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The messages framework¶
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 middleware class and corresponding context processor.
The default settings.py
created by django-admin startproject
already contains all the settings required to enable message functionality:
'django.contrib.messages'
is inINSTALLED_APPS
.MIDDLEWARE_CLASSES
contains'django.contrib.sessions.middleware.SessionMiddleware'
and'django.contrib.messages.middleware.MessageMiddleware'
.The default storage backend relies on sessions. That’s why
SessionMiddleware
must be enabled and appear beforeMessageMiddleware
inMIDDLEWARE_CLASSES
.The
'context_processors'
option of theDjangoTemplates
backend defined in yourTEMPLATES
setting contains'django.contrib.messages.context_processors.messages'
.
If you don’t want to use messages, you can remove
'django.contrib.messages'
from your INSTALLED_APPS
, the
MessageMiddleware
line from MIDDLEWARE_CLASSES
, and the
messages
context processor from TEMPLATES
.
Configuring the message engine¶
Storage backends¶
The messages framework can use different backends to store temporary messages.
Django provides three built-in storage classes in
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.
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 usingSessionStorage
for the messages that could not fit in a single cookie. It also requires Django’scontrib.sessions
application.This behavior avoids writing to the session whenever possible. It should provide the best performance in the general case.
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'
-
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 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.
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¶
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.
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
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
JSONResponseMixin instead of a
TemplateResponseMixin
.
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
andlevel_tag
) separated by spaces.extra_tags
: A string containing custom tags for this message, separated by spaces. It’s empty by default.
New in Django 1.7.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 theMESSAGE_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:
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.
Adding messages in class-based views¶
-
class
views.
SuccessMessageMixin
¶ Adds a success message attribute to
FormView
based classes-
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
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.