django1/docs/ref/contrib/messages.txt

======================
The messages framework
======================

.. module:: django.contrib.messages
   :synopsis: Provides cookie- and session-based temporary message storage.

Quite commonly in web applications, you need to display a one-time
notification message (also known as "flash message") to the user after
processing a form or some other types of user input.

For this, Django provides full support for cookie- and session-based
messaging, for both anonymous and authenticated users. The messages framework
allows you to temporarily store messages in one request and retrieve them for
display in a subsequent request (usually the next one). Every message is
tagged with a specific ``level`` that determines its priority (e.g., ``info``,
``warning``, or ``error``).

Enabling messages
=================

Messages are implemented through a :doc:`middleware </ref/middleware>`
class and corresponding :doc:`context processor </ref/templates/api>`.

The default ``settings.py`` created by ``django-admin startproject``
already contains all the settings required to enable message functionality:

* ``'django.contrib.messages'`` is in :setting:`INSTALLED_APPS`.

* :setting:`MIDDLEWARE` contains
  ``'django.contrib.sessions.middleware.SessionMiddleware'`` and
  ``'django.contrib.messages.middleware.MessageMiddleware'``.

  The default :ref:`storage backend <message-storage-backends>` relies on
  :doc:`sessions </topics/http/sessions>`. That's why ``SessionMiddleware``
  must be enabled and appear before ``MessageMiddleware`` in
  :setting:`MIDDLEWARE`.

* The ``'context_processors'`` option of the ``DjangoTemplates`` backend
  defined in your :setting:`TEMPLATES` setting contains
  ``'django.contrib.messages.context_processors.messages'``.

If you don't want to use messages, you can remove
``'django.contrib.messages'`` from your :setting:`INSTALLED_APPS`, the
``MessageMiddleware`` line from :setting:`MIDDLEWARE`, and the ``messages``
context processor from :setting:`TEMPLATES`.

Configuring the message engine
==============================

.. _message-storage-backends:

Storage backends
----------------

The messages framework can use different backends to store temporary messages.

Django provides three built-in storage classes in
:mod:`django.contrib.messages`:

.. class:: storage.session.SessionStorage

    This class stores all messages inside of the request's session. Therefore
    it requires Django's ``contrib.sessions`` application.

.. class:: storage.cookie.CookieStorage

    This class stores the message data in a cookie (signed with a secret hash
    to prevent manipulation) to persist notifications across requests. Old
    messages are dropped if the cookie data size would exceed 2048 bytes.

.. class:: storage.fallback.FallbackStorage

    This class first uses ``CookieStorage``, and falls back to using
    ``SessionStorage`` for the messages that could not fit in a single cookie.
    It also requires Django's ``contrib.sessions`` application.

    This behavior avoids writing to the session whenever possible. It should
    provide the best performance in the general case.

:class:`~django.contrib.messages.storage.fallback.FallbackStorage` is the
default storage class. If it isn't suitable to your needs, you can select
another storage class by setting :setting:`MESSAGE_STORAGE` to its full import
path, for example::

    MESSAGE_STORAGE = 'django.contrib.messages.storage.cookie.CookieStorage'

.. class:: storage.base.BaseStorage

To write your own storage class, subclass the ``BaseStorage`` class in
``django.contrib.messages.storage.base`` and implement the ``_get`` and
``_store`` methods.

.. _message-level:

Message levels
--------------

The messages framework is based on a configurable level architecture similar
to that of the Python logging module. Message levels allow you to group
messages by type so they can be filtered or displayed differently in views and
templates.

The built-in levels, which can be imported from ``django.contrib.messages``
directly, are:

=========== ========
Constant    Purpose
=========== ========
``DEBUG``   Development-related messages that will be ignored (or removed) in a production deployment
``INFO``    Informational messages for the user
``SUCCESS`` An action was successful, e.g. "Your profile was updated successfully"
``WARNING`` A failure did not occur but may be imminent
``ERROR``   An action was **not** successful or some other failure occurred
=========== ========

The :setting:`MESSAGE_LEVEL` setting can be used to change the minimum recorded level
(or it can be `changed per request`_). Attempts to add messages of a level less
than this will be ignored.

