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256 lines
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256 lines
9.8 KiB
Plaintext
=============
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Writing views
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=============
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A view function, or *view* for short, is simply a Python function that takes a
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Web request and returns a Web response. This response can be the HTML contents
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of a Web page, or a redirect, or a 404 error, or an XML document, or an image .
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. . or anything, really. The view itself contains whatever arbitrary logic is
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necessary to return that response. This code can live anywhere you want, as long
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as it's on your Python path. There's no other requirement--no "magic," so to
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speak. For the sake of putting the code *somewhere*, the convention is to
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put views in a file called ``views.py``, placed in your project or
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application directory.
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A simple view
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=============
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Here's a view that returns the current date and time, as an HTML document:
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.. code-block:: python
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from django.http import HttpResponse
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import datetime
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def current_datetime(request):
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now = datetime.datetime.now()
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html = "<html><body>It is now %s.</body></html>" % now
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return HttpResponse(html)
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Let's step through this code one line at a time:
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* First, we import the class :class:`~django.http.HttpResponse` from the
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:mod:`django.http` module, along with Python's ``datetime`` library.
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* Next, we define a function called ``current_datetime``. This is the view
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function. Each view function takes an :class:`~django.http.HttpRequest`
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object as its first parameter, which is typically named ``request``.
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Note that the name of the view function doesn't matter; it doesn't have to
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be named in a certain way in order for Django to recognize it. We're
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calling it ``current_datetime`` here, because that name clearly indicates
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what it does.
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* The view returns an :class:`~django.http.HttpResponse` object that
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contains the generated response. Each view function is responsible for
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returning an :class:`~django.http.HttpResponse` object. (There are
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exceptions, but we'll get to those later.)
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.. admonition:: Django's Time Zone
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Django includes a :setting:`TIME_ZONE` setting that defaults to
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``America/Chicago``. This probably isn't where you live, so you might want
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to change it in your settings file.
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Mapping URLs to views
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=====================
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So, to recap, this view function returns an HTML page that includes the current
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date and time. To display this view at a particular URL, you'll need to create a
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*URLconf*; see :doc:`/topics/http/urls` for instructions.
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Returning errors
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================
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Returning HTTP error codes in Django is easy. There are subclasses of
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:class:`~django.http.HttpResponse` for a number of common HTTP status codes
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other than 200 (which means *"OK"*). You can find the full list of available
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subclasses in the :ref:`request/response <ref-httpresponse-subclasses>`
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documentation. Just return an instance of one of those subclasses instead of
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a normal :class:`~django.http.HttpResponse` in order to signify an error. For
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example::
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from django.http import HttpResponse, HttpResponseNotFound
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def my_view(request):
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# ...
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if foo:
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return HttpResponseNotFound('<h1>Page not found</h1>')
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else:
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return HttpResponse('<h1>Page was found</h1>')
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There isn't a specialized subclass for every possible HTTP response code,
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since many of them aren't going to be that common. However, as documented in
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the :class:`~django.http.HttpResponse` documentation, you can also pass the
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HTTP status code into the constructor for :class:`~django.http.HttpResponse`
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to create a return class for any status code you like. For example::
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from django.http import HttpResponse
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def my_view(request):
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# ...
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# Return a "created" (201) response code.
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return HttpResponse(status=201)
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Because 404 errors are by far the most common HTTP error, there's an easier way
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to handle those errors.
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The Http404 exception
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---------------------
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.. class:: django.http.Http404()
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When you return an error such as :class:`~django.http.HttpResponseNotFound`,
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you're responsible for defining the HTML of the resulting error page::
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return HttpResponseNotFound('<h1>Page not found</h1>')
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For convenience, and because it's a good idea to have a consistent 404 error page
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across your site, Django provides an ``Http404`` exception. If you raise
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``Http404`` at any point in a view function, Django will catch it and return the
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standard error page for your application, along with an HTTP error code 404.
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Example usage::
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from django.http import Http404
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from django.shortcuts import render_to_response
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from polls.models import Poll
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def detail(request, poll_id):
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try:
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p = Poll.objects.get(pk=poll_id)
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except Poll.DoesNotExist:
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raise Http404
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return render_to_response('polls/detail.html', {'poll': p})
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In order to use the ``Http404`` exception to its fullest, you should create a
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template that is displayed when a 404 error is raised. This template should be
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called ``404.html`` and located in the top level of your template tree.
