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267 lines
11 KiB
Plaintext
==========
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Middleware
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==========
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Middleware is a framework of hooks into Django's request/response processing.
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It's a light, low-level "plugin" system for globally altering Django's input
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and/or output.
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Each middleware component is responsible for doing some specific function. For
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example, Django includes a middleware component, ``XViewMiddleware``, that adds
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an ``"X-View"`` HTTP header to every response to a ``HEAD`` request.
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This document explains all middleware components that come with Django, how to
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use them, and how to write your own middleware.
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Activating middleware
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=====================
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To activate a middleware component, add it to the ``MIDDLEWARE_CLASSES`` list
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in your Django settings. In ``MIDDLEWARE_CLASSES``, each middleware component
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is represented by a string: the full Python path to the middleware's class
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name. For example, here's the default ``MIDDLEWARE_CLASSES`` created by
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``django-admin.py startproject``::
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MIDDLEWARE_CLASSES = (
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'django.middleware.common.CommonMiddleware',
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'django.contrib.sessions.middleware.SessionMiddleware',
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'django.contrib.auth.middleware.AuthenticationMiddleware',
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'django.middleware.doc.XViewMiddleware',
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)
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Django applies middleware in the order it's defined in ``MIDDLEWARE_CLASSES``,
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except in the case of response and exception middleware, which is applied in
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reverse order.
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A Django installation doesn't require any middleware -- e.g.,
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``MIDDLEWARE_CLASSES`` can be empty, if you'd like -- but it's strongly
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suggested that you use ``CommonMiddleware``.
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Available middleware
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====================
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django.middleware.cache.CacheMiddleware
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---------------------------------------
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Enables site-wide cache. If this is enabled, each Django-powered page will be
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cached for as long as the ``CACHE_MIDDLEWARE_SECONDS`` setting defines. See
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the `cache documentation`_.
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.. _`cache documentation`: ../cache/#the-per-site-cache
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django.middleware.common.CommonMiddleware
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-----------------------------------------
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Adds a few conveniences for perfectionists:
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* Forbids access to user agents in the ``DISALLOWED_USER_AGENTS`` setting,
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which should be a list of strings.
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* Performs URL rewriting based on the ``APPEND_SLASH`` and ``PREPEND_WWW``
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settings.
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If ``APPEND_SLASH`` is ``True`` and the initial URL doesn't end with a slash,
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and it is not found in the URLconf, then a new URL is formed by appending a
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slash at the end. If this new URL is found in the URLconf, then Django
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redirects the request to this new URL. Otherwise, the initial URL is
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processed as usual.
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For example, ``foo.com/bar`` will be redirected to ``foo.com/bar/`` if you
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don't have a valid URL pattern for ``foo.com/bar`` but *do* have a valid
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pattern for ``foo.com/bar/``.
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**New in Django development version:** The behavior of ``APPEND_SLASH`` has
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changed slightly in the development version. It didn't used to check whether
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the pattern was matched in the URLconf.
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If ``PREPEND_WWW`` is ``True``, URLs that lack a leading "www." will be
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redirected to the same URL with a leading "www."
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Both of these options are meant to normalize URLs. The philosophy is that
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each URL should exist in one, and only one, place. Technically a URL
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``foo.com/bar`` is distinct from ``foo.com/bar/`` -- a search-engine
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indexer would treat them as separate URLs -- so it's best practice to
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normalize URLs.
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* Handles ETags based on the ``USE_ETAGS`` setting. If ``USE_ETAGS`` is set
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to ``True``, Django will calculate an ETag for each request by
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MD5-hashing the page content, and it'll take care of sending
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``Not Modified`` responses, if appropriate.
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django.middleware.doc.XViewMiddleware
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-------------------------------------
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Sends custom ``X-View`` HTTP headers to HEAD requests that come from IP
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addresses defined in the ``INTERNAL_IPS`` setting. This is used by Django's
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automatic documentation system.
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django.middleware.gzip.GZipMiddleware
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-------------------------------------
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Compresses content for browsers that understand gzip compression (all modern
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browsers).
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It is suggested to place this first in the middleware list, so that the
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compression of the response content is the last thing that happens. Will not
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compress content bodies less than 200 bytes long, when the response code is
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something other than 200, Javascript files (for IE compatibitility), or
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responses that have the ``Content-Encoding`` header already specified.
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django.middleware.http.ConditionalGetMiddleware
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-----------------------------------------------
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Handles conditional GET operations. If the response has a ``ETag`` or
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``Last-Modified`` header, and the request has ``If-None-Match`` or
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``If-Modified-Since``, the response is replaced by an HttpNotModified.
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Also sets the ``Date`` and ``Content-Length`` response-headers.
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django.middleware.http.SetRemoteAddrFromForwardedFor
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----------------------------------------------------
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Sets ``request.META['REMOTE_ADDR']`` based on
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``request.META['HTTP_X_FORWARDED_FOR']``, if the latter is set. This is useful
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if you're sitting behind a reverse proxy that causes each request's
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``REMOTE_ADDR`` to be set to ``127.0.0.1``.