.. _`changed per request`: `Changing the minimum recorded level per-request`_

Message tags
------------

Message tags are a string representation of the message level plus any
extra tags that were added directly in the view (see
`Adding extra message tags`_ below for more details). Tags are stored in a
string and are separated by spaces. Typically, message tags
are used as CSS classes to customize message style based on message type. By
default, each level has a single tag that's a lowercase version of its own
constant:

==============  ===========
Level Constant  Tag
==============  ===========
``DEBUG``       ``debug``
``INFO``        ``info``
``SUCCESS``     ``success``
``WARNING``     ``warning``
``ERROR``       ``error``
==============  ===========

To change the default tags for a message level (either built-in or custom),
set the :setting:`MESSAGE_TAGS` setting to a dictionary containing the levels
you wish to change. As this extends the default tags, you only need to provide
tags for the levels you wish to override::

    from django.contrib.messages import constants as messages
    MESSAGE_TAGS = {
        messages.INFO: '',
        50: 'critical',
    }

Using messages in views and templates
=====================================

.. function:: add_message(request, level, message, extra_tags='', fail_silently=False)

Adding a message
----------------

To add a message, call::

    from django.contrib import messages
    messages.add_message(request, messages.INFO, 'Hello world.')

Some shortcut methods provide a standard way to add messages with commonly
used tags (which are usually represented as HTML classes for the message)::

    messages.debug(request, '%s SQL statements were executed.' % count)
    messages.info(request, 'Three credits remain in your account.')
    messages.success(request, 'Profile details updated.')
    messages.warning(request, 'Your account expires in three days.')
    messages.error(request, 'Document deleted.')

.. _message-displaying:

Displaying messages
-------------------
.. function:: get_messages(request)

**In your template**, use something like::

    {% if messages %}
    <ul class="messages">
        {% for message in messages %}
        <li{% if message.tags %} class="{{ message.tags }}"{% endif %}>{{ message }}</li>
        {% endfor %}
    </ul>
    {% endif %}

If you're using the context processor, your template should be rendered with a
``RequestContext``. Otherwise, ensure ``messages`` is available to
the template context.

Even if you know there is only just one message, you should still iterate over
the ``messages`` sequence, because otherwise the message storage will not be cleared
for the next request.

The context processor also provides a ``DEFAULT_MESSAGE_LEVELS`` variable which
is a mapping of the message level names to their numeric value::

    {% if messages %}
    <ul class="messages">
        {% for message in messages %}
        <li{% if message.tags %} class="{{ message.tags }}"{% endif %}>
            {% if message.level == DEFAULT_MESSAGE_LEVELS.ERROR %}Important: {% endif %}
            {{ message }}
        </li>
        {% endfor %}
    </ul>
    {% endif %}

**Outside of templates**, you can use
:func:`~django.contrib.messages.get_messages`::

    from django.contrib.messages import get_messages

    storage = get_messages(request)
    for message in storage:
        do_something_with_the_message(message)

For instance, you can fetch all the messages to return them in a
:ref:`JSONResponseMixin <jsonresponsemixin-example>` instead of a
:class:`~django.views.generic.base.TemplateResponseMixin`.

:func:`~django.contrib.messages.get_messages` will return an
instance of the configured storage backend.


The ``Message`` class
---------------------

.. class:: storage.base.Message

    When you loop over the list of messages in a template, what you get are
    instances of the ``Message`` class. It's quite a simple object, with only a
    few attributes:

    * ``message``: The actual text of the message.

    * ``level``: An integer describing the type of the message (see the
      `message levels`_ section above).

    * ``tags``: A string combining all the message's tags (``extra_tags`` and
      ``level_tag``) separated by spaces.

    * ``extra_tags``: A string containing custom tags for this message,
      separated by spaces. It's empty by default.

    * ``level_tag``: The string representation of the level. By default, it's
      the lowercase version of the name of the associated constant, but this
      can be changed if you need by using the :setting:`MESSAGE_TAGS` setting.

Creating custom message levels
------------------------------

Messages levels are nothing more than integers, so you can define your own
level constants and use them to create more customized user feedback, e.g.::

    CRITICAL = 50

    def my_view(request):
        messages.add_message(request, CRITICAL, 'A serious error occurred.')

When creating custom message levels you should be careful to avoid overloading
existing levels. The values for the built-in levels are:

.. _message-level-constants:

==============  =====
Level Constant  Value
==============  =====
``DEBUG``       10
``INFO``        20
``SUCCESS``     25
``WARNING``     30
``ERROR``       40
==============  =====

If you need to identify the custom levels in your HTML or CSS, you need to
provide a mapping via the :setting:`MESSAGE_TAGS` setting.

.. note::
   If you are creating a reusable application, it is recommended to use
   only the built-in `message levels`_ and not rely on any custom levels.

Changing the minimum recorded level per-request
-----------------------------------------------

The minimum recorded level can be set per request via the ``set_level``
method::

    from django.contrib import messages

    # Change the messages level to ensure the debug message is added.
    messages.set_level(request, messages.DEBUG)
    messages.debug(request, 'Test message...')

