django/docs/topics/http/middleware.txt

==========
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>`.

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

A middleware factory is a callable that takes a ``get_response`` callable and
returns a middleware. A middleware is a callable that takes a request and
returns a response, just like a view.

A middleware can be written as a function that looks like this::

    def simple_middleware(get_response):
        # One-time configuration and initialization.

        def middleware(request):
            # Code to be executed for each request before
            # the view (and later middleware) are called.

            response = get_response(request)

            # Code to be executed for each request/response after
            # the view is called.

            return response

        return middleware

Or it can be written as a class whose instances are callable, like this::

    class SimpleMiddleware(object):
        def __init__(self, get_response):
            self.get_response = get_response
            # One-time configuration and initialization.

        def __call__(self, request):
            # Code to be executed for each request before
            # the view (and later middleware) are called.

            response = self.get_response(request)

            # Code to be executed for each request/response after
            # the view is called.

            return response

The ``get_response`` callable provided by Django might be the actual view (if
this is the last listed middleware) or it might be the next middleware in the
chain. The current middleware doesn't need to know or care what exactly it is,
just that it represents whatever comes next.

The above is a slight simplification -- the ``get_response`` callable for the
last middleware in the chain won't be the actual view but rather a wrapper
method from the handler which takes care of applying :ref:`view middleware
<view-middleware>`, calling the view with appropriate URL arguments, and
applying :ref:`template-response <template-response-middleware>` and
:ref:`exception <exception-middleware>` middleware.

Middleware can live anywhere on your Python path.

``__init__(get_response)``
--------------------------

Middleware factories must accept a ``get_response`` argument. You can also
initialize some global state for the middleware. Keep in mind a couple of
caveats:

* Django initializes your middleware with only the ``get_response`` argument,
  so you can't define ``__init__()`` as requiring any other arguments.

* Unlike the ``__call__()`` method which is called once per request,
  ``__init__()`` is called only *once*, when the Web server starts.

.. versionchanged:: 1.10

    In older versions, ``__init__()`` wasn't called until the Web server
    responded to its first request.

    In older versions, ``__init__()`` didn't accept any arguments. To allow
    your middleware to be used in Django 1.9 and earlier, make ``get_response``
    an optional argument (``get_response=None``).

Marking middleware as unused
----------------------------

It's sometimes useful to determine at startup 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 middleware from the middleware process and log a debug message
to the :ref:`django-request-logger` logger when :setting:`DEBUG` is ``True``.

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

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

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

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

A Django installation doesn't require any middleware — :setting:`MIDDLEWARE`
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` 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.

Middleware order and layering
=============================

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

You can think of it like an onion: each middleware class is a "layer" that
wraps the view, which is in the core of the onion. If the request passes
through all the layers of the onion (each one calls ``get_response`` to pass
the request in to the next layer), all the way to the view at the core, the
response will then pass through every layer (in reverse order) on the way back
out.

If one of the layers decides to short-circuit and return a response without
ever calling its ``get_response``, none of the layers of the onion inside that
layer (including the view) will see the request or the response. The response
will only return through the same layers that the request passed in through.

Other middleware hooks
======================

Besides the basic request/response middleware pattern described earlier, you
can add three other special methods to class-based middleware:

.. _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 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 before the view runs or in ``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.

.. _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, :ref:`default exception handling <error-views>` 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 ``process_exception`` methods of the middleware classes above that
middleware won't be called at all.

.. _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()``.

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 handling
==================

Django automatically converts exceptions raised by the view or by middleware
into an appropriate HTTP response with an error status code. :ref:`Certain
exceptions <error-views>` are converted to 4xx status codes, while an unknown
exception is converted to a 500 status code.

This conversion takes place before and after each middleware (you can think of
it as the thin film in between each layer of the onion), so that every
middleware can always rely on getting some kind of HTTP response back from
calling its ``get_response`` callable. Middleware don't need to worry about
wrapping their call to ``get_response`` in a ``try/except`` and handling an
exception that might have been raised by a later middleware or the view. Even
if the very next middleware in the chain raises an
:class:`~django.http.Http404` exception, for example, your middleware won't see
that exception; instead it will get an :class:`~django.http.HttpResponse`
object with a :attr:`~django.http.HttpResponse.status_code` of 404.

.. _upgrading-middleware:

Upgrading pre-Django 1.10-style middleware
==========================================

.. class:: django.utils.deprecation.MiddlewareMixin
    :module:

Django provides ``django.utils.deprecation.MiddlewareMixin`` to ease creating
middleware classes that are compatible with both :setting:`MIDDLEWARE` and the
old ``MIDDLEWARE_CLASSES``. All middleware classes included with Django
are compatible with both settings.

The mixin provides an ``__init__()`` method that accepts an optional
``get_response`` argument and stores it in ``self.get_response``.

The ``__call__()`` method:

#. Calls ``self.process_request(request)`` (if defined).
#. Calls ``self.get_response(request)`` to get the response from later
   middleware and the view.
#. Calls ``self.process_response(request, response)`` (if defined).
#. Returns the response.

If used with ``MIDDLEWARE_CLASSES``, the ``__call__()`` method will
never be used; Django calls ``process_request()`` and ``process_response()``
directly.

In most cases, inheriting from this mixin will be sufficient to make an
old-style middleware compatible with the new system with sufficient
backwards-compatibility. The new short-circuiting semantics will be harmless or
even beneficial to the existing middleware. In a few cases, a middleware class
may need some changes to adjust to the new semantics.

These are the behavioral differences between using :setting:`MIDDLEWARE` and
``MIDDLEWARE_CLASSES``:

1. Under ``MIDDLEWARE_CLASSES``, every middleware will always have its
   ``process_response`` method called, even if an earlier middleware
   short-circuited by returning a response from its ``process_request``
   method. Under :setting:`MIDDLEWARE`, middleware behaves more like an onion:
   the layers that a response goes through on the way out are the same layers
   that saw the request on the way in. If a middleware short-circuits, only
   that middleware and the ones before it in :setting:`MIDDLEWARE` will see the
   response.

2. Under ``MIDDLEWARE_CLASSES``, ``process_exception`` is applied to
   exceptions raised from a middleware ``process_request`` method. Under
   :setting:`MIDDLEWARE`, ``process_exception`` applies only to exceptions
   raised from the view (or from the ``render`` method of a
   :class:`~django.template.response.TemplateResponse`). Exceptions raised from
   a middleware are converted to the appropriate HTTP response and then passed
   to the next middleware.

3. Under ``MIDDLEWARE_CLASSES``, if a ``process_response`` method raises
   an exception, the ``process_response`` methods of all earlier middleware are
   skipped and a ``500 Internal Server Error`` HTTP response is always
   returned (even if the exception raised was e.g. an
   :class:`~django.http.Http404`). Under :setting:`MIDDLEWARE`, an exception
   raised from a middleware will immediately be converted to the appropriate
   HTTP response, and then the next middleware in line will see that
   response. Middleware are never skipped due to a middleware raising an
   exception.