==========
Middleware
==========

Middleware is a framework of hooks into Django's request/response processing.
It's a light, low-level "plugin" system for globally altering Django's input
or output.

Each middleware component is responsible for doing some specific function. For
example, Django includes a middleware component,
:class:`~django.contrib.auth.middleware.AuthenticationMiddleware`, that
associates users with requests using sessions.

This document explains how middleware works, how you activate middleware, and
how to write your own middleware. Django ships with some built-in middleware
you can use right out of the box. They're documented in the :doc:`built-in
middleware reference </ref/middleware>`.

Activating middleware
=====================

To activate a middleware component, add it to the
:setting:`MIDDLEWARE_CLASSES` list in your Django settings.

In :setting:`MIDDLEWARE_CLASSES`, each middleware component is represented by
a string: the full Python path to the middleware's class name. For example,
here's the default value created by :djadmin:`django-admin startproject
<startproject>`::

    MIDDLEWARE_CLASSES = [
        'django.contrib.sessions.middleware.SessionMiddleware',
        'django.middleware.common.CommonMiddleware',
        'django.middleware.csrf.CsrfViewMiddleware',
        'django.contrib.auth.middleware.AuthenticationMiddleware',
        'django.contrib.auth.middleware.SessionAuthenticationMiddleware',
        'django.contrib.messages.middleware.MessageMiddleware',
        'django.middleware.clickjacking.XFrameOptionsMiddleware',
        'django.middleware.security.SecurityMiddleware',
    ]

A Django installation doesn't require any middleware —
:setting:`MIDDLEWARE_CLASSES` can be empty, if you'd like — but it's strongly
suggested that you at least use
:class:`~django.middleware.common.CommonMiddleware`.

The order in :setting:`MIDDLEWARE_CLASSES` matters because a middleware can
depend on other middleware. For instance,
:class:`~django.contrib.auth.middleware.AuthenticationMiddleware` stores the
authenticated user in the session; therefore, it must run after
:class:`~django.contrib.sessions.middleware.SessionMiddleware`. See
:ref:`middleware-ordering` for some common hints about ordering of Django
middleware classes.

Hooks and application order
===========================

During the request phase, before calling the view, Django applies middleware
in the order it's defined in :setting:`MIDDLEWARE_CLASSES`, top-down. Two
hooks are available:

* :meth:`process_request`
* :meth:`process_view`

During the response phase, after calling the view, middleware are applied in
reverse order, from the bottom up. Three hooks are available:

* :meth:`process_exception` (only if the view raised an exception)
* :meth:`process_template_response` (only for template responses)
* :meth:`process_response`

.. image:: _images/middleware.*
   :alt: middleware application order
   :width: 481
   :height: 409

If you prefer, you can also think of it like an onion: each middleware class
is a "layer" that wraps the view.

The behavior of each hook is described below.

Writing your own middleware
===========================

Writing your own middleware is easy. Each middleware component is a single
Python class that defines one or more of the following methods:

.. _request-middleware:

``process_request``
-------------------

.. method:: process_request(request)

``request`` is an :class:`~django.http.HttpRequest` object.

``process_request()`` is called on each request, before Django decides which
view to execute.

It should return either ``None`` or an :class:`~django.http.HttpResponse`
object. If it returns ``None``, Django will continue processing this request,
executing any other ``process_request()`` middleware, then, ``process_view()``
middleware, and finally, the appropriate view. If it returns an
:class:`~django.http.HttpResponse` object, Django won't bother calling any
other request, view or exception middleware, or the appropriate view; it'll
apply response middleware to that :class:`~django.http.HttpResponse`, and
return the result.

.. _view-middleware:

``process_view``
----------------

.. method:: process_view(request, view_func, view_args, view_kwargs)

``request`` is an :class:`~django.http.HttpRequest` object. ``view_func`` is
the Python function that Django is about to use. (It's the actual function
object, not the name of the function as a string.) ``view_args`` is a list of
positional arguments that will be passed to the view, and ``view_kwargs`` is a
dictionary of keyword arguments that will be passed to the view. Neither
``view_args`` nor ``view_kwargs`` include the first view argument
(``request``).

``process_view()`` is called just before Django calls the view.

It should return either ``None`` or an :class:`~django.http.HttpResponse`
object. If it returns ``None``, Django will continue processing this request,
executing any other ``process_view()`` middleware and, then, the appropriate
view. If it returns an :class:`~django.http.HttpResponse` object, Django won't
bother calling any other view or exception middleware, or the appropriate
view; it'll apply response middleware to that
:class:`~django.http.HttpResponse`, and return the result.

.. note::

    Accessing :attr:`request.POST <django.http.HttpRequest.POST>` inside
    middleware from ``process_request`` or ``process_view`` will prevent any
    view running after the middleware from being able to :ref:`modify the
    upload handlers for the request <modifying_upload_handlers_on_the_fly>`,
    and should normally be avoided.

    The :class:`~django.middleware.csrf.CsrfViewMiddleware` class can be
    considered an exception, as it provides the
    :func:`~django.views.decorators.csrf.csrf_exempt` and
    :func:`~django.views.decorators.csrf.csrf_protect` decorators which allow
    views to explicitly control at what point the CSRF validation should occur.

