django/docs/ref/signals.txt

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

A list of all the signals that Django sends.

.. seealso::

    See the documentation on the :doc:`signal dispatcher </topics/signals>` for
    information regarding how to register for and receive signals.

    The :doc:`comment framework </ref/contrib/comments/index>` sends a :doc:`set
    of comment-related signals </ref/contrib/comments/signals>`.

    The :doc:`authentication framework </topics/auth>` sends :ref:`signals when
    a user is logged in / out <topics-auth-signals>`.

Model signals
=============

.. module:: django.db.models.signals
   :synopsis: Signals sent by the model system.

The :mod:`django.db.models.signals` module defines a set of signals sent by the
module system.

.. warning::

    Many of these signals are sent by various model methods like
    :meth:`~django.db.models.Model.__init__` or
    :meth:`~django.db.models.Model.save` that you can overwrite in your own
    code.

    If you override these methods on your model, you must call the parent class'
    methods for this signals to be sent.

    Note also that Django stores signal handlers as weak references by default,
    so if your handler is a local function, it may be garbage collected.  To
    prevent this, pass ``weak=False`` when you call the signal's :meth:`~django.dispatch.Signal.connect`.

pre_init
--------

.. attribute:: django.db.models.signals.pre_init
   :module:

.. ^^^^^^^ this :module: hack keeps Sphinx from prepending the module.

Whenever you instantiate a Django model, this signal is sent at the beginning
of the model's :meth:`~django.db.models.Model.__init__` method.

Arguments sent with this signal:

``sender``
    The model class that just had an instance created.

``args``
    A list of positional arguments passed to
    :meth:`~django.db.models.Model.__init__`:

``kwargs``
    A dictionary of keyword arguments passed to
    :meth:`~django.db.models.Model.__init__`:.

For example, the :doc:`tutorial </intro/tutorial01>` has this line::

    p = Poll(question="What's up?", pub_date=datetime.now())

The arguments sent to a :data:`pre_init` handler would be:

==========  ===============================================================
Argument    Value
==========  ===============================================================
``sender``  ``Poll`` (the class itself)

``args``    ``[]`` (an empty list because there were no positional
            arguments passed to ``__init__``.)

``kwargs``  ``{'question': "What's up?", 'pub_date': datetime.now()}``
==========  ===============================================================

post_init
---------

.. data:: django.db.models.signals.post_init
   :module:

Like pre_init, but this one is sent when the :meth:`~django.db.models.Model.__init__`: method finishes.

Arguments sent with this signal:

``sender``
    As above: the model class that just had an instance created.

``instance``
    The actual instance of the model that's just been created.

pre_save
--------

.. data:: django.db.models.signals.pre_save
   :module:

This is sent at the beginning of a model's :meth:`~django.db.models.Model.save`
method.

Arguments sent with this signal:

``sender``
    The model class.

``instance``
    The actual instance being saved.

``raw``
    A boolean; ``True`` if the model is saved exactly as presented
    (i.e. when loading a fixture). One should not query/modify other
    records in the database as the database might not be in a
    consistent state yet.

``using``
    The database alias being used.

.. versionadded:: 1.5

``update_fields``
    The set of fields to update explicitly specified in the ``save()`` method.
    ``None`` if this argument was not used in the ``save()`` call.

post_save
---------

.. data:: django.db.models.signals.post_save
   :module:

Like :data:`pre_save`, but sent at the end of the
:meth:`~django.db.models.Model.save` method.

Arguments sent with this signal:

``sender``
    The model class.

``instance``
    The actual instance being saved.

``created``
    A boolean; ``True`` if a new record was created.

``raw``
    A boolean; ``True`` if the model is saved exactly as presented
    (i.e. when loading a fixture). One should not query/modify other
    records in the database as the database might not be in a
    consistent state yet.

``using``
    The database alias being used.

.. versionadded:: 1.5

``update_fields``
    The set of fields to update explicitly specified in the ``save()`` method.
    ``None`` if this argument was not used in the ``save()`` call.

pre_delete
----------

.. data:: django.db.models.signals.pre_delete
   :module:

Sent at the beginning of a model's :meth:`~django.db.models.Model.delete`
method and a queryset's :meth:`~django.db.models.query.QuerySet.delete` method.

Arguments sent with this signal:

``sender``
    The model class.

``instance``
    The actual instance being deleted.

``using``
    The database alias being used.

post_delete
-----------

.. data:: django.db.models.signals.post_delete
   :module:

Like :data:`pre_delete`, but sent at the end of a model's
:meth:`~django.db.models.Model.delete` method and a queryset's
:meth:`~django.db.models.query.QuerySet.delete` method.