    # In another request, record only messages with a level of WARNING and higher
    messages.set_level(request, messages.WARNING)
    messages.success(request, 'Your profile was updated.') # ignored
    messages.warning(request, 'Your account is about to expire.') # recorded

    # Set the messages level back to default.
    messages.set_level(request, None)

Similarly, the current effective level can be retrieved with ``get_level``::

    from django.contrib import messages
    current_level = messages.get_level(request)

For more information on how the minimum recorded level functions, see
`Message levels`_ above.

Adding extra message tags
-------------------------

For more direct control over message tags, you can optionally provide a string
containing extra tags to any of the add methods::

    messages.add_message(request, messages.INFO, 'Over 9000!', extra_tags='dragonball')
    messages.error(request, 'Email box full', extra_tags='email')

Extra tags are added before the default tag for that level and are space
separated.

Failing silently when the message framework is disabled
-------------------------------------------------------

If you're writing a reusable app (or other piece of code) and want to include
messaging functionality, but don't want to require your users to enable it
if they don't want to, you may pass an additional keyword argument
``fail_silently=True`` to any of the ``add_message`` family of methods. For
example::

    messages.add_message(
        request, messages.SUCCESS, 'Profile details updated.',
        fail_silently=True,
    )
    messages.info(request, 'Hello world.', fail_silently=True)

.. note::
   Setting ``fail_silently=True`` only hides the ``MessageFailure`` that would
   otherwise occur when the messages framework disabled and one attempts to
   use one of the ``add_message`` family of methods. It does not hide failures
   that may occur for other reasons.

Adding messages in class-based views
------------------------------------

.. class:: views.SuccessMessageMixin

    Adds a success message attribute to
    :class:`~django.views.generic.edit.FormView` based classes

    .. method:: get_success_message(cleaned_data)

        ``cleaned_data`` is the cleaned data from the form which is used for
        string formatting

**Example views.py**::

    from django.contrib.messages.views import SuccessMessageMixin
    from django.views.generic.edit import CreateView
    from myapp.models import Author

    class AuthorCreate(SuccessMessageMixin, CreateView):
        model = Author
        success_url = '/success/'
        success_message = "%(name)s was created successfully"

The cleaned data from the ``form`` is available for string interpolation using
the ``%(field_name)s`` syntax. For ModelForms, if you need access to fields
from the saved ``object`` override the
:meth:`~django.contrib.messages.views.SuccessMessageMixin.get_success_message`
method.

**Example views.py for ModelForms**::

    from django.contrib.messages.views import SuccessMessageMixin
    from django.views.generic.edit import CreateView
    from myapp.models import ComplicatedModel

    class ComplicatedCreate(SuccessMessageMixin, CreateView):
        model = ComplicatedModel
        success_url = '/success/'
        success_message = "%(calculated_field)s was created successfully"

        def get_success_message(self, cleaned_data):
            return self.success_message % dict(
                cleaned_data,
                calculated_field=self.object.calculated_field,
            )

Expiration of messages
======================

The messages are marked to be cleared when the storage instance is iterated
(and cleared when the response is processed).

To avoid the messages being cleared, you can set the messages storage to
``False`` after iterating::

    storage = messages.get_messages(request)
    for message in storage:
        do_something_with(message)
    storage.used = False

Behavior of parallel requests
=============================

Due to the way cookies (and hence sessions) work, **the behavior of any
backends that make use of cookies or sessions is undefined when the same
client makes multiple requests that set or get messages in parallel**. For
example, if a client initiates a request that creates a message in one window
(or tab) and then another that fetches any uniterated messages in another
window, before the first window redirects, the message may appear in the
second window instead of the first window where it may be expected.

In short, when multiple simultaneous requests from the same client are
involved, messages are not guaranteed to be delivered to the same window that
created them nor, in some cases, at all. Note that this is typically not a
problem in most applications and will become a non-issue in HTML5, where each
window/tab will have its own browsing context.

Settings
========

A few :ref:`settings<settings-messages>` give you control over message
behavior:

* :setting:`MESSAGE_LEVEL`
* :setting:`MESSAGE_STORAGE`
* :setting:`MESSAGE_TAGS`

For backends that use cookies, the settings for the cookie are taken from
the session cookie settings:

* :setting:`SESSION_COOKIE_DOMAIN`
* :setting:`SESSION_COOKIE_SECURE`
* :setting:`SESSION_COOKIE_HTTPONLY`