Fixed #7820: MiddlewareNotUsed is finally documented somewhere else besides my brain.

Writing your own middleware is easy. Each middleware component is a single

Python class that defines one or more of the following methods:

process_request

---------------

``process_request``

-------------------

Interface: ``process_request(self, request)``

view or exception middleware, or the appropriate view; it'll return that

``HttpResponse``. Response middleware is always called on every response.

process_view

------------

``process_view``

----------------

Interface: ``process_view(self, request, view_func, view_args, view_kwargs)``

or exception middleware, or the appropriate view; it'll return that

``HttpResponse``. Response middleware is always called on every response.

process_response

----------------

``process_response``

--------------------

Interface: ``process_response(self, request, response)``

the given ``response``, or it could create and return a brand-new

``HttpResponse``.

process_exception

-----------------

``process_exception``

---------------------

Interface: ``process_exception(self, request, exception)``

object. If it returns an ``HttpResponse`` object, the response will be returned

to the browser. Otherwise, default exception handling kicks in.

``__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

----------