Arguments sent with this signal:

``sender``
    The model class.

``instance``
    The actual instance being deleted.

    Note that the object will no longer be in the database, so be very
    careful what you do with this instance.

``using``
    The database alias being used.

m2m_changed
-----------

.. data:: django.db.models.signals.m2m_changed
   :module:

Sent when a :class:`~django.db.models.ManyToManyField` is changed on a model
instance. Strictly speaking, this is not a model signal since it is sent by the
:class:`~django.db.models.ManyToManyField`, but since it complements the
:data:`pre_save`/:data:`post_save` and :data:`pre_delete`/:data:`post_delete`
when it comes to tracking changes to models, it is included here.

Arguments sent with this signal:

``sender``
    The intermediate model class describing the
    :class:`~django.db.models.ManyToManyField`. This class is automatically
    created when a many-to-many field is defined; you can access it using the
    ``through`` attribute on the many-to-many field.

``instance``
    The instance whose many-to-many relation is updated. This can be an
    instance of the ``sender``, or of the class the
    :class:`~django.db.models.ManyToManyField` is related to.

``action``
    A string indicating the type of update that is done on the relation.
    This can be one of the following:

    ``"pre_add"``
        Sent *before* one or more objects are added to the relation.
    ``"post_add"``
        Sent *after* one or more objects are added to the relation.
    ``"pre_remove"``
        Sent *before* one or more objects are removed from the relation.
    ``"post_remove"``
        Sent *after* one or more objects are removed from the relation.
    ``"pre_clear"``
        Sent *before* the relation is cleared.
    ``"post_clear"``
        Sent *after* the relation is cleared.

``reverse``
    Indicates which side of the relation is updated (i.e., if it is the
    forward or reverse relation that is being modified).

``model``
    The class of the objects that are added to, removed from or cleared
    from the relation.

``pk_set``
    For the ``pre_add``, ``post_add``, ``pre_remove`` and ``post_remove``
    actions, this is a list of primary key values that have been added to
    or removed from the relation.

    For the ``pre_clear`` and ``post_clear`` actions, this is ``None``.

``using``
    The database alias being used.

For example, if a ``Pizza`` can have multiple ``Topping`` objects, modeled
like this::

    class Topping(models.Model):
        # ...
        pass

    class Pizza(models.Model):
        # ...
        toppings = models.ManyToManyField(Topping)

If we connected a handler like this::

    def toppings_changed(sender, **kwargs):
        # Do something
        pass

    m2m_changed.connect(toppings_changed, sender=Pizza.toppings.through)

and then did something like this::

    >>> p = Pizza.object.create(...)
    >>> t = Topping.objects.create(...)
    >>> p.toppings.add(t)

the arguments sent to a :data:`m2m_changed` handler (``topppings_changed`` in
the example above) would be:

==============  ============================================================
Argument        Value
==============  ============================================================
``sender``      ``Pizza.toppings.through`` (the intermediate m2m class)

``instance``    ``p`` (the ``Pizza`` instance being modified)

``action``      ``"pre_add"`` (followed by a separate signal with ``"post_add"``)

``reverse``     ``False`` (``Pizza`` contains the
                :class:`~django.db.models.ManyToManyField`, so this call
                modifies the forward relation)

``model``       ``Topping`` (the class of the objects added to the
                ``Pizza``)

``pk_set``      ``[t.id]`` (since only ``Topping t`` was added to the relation)

``using``       ``"default"`` (since the default router sends writes here)
==============  ============================================================

And if we would then do something like this::

    >>> t.pizza_set.remove(p)

the arguments sent to a :data:`m2m_changed` handler would be:

==============  ============================================================
Argument        Value
==============  ============================================================
``sender``      ``Pizza.toppings.through`` (the intermediate m2m class)

``instance``    ``t`` (the ``Topping`` instance being modified)

``action``      ``"pre_remove"`` (followed by a separate signal with ``"post_remove"``)

``reverse``     ``True`` (``Pizza`` contains the
                :class:`~django.db.models.ManyToManyField`, so this call
                modifies the reverse relation)

``model``       ``Pizza`` (the class of the objects removed from the
                ``Topping``)

``pk_set``      ``[p.id]`` (since only ``Pizza p`` was removed from the
                relation)

``using``       ``"default"`` (since the default router sends writes here)
==============  ============================================================

class_prepared
--------------

.. data:: django.db.models.signals.class_prepared
   :module:

Sent whenever a model class has been "prepared" -- that is, once model has
been defined and registered with Django's model system. Django uses this
signal internally; it's not generally used in third-party applications.

Arguments that are sent with this signal:

``sender``
    The model class which was just prepared.