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**Important note:** This does NOT validate ``HTTP_X_FORWARDED_FOR``. If you're
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not behind a reverse proxy that sets ``HTTP_X_FORWARDED_FOR`` automatically, do
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not use this middleware. Anybody can spoof the value of
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``HTTP_X_FORWARDED_FOR``, and because this sets ``REMOTE_ADDR`` based on
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``HTTP_X_FORWARDED_FOR``, that means anybody can "fake" their IP address. Only
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use this when you can absolutely trust the value of ``HTTP_X_FORWARDED_FOR``.
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django.middleware.locale.LocaleMiddleware
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-----------------------------------------
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Enables language selection based on data from the request. It customizes content
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for each user. See the `internationalization documentation`_.
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.. _`internationalization documentation`: ../i18n/
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django.contrib.sessions.middleware.SessionMiddleware
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----------------------------------------------------
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Enables session support. See the `session documentation`_.
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.. _`session documentation`: ../sessions/
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django.contrib.auth.middleware.AuthenticationMiddleware
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-------------------------------------------------------
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Adds the ``user`` attribute, representing the currently-logged-in user, to
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every incoming ``HttpRequest`` object. See `Authentication in Web requests`_.
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.. _Authentication in Web requests: ../authentication/#authentication-in-web-requests
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django.contrib.csrf.middleware.CsrfMiddleware
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---------------------------------------------
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**New in Django development version**
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Adds protection against Cross Site Request Forgeries by adding hidden form
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fields to POST forms and checking requests for the correct value. See the
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`Cross Site Request Forgery protection documentation`_.
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.. _`Cross Site Request Forgery protection documentation`: ../csrf/
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django.middleware.transaction.TransactionMiddleware
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---------------------------------------------------
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Binds commit and rollback to the request/response phase. If a view function runs
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successfully, a commit is done. If it fails with an exception, a rollback is
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done.
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The order of this middleware in the stack is important: middleware modules
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running outside of it run with commit-on-save - the default Django behavior.
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Middleware modules running inside it (coming later in the stack) will be under
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the same transaction control as the view functions.
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See the `transaction management documentation`_.
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.. _`transaction management documentation`: ../transactions/
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Writing your own middleware
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===========================
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Writing your own middleware is easy. Each middleware component is a single
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Python class that defines one or more of the following methods:
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process_request
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---------------
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Interface: ``process_request(self, request)``
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``request`` is an ``HttpRequest`` object. This method is called on each
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request, before Django decides which view to execute.
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``process_request()`` should return either ``None`` or an ``HttpResponse``
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object. If it returns ``None``, Django will continue processing this request,
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executing any other middleware and, then, the appropriate view. If it returns
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an ``HttpResponse`` object, Django won't bother calling ANY other request,
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view or exception middleware, or the appropriate view; it'll return that
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``HttpResponse``. Response middleware is always called on every response.
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process_view
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------------
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Interface: ``process_view(self, request, view_func, view_args, view_kwargs)``
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``request`` is an ``HttpRequest`` object. ``view_func`` is the Python function
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that Django is about to use. (It's the actual function object, not the name of
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the function as a string.) ``view_args`` is a list of positional arguments that
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will be passed to the view, and ``view_kwargs`` is a dictionary of keyword
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arguments that will be passed to the view. Neither ``view_args`` nor
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``view_kwargs`` include the first view argument (``request``).
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``process_view()`` is called just before Django calls the view. It should
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return either ``None`` or an ``HttpResponse`` object. If it returns ``None``,
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Django will continue processing this request, executing any other
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``process_view()`` middleware and, then, the appropriate view. If it returns an
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``HttpResponse`` object, Django won't bother calling ANY other request, view
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or exception middleware, or the appropriate view; it'll return that
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``HttpResponse``. Response middleware is always called on every response.
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process_response
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----------------
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Interface: ``process_response(self, request, response)``
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``request`` is an ``HttpRequest`` object. ``response`` is the ``HttpResponse``
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object returned by a Django view.
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``process_response()`` should return an ``HttpResponse`` object. It could alter
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the given ``response``, or it could create and return a brand-new
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``HttpResponse``.
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process_exception
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-----------------
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Interface: ``process_exception(self, request, exception)``
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``request`` is an ``HttpRequest`` object. ``exception`` is an ``Exception``
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object raised by the view function.
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Django calls ``process_exception()`` when a view raises an exception.
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``process_exception()`` should return either ``None`` or an ``HttpResponse``
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object. If it returns an ``HttpResponse`` object, the response will be returned
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to the browser. Otherwise, default exception handling kicks in.
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Guidelines
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----------
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* Middleware classes don't have to subclass anything.
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* The middleware class can live anywhere on your Python path. All Django
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cares about is that the ``MIDDLEWARE_CLASSES`` setting includes the path
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to it.
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* Feel free to look at Django's available middleware for examples. The
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core Django middleware classes are in ``django/middleware/`` in the
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Django distribution. The session middleware is in
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``django/contrib/sessions``.
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* If you write a middleware component that you think would be useful to
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other people, contribute to the community! Let us know, and we'll
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consider adding it to Django.
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