePool(connect)
    my_engine = create_engine('postgresql://ed@localhost/test')

    # associate listener with all instances of Pool
    listen(Pool, 'connect', my_on_connect)

    # associate listener with all instances of Pool
    # via the Engine class
    listen(Engine, 'connect', my_on_connect)

    # associate listener with my_pool
    listen(my_pool, 'connect', my_on_connect)

    # associate listener with my_engine.pool
    listen(my_engine, 'connect', my_on_connect)

## Modifiers

Some listeners allow modifiers to be passed to `listen()`. These modifiers sometimes provide alternate calling signatures for listeners. Such as with ORM events, some event listeners can have a return value which modifies the subsequent handling. By default, no listener ever requires a return value, but by passing `retval=True` this value can be supported:

    def validate_phone(target, value, oldvalue, initiator):
        """Strip non-numeric characters from a phone number"""

        return re.sub(r'\D', '', value)

    # setup listener on UserContact.phone attribute, instructing
    # it to use the return value
    listen(UserContact.phone, 'set', validate_phone, retval=True)

## Event Reference

Both SQLAlchemy Core and SQLAlchemy ORM feature a wide variety of event hooks:

- Core Events - these are described in Core Events and include event hooks specific to connection pool lifecycle, SQL statement execution, transaction lifecycle, and schema creation and teardown.

- ORM Events - these are described in ORM Events, and include event hooks specific to class and attribute instrumentation, object initialization hooks, attribute on-change hooks, session state, flush, and commit hooks, mapper initialization, object/result population, and per-instance persistence hooks.

## API Reference

`sqlalchemy.event.``listen`(target, identifier, fn, *args, **kw)

  Register a listener function for the given target.

  The `listen()` function is part of the primary interface for the SQLAlchemy event system, documented at Events.

  e.g.:

      from sqlalchemy import event
      from sqlalchemy.schema import UniqueConstraint

      def unique_constraint_name(const, table):
          const.name = "uq_%s_%s" % (
              table.name,
              list(const.columns)[0].name
          )
      event.listen(
              UniqueConstraint,
              "after_parent_attach",
              unique_constraint_name)

  A given function can also be invoked for only the first invocation of the event using the `once` argument:

      def on_config():
          do_config()

      event.listen(Mapper, "before_configure", on_config, once=True)

  New in version 0.9.4: Added `once=True` to `event.listen()` and `event.listens_for()`.

  Note

  The `listen()` function cannot be called at the same time that the target event is being run. This has implications for thread safety, and also means an event cannot be added from inside the listener function for itself. The list of events to be run are present inside of a mutable collection that can’t be changed during iteration.

  Event registration and removal is not intended to be a “high velocity” operation; it is a configurational operation. For systems that need to quickly associate and deassociate with events at high scale, use a mutable structure that is handled from inside of a single listener.

  Changed in version 1.0.0: - a `collections.deque()` object is now used as the container for the list of events, which explicitly disallows collection mutation while the collection is being iterated.

  See also

  `listens_for()`

  `remove()`

`sqlalchemy.event.``listens_for`(target, identifier, *args, **kw)

  Decorate a function as a listener for the given target + identifier.

  The `listens_for()` decorator is part of the primary interface for the SQLAlchemy event system, documented at Events.

  e.g.:

      from sqlalchemy import event
      from sqlalchemy.schema import UniqueConstraint

      @event.listens_for(UniqueConstraint, "after_parent_attach")
      def unique_constraint_name(const, table):
          const.name = "uq_%s_%s" % (
              table.name,
              list(const.columns)[0].name
          )

  A given function can also be invoked for only the first invocation of the event using the `once` argument:

      @event.listens_for(Mapper, "before_configure", once=True)
      def on_config():
          do_config()

  New in version 0.9.4: Added `once=True` to `event.listen()` and `event.listens_for()`.

  See also

  `listen()` - general description of event listening

`sqlalchemy.event.``remove`(target, identifier, fn)

  Remove an event listener.

  The arguments here should match exactly those which were sent to `listen()`; all the event registration which proceeded as a result of this call will be reverted by calling `remove()` with the same arguments.

  e.g.:

      # if a function was registered like this...
      @event.listens_for(SomeMappedClass, "before_insert", propagate=True)
      def my_listener_function(*arg):
          pass

      # ... it's removed like this
      event.remove(SomeMappedClass, "before_insert", my_listener_function)

  Above, the listener function associated with `SomeMappedClass` was also propagated to subclasses of `SomeMappedClass`; the `remove()` function will revert all of these operations.

  New in version 0.9.0.

  Note

  The `remove()` function cannot be called at the same time that the target event is being run. This has implications for thread safety, and also means an event cannot be removed from inside the listener function for itself. The list of events to be run are present inside of a mutable collection that can’t be changed during iteration.

  Event registration and removal is not intended to be a “high velocity” operation; it is a configurational operation. For systems that need to quickly associate and deassociate with events at high scale, use a mutable structure that is handled from inside of a single listener.

  Changed in version 1.0.0: - a `collections.deque()` object is now used as the container for the list of events, which explicitly disallows collection mutation while the collection is being iterated.

  See also

  `listen()`

`sqlalchemy.event.``contains`(target, identifier, fn)

  Return True if the given target/ident/fn is set up to listen.

  New in version 0.9.0.