.. _event_toplevel:

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 :func:`.listen` function,
or alternatively the :func:`.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 :meth:`_events.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 :func:`.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)

.. _event_named_argument_styles:

Named Argument Styles
---------------------

There are some varieties of argument styles which can be accepted by listener
functions.  Taking the example of :meth:`_events.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
:func:`.listen` or :func:`.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 :func:`.listens_for` that
arguments should be passed to the function by name, rather than positionally.

Targets
-------

The :func:`.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
:class:`_engine.Engine` classes and objects as well as :class:`_pool.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(user="ed", host="127.0.0.1", dbname="test")


    my_pool = QueuePool(connect)
    my_engine = create_engine("postgresql+psycopg2://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)

.. _event_modifiers:

Modifiers
---------

Some listeners allow modifiers to be passed to :func:`.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)

Events and Multiprocessing
--------------------------

SQLAlchemy's event hooks are implemented with Python functions and objects,
so events propagate via Python function calls.
Python multiprocessing follows the
same way we think about OS multiprocessing,
such as a parent process forking a child process,
thus we can describe the SQLAlchemy event system's behavior using the same model.

Event hooks registered in a parent process
will be present in new child processes
that are forked from that parent after the hooks have been registered,
since the child process starts with
a copy of all existing Python structures from the parent when spawned.
Child processes that already exist before the hooks are registered
will not receive those new event hooks,
as changes made to Python structures in a parent process
do not propagate to child processes.

For the events themselves, these are Python function calls,
which do not have any ability to propagate between processes.
SQLAlchemy's event system does not implement any inter-process communication.
It is possible to implement event hooks
that use Python inter-process messaging within them,
however this would need to be implemented by the user.

Event Reference
---------------

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

* **Core Events** - these are described in
  :ref:`core_event_toplevel` 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
  :ref:`orm_event_toplevel`, 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
-------------

.. autofunction:: sqlalchemy.event.listen

.. autofunction:: sqlalchemy.event.listens_for

.. autofunction:: sqlalchemy.event.remove

.. autofunction:: sqlalchemy.event.contains
