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.py startproject``
already contains all the settings required to enable message functionality:

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

* :setting:`MIDDLEWARE_CLASSES` 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_CLASSES`.

* :setting:`TEMPLATE_CONTEXT_PROCESSORS`   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_CLASSES`, and the
``messages`` context processor from :setting:`TEMPLATE_CONTEXT_PROCESSORS`.

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:

.. class:: django.contrib.messages.storage.session.SessionStorage

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

.. class:: django.contrib.messages.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:: django.contrib.messages.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 `MESSAGE_STORAGE`_ to its full import path,
for example::

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

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

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 `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 `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.')

Displaying messages
-------------------

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.

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 `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.

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 :doc:`Django settings </ref/settings>` give you control over message
behavior:

MESSAGE_LEVEL
-------------

Default: ``messages.INFO``

This sets the minimum message that will be saved in the message storage. See
`Message levels`_ above for more details.

.. admonition:: Important

   If you override ``MESSAGE_LEVEL`` in your settings file and rely on any of
   the built-in constants, you must import the constants module directly to
   avoid the potential for circular imports, e.g.::

       from django.contrib.messages import constants as message_constants
       MESSAGE_LEVEL = message_constants.DEBUG

   If desired, you may specify the numeric values for the constants directly
   according to the values in the above :ref:`constants table
   <message-level-constants>`.

MESSAGE_STORAGE
---------------

Default: ``'django.contrib.messages.storage.fallback.FallbackStorage'``

Controls where Django stores message data. Valid values are:

* ``'django.contrib.messages.storage.fallback.FallbackStorage'``
* ``'django.contrib.messages.storage.session.SessionStorage'``
* ``'django.contrib.messages.storage.cookie.CookieStorage'``

See `Storage backends`_ for more details.

MESSAGE_TAGS
------------

Default::

        {messages.DEBUG: 'debug',
        messages.INFO: 'info',
        messages.SUCCESS: 'success',
        messages.WARNING: 'warning',
        messages.ERROR: 'error',}

This sets the mapping of message level to message tag, which is typically
rendered as a CSS class in HTML. If you specify a value, it will extend
the default. This means you only have to specify those values which you need
to override. See `Displaying messages`_ above for more details.

.. admonition:: Important

   If you override ``MESSAGE_TAGS`` in your settings file and rely on any of
   the built-in constants, you must import the ``constants`` module directly to
   avoid the potential for circular imports, e.g.::

       from django.contrib.messages import constants as message_constants
       MESSAGE_TAGS = {message_constants.INFO: ''}

   If desired, you may specify the numeric values for the constants directly
   according to the values in the above :ref:`constants table
   <message-level-constants>`.

SESSION_COOKIE_DOMAIN
---------------------

Default: ``None``

The storage backends that use cookies -- ``CookieStorage`` and
``FallbackStorage`` -- use the value of :setting:`SESSION_COOKIE_DOMAIN` in
setting their cookies. See the :doc:`settings documentation </ref/settings>`
for more information on how this works and why you might need to set it.

.. _Django settings: ../settings/