==========
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
and/or output.

Each middleware component is responsible for doing some specific function. For
example, Django includes a middleware component, ``XViewMiddleware``, that adds
an ``"X-View"`` HTTP header to every response to a ``HEAD`` request.

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 :setting:`MIDDLEWARE_CLASSES`
created by :djadmin:`django-admin.py startproject <startproject>`::

    MIDDLEWARE_CLASSES = (
        'django.middleware.common.CommonMiddleware',
        'django.contrib.sessions.middleware.SessionMiddleware',
        'django.middleware.csrf.CsrfViewMiddleware',
        'django.contrib.auth.middleware.AuthenticationMiddleware',
        'django.contrib.messages.middleware.MessageMiddleware',
    )

During the request phases (:meth:`process_request` and :meth:`process_view`
middleware), Django applies middleware in the order it's defined in
:setting:`MIDDLEWARE_CLASSES`, top-down. During the response phases
(:meth:`process_response` and :meth:`process_exception` middleware), the
classes are applied in reverse order, from the bottom up. You can think of it
like an onion: each middleware class is a "layer" that wraps the view:

.. image:: _images/middleware.png
   :width: 502
   :height: 417
   :alt: Middleware application order.

A Django installation doesn't require any middleware -- e.g.,
: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`.

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(self, request)

``request`` is an :class:`~django.http.HttpRequest` object. This method is
called on each request, before Django decides which view to execute.

``process_request()`` should return either ``None`` or an
:class:`~django.http.HttpResponse` object. If it returns ``None``, Django will
continue processing this request, executing any other middleware and, then, 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 return that :class:`~django.http.HttpResponse`.
Response middleware is always called on every response.

.. _view-middleware:

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

.. method:: process_view(self, 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 request, view or exception middleware, or the appropriate
view; it'll return that :class:`~django.http.HttpResponse`. Response
middleware is always called on every response.

.. _response-middleware:

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

.. method:: process_response(self, request, response)

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

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

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`
(this means that your ``process_response()`` method cannot rely on setup done in
``process_request()``, for example). In addition, during the response phase the
classes are applied in reverse order, from the bottom up. This means classes
defined at the end of :setting:`MIDDLEWARE_CLASSES` will be run first.

.. _exception-middleware:

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

.. method:: process_exception(self, 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 response will be 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 return 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 starts up.

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
``django.core.exceptions.MiddlewareNotUsed``. Django will then remove that
piece of middleware from the middleware process.

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>`, and we'll consider adding it to Django.