Management signals
==================

Signals sent by :doc:`django-admin </ref/django-admin>`.

post_syncdb
-----------

.. data:: django.db.models.signals.post_syncdb
   :module:

Sent by the :djadmin:`syncdb` command after it installs an application, and the
:djadmin:`flush` command.

Any handlers that listen to this signal need to be written in a particular
place: a ``management`` module in one of your :setting:`INSTALLED_APPS`. If
handlers are registered anywhere else they may not be loaded by
:djadmin:`syncdb`. It is important that handlers of this signal perform
idempotent changes (e.g. no database alterations) as this may cause the
:djadmin:`flush` management command to fail if it also ran during the
:djadmin:`syncdb` command.

Arguments sent with this signal:

``sender``
    The ``models`` module that was just installed. That is, if
    :djadmin:`syncdb` just installed an app called ``"foo.bar.myapp"``,
    ``sender`` will be the ``foo.bar.myapp.models`` module.

``app``
    Same as ``sender``.

``created_models``
    A list of the model classes from any app which :djadmin:`syncdb` has
    created so far.

``verbosity``
    Indicates how much information manage.py is printing on screen. See
    the :djadminopt:`--verbosity` flag for details.

    Functions which listen for :data:`post_syncdb` should adjust what they
    output to the screen based on the value of this argument.

``interactive``
    If ``interactive`` is ``True``, it's safe to prompt the user to input
    things on the command line. If ``interactive`` is ``False``, functions
    which listen for this signal should not try to prompt for anything.

    For example, the :mod:`django.contrib.auth` app only prompts to create a
    superuser when ``interactive`` is ``True``.

For example, ``yourapp/management/__init__.py`` could be written like::

    from django.db.models.signals import post_syncdb
    import yourapp.models

    def my_callback(sender, **kwargs):
        # Your specific logic here
        pass

    post_syncdb.connect(my_callback, sender=yourapp.models)

Request/response signals
========================

.. module:: django.core.signals
   :synopsis: Core signals sent by the request/response system.

Signals sent by the core framework when processing a request.

request_started
---------------

.. data:: django.core.signals.request_started
   :module:

Sent when Django begins processing an HTTP request.

Arguments sent with this signal:

``sender``
    The handler class -- e.g.
    :class:`django.core.handlers.wsgi.WsgiHandler` -- that handled
    the request.

request_finished
----------------

.. data:: django.core.signals.request_finished
   :module:

Sent when Django finishes processing an HTTP request.

Arguments sent with this signal:

``sender``
    The handler class, as above.

got_request_exception
---------------------

.. data:: django.core.signals.got_request_exception
   :module:

This signal is sent whenever Django encounters an exception while processing an incoming HTTP request.

Arguments sent with this signal:

``sender``
    The handler class, as above.

``request``
    The :class:`~django.http.HttpRequest` object.

Test signals
============

.. module:: django.test.signals
   :synopsis: Signals sent during testing.

Signals only sent when :ref:`running tests <running-tests>`.

setting_changed
---------------

.. versionadded:: 1.4

.. data:: django.test.signals.setting_changed
   :module:

This signal is sent when the value of a setting is changed through the
:meth:`django.test.TestCase.setting` context manager or the
:func:`django.test.utils.override_settings` decorator/context manager.

It's actually sent twice: when the new value is applied ("setup") and when the
original value is restored ("teardown").

Arguments sent with this signal:

``sender``
    The settings handler.

``setting``
    The name of the setting.

``value``
    The value of the setting after the change. For settings that initially
    don't exist, in the "teardown" phase, ``value`` is ``None``.

template_rendered
-----------------

.. data:: django.test.signals.template_rendered
   :module:

Sent when the test system renders a template. This signal is not emitted during
normal operation of a Django server -- it is only available during testing.

Arguments sent with this signal:

``sender``
    The :class:`~django.template.Template` object which was rendered.

``template``
    Same as sender

``context``
    The :class:`~django.template.Context` with which the template was
    rendered.

Database Wrappers
=================

.. module:: django.db.backends
   :synopsis: Core signals sent by the database wrapper.

Signals sent by the database wrapper when a database connection is
initiated.

connection_created
------------------

.. data:: django.db.backends.signals.connection_created
   :module:

Sent when the database wrapper makes the initial connection to the
database.  This is particularly useful if you'd like to send any post
connection commands to the SQL backend.

Arguments sent with this signal:

``sender``
    The database wrapper class -- i.e.
    :class:`django.db.backends.postgresql_psycopg2.DatabaseWrapper` or
    :class:`django.db.backends.mysql.DatabaseWrapper`, etc.

``connection``
    The database connection that was opened. This can be used in a
    multiple-database configuration to differentiate connection signals
    from different databases.