django/docs/topics/signals.txt

=======
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 :doc:`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 or queryset's :meth:`~django.db.models.query.QuerySet.delete`
  method is called.

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

  Sent when a :class:`~django.db.models.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 :doc:`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, register a *receiver* function using the
:meth:`Signal.connect` method. The receiver function is called when the signal
is sent.

.. method:: Signal.connect(receiver, sender=None, weak=True, dispatch_uid=None)

    :param receiver: The callback function which will be connected to this
        signal. See :ref:`receiver-functions` for more information.

    :param sender: Specifies a particular sender to receive signals from. See
        :ref:`connecting-to-specific-signals` for more information.

    :param weak: Django stores signal handlers as weak references by
        default. Thus, if your receiver is a local function, it may be
        garbage collected. To prevent this, pass ``weak=False`` when you call
        the signal's ``connect()`` method.

    :param dispatch_uid: A unique identifier for a signal receiver in cases
        where duplicate signals may be sent. See
        :ref:`preventing-duplicate-signals` for more information.

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:

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

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

    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:

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

There are two ways you can connect a receiver to a signal. You can take the
manual connect route::

    from django.core.signals import request_finished

    request_finished.connect(my_callback)

Alternatively, you can use a :func:`receiver` decorator:

.. function:: receiver(signal)

    :param signal: A signal or a list of signals to connect a function to.

Here's how you connect with the decorator::

    from django.core.signals import request_finished
    from django.dispatch import receiver

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

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

.. admonition:: Where should this code live?

    Strictly speaking, signal handling and registration code can live anywhere
    you like, although it's recommended to avoid the application's root module
    and its ``models`` module to minimize side-effects of importing code.

    In practice, signal handlers are usually defined in a ``signals``
    submodule of the application they relate to. Signal receivers are
    connected in the :meth:`~django.apps.AppConfig.ready` method of your
    application configuration class. If you're using the :func:`receiver`
    decorator, import the ``signals`` submodule inside
    :meth:`~django.apps.AppConfig.ready`.

.. note::

    The :meth:`~django.apps.AppConfig.ready` method may be executed more than
    once during testing, so you may want to :ref:`guard your signals from
    duplication <preventing-duplicate-signals>`, especially if you're planning
    to send them within tests.

.. _connecting-to-specific-signals:

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::

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


    @receiver(pre_save, sender=MyModel)
    def my_handler(sender, **kwargs):
        ...

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 :doc:`built-in signal documentation </ref/signals>` for details of each
particular signal.

.. _preventing-duplicate-signals:

Preventing duplicate signals
----------------------------

In some circumstances, the code connecting receivers to signals may run
multiple times. This can cause your receiver function to be registered more
than once, and thus called multiple times for a single signal event.

If this behavior is problematic (such as when using signals to
send an email whenever a model is saved), pass a unique identifier as
the ``dispatch_uid`` argument to identify your receiver function. This
identifier will usually be a string, although any hashable object will
suffice. The end result is that your receiver function will only be
bound to the signal once for each unique ``dispatch_uid`` value::

    from django.core.signals import request_finished

    request_finished.connect(my_callback, dispatch_uid="my_unique_identifier")

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

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

.. admonition:: When to use custom signals

    Signals are implicit function calls which make debugging harder. If the
    sender and receiver of your custom signal are both within your project,
    you're better off using an explicit function call.

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. This is purely documentational, however, as there is nothing that
checks that the signal actually provides these arguments to its listeners.

For example::

    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
---------------

There are two ways to send signals in Django.

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

To send a signal, call either :meth:`Signal.send` (all built-in signals use
this) or :meth:`Signal.send_robust`. You must provide the ``sender`` argument
(which is a class most of the time) and may provide as many other keyword
arguments as you like.

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

    class PizzaStore:
        ...

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

Both ``send()`` and ``send_robust()`` return a list of tuple pairs
``[(receiver, response), ... ]``, representing the list of called receiver
functions and their response values.

``send()`` differs from ``send_robust()`` in how exceptions raised by receiver
functions are handled. ``send()`` does *not* catch any exceptions raised by
receivers; it simply allows errors to propagate. Thus not all receivers may
be notified of a signal in the face of an error.

``send_robust()`` catches all errors derived from Python's ``Exception`` class,
and ensures all receivers are notified of the signal. If an error occurs, the
error instance is returned in the tuple pair for the receiver that raised the error.

The tracebacks are present on the ``__traceback__`` attribute of the errors
returned when calling ``send_robust()``.

Disconnecting signals
=====================

.. method:: Signal.disconnect(receiver=None, sender=None, dispatch_uid=None)

To disconnect a receiver from a signal, call :meth:`Signal.disconnect`. The
arguments are as described in :meth:`.Signal.connect`. The method returns
``True`` if a receiver was disconnected and ``False`` if not.

The ``receiver`` argument indicates the registered receiver to disconnect. It
may be ``None`` if ``dispatch_uid`` is used to identify the receiver.