.. _template-response-middleware:

``process_template_response``
-----------------------------

.. method:: process_template_response(request, response)

``request`` is an :class:`~django.http.HttpRequest` object. ``response`` is
the :class:`~django.template.response.TemplateResponse` object (or equivalent)
returned by a Django view or by a middleware.

``process_template_response()`` is called just after the view has finished
executing, if the response instance has a ``render()`` method, indicating that
it is a :class:`~django.template.response.TemplateResponse` or equivalent.

It must return a response object that implements a ``render`` method. It could
alter the given ``response`` by changing ``response.template_name`` and
``response.context_data``, or it could create and return a brand-new
:class:`~django.template.response.TemplateResponse` or equivalent.

You don't need to explicitly render responses -- responses will be
automatically rendered once all template response middleware has been
called.

Middleware are run in reverse order during the response phase, which
includes ``process_template_response()``.

.. _response-middleware:

``process_response``
--------------------

.. method:: process_response(request, response)

``request`` is an :class:`~django.http.HttpRequest` object. ``response`` is
the :class:`~django.http.HttpResponse` or
:class:`~django.http.StreamingHttpResponse` object returned by a Django view
or by a middleware.

``process_response()`` is called on all responses before they're returned to
the browser.

It must return an :class:`~django.http.HttpResponse` or
:class:`~django.http.StreamingHttpResponse` object. It could alter the given
``response``, or it could create and return a brand-new
:class:`~django.http.HttpResponse` or
:class:`~django.http.StreamingHttpResponse`.

Unlike the ``process_request()`` and ``process_view()`` methods, the
``process_response()`` method is always called, even if the
``process_request()`` and ``process_view()`` methods of the same middleware
class were skipped (because an earlier middleware method returned an
:class:`~django.http.HttpResponse`). In particular, this means that your
``process_response()`` method cannot rely on setup done in
``process_request()``.

Finally, remember that during the response phase, middleware are applied in
reverse order, from the bottom up. This means classes defined at the end of
:setting:`MIDDLEWARE_CLASSES` will be run first.

Dealing with streaming responses
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Unlike :class:`~django.http.HttpResponse`,
:class:`~django.http.StreamingHttpResponse` does not have a ``content``
attribute. As a result, middleware can no longer assume that all responses
will have a ``content`` attribute. If they need access to the content, they
must test for streaming responses and adjust their behavior accordingly::

    if response.streaming:
        response.streaming_content = wrap_streaming_content(response.streaming_content)
    else:
        response.content = alter_content(response.content)

.. note::

    ``streaming_content`` should be assumed to be too large to hold in memory.
    Response middleware may wrap it in a new generator, but must not consume
    it. Wrapping is typically implemented as follows::

        def wrap_streaming_content(content):
            for chunk in content:
                yield alter_content(chunk)

.. _exception-middleware:

``process_exception``
---------------------

.. method:: process_exception(request, exception)

``request`` is an :class:`~django.http.HttpRequest` object. ``exception`` is an
``Exception`` object raised by the view function.

Django calls ``process_exception()`` when a view raises an exception.
``process_exception()`` should return either ``None`` or an
:class:`~django.http.HttpResponse` object. If it returns an
:class:`~django.http.HttpResponse` object, the template response and response
middleware will be applied, and the resulting response returned to the
browser. Otherwise, default exception handling kicks in.

Again, middleware are run in reverse order during the response phase, which
includes ``process_exception``. If an exception middleware returns a response,
the middleware classes above that middleware will not be called at all.

``__init__``
------------

Most middleware classes won't need an initializer since middleware classes are
essentially placeholders for the ``process_*`` methods. If you do need some
global state you may use ``__init__`` to set up. However, keep in mind a couple
of caveats:

* Django initializes your middleware without any arguments, so you can't
  define ``__init__`` as requiring any arguments.

* Unlike the ``process_*`` methods which get called once per request,
  ``__init__`` gets called only *once*, when the Web server responds to the
  first request.

Marking middleware as unused
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It's sometimes useful to determine at run-time whether a piece of middleware
should be used. In these cases, your middleware's ``__init__`` method may
raise :exc:`django.core.exceptions.MiddlewareNotUsed`. Django will then remove
that piece of middleware from the middleware process and a debug message will
be logged to the ``django.request`` logger when :setting:`DEBUG` is set to
``True``.

.. versionchanged:: 1.8

    Previously, :exc:`~django.core.exceptions.MiddlewareNotUsed` exceptions
    weren't logged.

Guidelines
----------

* Middleware classes don't have to subclass anything.

* The middleware class can live anywhere on your Python path. All Django
  cares about is that the :setting:`MIDDLEWARE_CLASSES` setting includes
  the path to it.

* Feel free to look at :doc:`Django's available middleware
  </ref/middleware>` for examples.

* If you write a middleware component that you think would be useful to
  other people, contribute to the community! :doc:`Let us know
  </internals/contributing/index>`, and we'll consider adding it to Django.
