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 :ref:`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:

.. code-block:: python

    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.

    ``using``
        The database alias being used.

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.

    ``using``
        The database alias being used.

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.

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 the
:meth:`~django.db.models.Model.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:

.. versionadded:: 1.2

Sent when a :class:`ManyToManyField` is changed on a model instance.
Strictly speaking, this is not a model signal since it is sent by the
:class:`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:`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:`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 *after* 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:

.. code-block:: python

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

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

If we would do something like this:

.. code-block:: python

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

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

.. code-block:: python

    >>> 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:`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 :djadmin:`syncdb` after it installs an application.

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`.

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/signals/__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 :doc:`running tests </topics/testing>`.

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:

.. versionchanged:: 1.2
   The connection argument was added

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.