.. _topics-signals:

=======
Signals
=======

.. module:: django.dispatch
   :synopsis: Signal dispatch

Django includes a "signal dispatcher" which helps allow decoupled applications
get notified when actions occur elsewhere in the framework. In a nutshell,
signals allow certain *senders* to notify a set of *receivers* that some action
has taken place. They're especially useful when many pieces of code may be
interested in the same events.

Django provides a :ref:`set of built-in signals <ref-signals>` that let user
code get notified by Django itself of certain actions. These include some useful
notifications:

    * :data:`django.db.models.signals.pre_save` &
      :data:`django.db.models.signals.post_save`

      Sent before or after a model's :meth:`~django.db.models.Model.save` method
      is called.

    * :data:`django.db.models.signals.pre_delete` &
      :data:`django.db.models.signals.post_delete`

      Sent before or after a model's :meth:`~django.db.models.Model.delete`
      method is called.

    * :data:`django.db.models.signals.m2m_changed`

      Sent when a :class:`ManyToManyField` on a model is changed.

    * :data:`django.core.signals.request_started` &
      :data:`django.core.signals.request_finished`

      Sent when Django starts or finishes an HTTP request.

See the :ref:`built-in signal documentation <ref-signals>` for a complete list,
and a complete explanation of each signal.

You can also `define and send your own custom signals`_; see below.

.. _define and send your own custom signals: `defining and sending signals`_

Listening to signals
====================

To receive a signal, you need to register a *receiver* function that gets called
when the signal is sent. Let's see how this works by registering a signal that
gets called after each HTTP request is finished. We'll be connecting to the
:data:`~django.core.signals.request_finished` signal.

Receiver functions
------------------

First, we need to define a receiver function. A receiver can be any Python function or method:

.. code-block:: python

    def my_callback(sender, **kwargs):
        print "Request finished!"

Notice that the function takes a ``sender`` argument, along with wildcard
keyword arguments (``**kwargs``); all signal handlers must take these arguments.

We'll look at senders `a bit later`_, but right now look at the ``**kwargs``
argument. All signals send keyword arguments, and may change those keyword
arguments at any time. In the case of
:data:`~django.core.signals.request_finished`, it's documented as sending no
arguments, which means we might be tempted to write our signal handling as
``my_callback(sender)``.

.. _a bit later: `connecting to signals sent by specific senders`_

This would be wrong -- in fact, Django will throw an error if you do so. That's
because at any point arguments could get added to the signal and your receiver
must be able to handle those new arguments.

Connecting receiver functions
-----------------------------

Next, we'll need to connect our receiver to the signal:

.. code-block:: python

    from django.core.signals import request_finished

    request_finished.connect(my_callback)

Now, our ``my_callback`` function will be called each time a request finishes.

.. admonition:: Where should this code live?

    You can put signal handling and registration code anywhere you like.
    However, you'll need to make sure that the module it's in gets imported
    early on so that the signal handling gets registered before any signals need
    to be sent. This makes your app's ``models.py`` a good place to put
    registration of signal handlers.

Connecting to signals sent by specific senders
----------------------------------------------

Some signals get sent many times, but you'll only be interested in receiving a
certain subset of those signals. For example, consider the
:data:`django.db.models.signals.pre_save` signal sent before a model gets saved.
Most of the time, you don't need to know when *any* model gets saved -- just
when one *specific* model is saved.

In these cases, you can register to receive signals sent only by particular
senders. In the case of :data:`django.db.models.signals.pre_save`, the sender
will be the model class being saved, so you can indicate that you only want
signals sent by some model:

.. code-block:: python

    from django.db.models.signals import pre_save
    from myapp.models import MyModel

    def my_handler(sender, **kwargs):
        ...

    pre_save.connect(my_handler, sender=MyModel)

The ``my_handler`` function will only be called when an instance of ``MyModel``
is saved.

Different signals use different objects as their senders; you'll need to consult
the :ref:`built-in signal documentation <ref-signals>` for details of each
particular signal.

Defining and sending signals
============================

Your applications can take advantage of the signal infrastructure and provide its own signals.

Defining signals
----------------

.. class:: Signal([providing_args=list])

All signals are :class:`django.dispatch.Signal` instances. The
``providing_args`` is a list of the names of arguments the signal will provide
to listeners.

For example:

.. code-block:: python

    import django.dispatch

    pizza_done = django.dispatch.Signal(providing_args=["toppings", "size"])

This declares a ``pizza_done`` signal that will provide receivers with
``toppings`` and ``size`` arguments.

Remember that you're allowed to change this list of arguments at any time, so getting the API right on the first try isn't necessary.

Sending signals
---------------

.. method:: Signal.send(sender, **kwargs)

To send a signal, call :meth:`Signal.send`. You must provide the ``sender`` argument, and may provide as many other keyword arguments as you like.

For example, here's how sending our ``pizza_done`` signal might look:

.. code-block:: python

    class PizzaStore(object):
        ...

        def send_pizza(self, toppings, size):
            pizza_done.send(sender=self, toppings=toppings, size=size)
            ...