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.. _customizing-error-views:
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Customizing error views
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=======================
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.. _http_not_found_view:
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The 404 (page not found) view
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-----------------------------
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.. function:: django.views.defaults.page_not_found(request, template_name='404.html')
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When you raise :exc:`~django.http.Http404` from within a view, Django loads a
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special view devoted to handling 404 errors. By default, it's the view
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:func:`django.views.defaults.page_not_found`, which either produces a very
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simple "Not Found" message or loads and renders the template ``404.html`` if
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you created it in your root template directory.
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The default 404 view will pass one variable to the template: ``request_path``,
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which is the URL that resulted in the error.
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The ``page_not_found`` view should suffice for 99% of Web applications, but if
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you want to override it, you can specify ``handler404`` in your root URLconf
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(setting ``handler404`` anywhere else will have no effect), like so::
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handler404 = 'mysite.views.my_custom_404_view'
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Behind the scenes, Django determines the 404 view by looking for
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``handler404`` in your root URLconf, and falling back to
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``django.views.defaults.page_not_found`` if you did not define one.
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Three things to note about 404 views:
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* The 404 view is also called if Django doesn't find a match after
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checking every regular expression in the URLconf.
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* The 404 view is passed a :class:`~django.template.RequestContext` and
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will have access to variables supplied by your
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:setting:`TEMPLATE_CONTEXT_PROCESSORS` setting (e.g., ``MEDIA_URL``).
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* If :setting:`DEBUG` is set to ``True`` (in your settings module), then
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your 404 view will never be used, and your URLconf will be displayed
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instead, with some debug information.
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.. _http_internal_server_error_view:
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The 500 (server error) view
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----------------------------
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Similarly, Django executes special-case behavior in the case of runtime errors
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in view code. If a view results in an exception, Django will, by default, call
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the view ``django.views.defaults.server_error``, which either produces a very
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simple "Server Error" message or loads and renders the template ``500.html`` if
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you created it in your root template directory.
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The default 500 view passes no variables to the ``500.html`` template and is
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rendered with an empty ``Context`` to lessen the chance of additional errors.
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This ``server_error`` view should suffice for 99% of Web applications, but if
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you want to override the view, you can specify ``handler500`` in your URLconf,
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like so::
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handler500 = 'mysite.views.my_custom_error_view'
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Behind the scenes, Django determines the 500 view by looking for
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``handler500`` in your root URLconf, and falling back to
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``django.views.defaults.server_error`` if you did not define one.
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One thing to note about 500 views:
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* If :setting:`DEBUG` is set to ``True`` (in your settings module), then
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your 500 view will never be used, and the traceback will be displayed
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instead, with some debug information.
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.. _http_forbidden_view:
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The 403 (HTTP Forbidden) view
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-----------------------------
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In the same vein as the 404 and 500 views, Django has a view to handle 403
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Forbidden errors. If a view results in a 403 exception then Django will, by
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default, call the view ``django.views.defaults.permission_denied``.
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This view loads and renders the template ``403.html`` in your root template
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directory, or if this file does not exist, instead serves the text
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"403 Forbidden", as per :rfc:`2616` (the HTTP 1.1 Specification).
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``django.views.defaults.permission_denied`` is triggered by a
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:exc:`~django.core.exceptions.PermissionDenied` exception. To deny access in a
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view you can use code like this::
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from django.core.exceptions import PermissionDenied
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def edit(request, pk):
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if not request.user.is_staff:
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raise PermissionDenied
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# ...
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It is possible to override ``django.views.defaults.permission_denied`` in the
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same way you can for the 404 and 500 views by specifying a ``handler403`` in
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your URLconf::
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handler403 = 'mysite.views.my_custom_permission_denied_view'
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.. _http_bad_request_view:
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The 400 (bad request) view
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--------------------------
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When a :exc:`~django.core.exceptions.SuspiciousOperation` is raised in Django,
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the it may be handled by a component of Django (for example resetting the
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session data). If not specifically handled, Django will consider the current
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request a 'bad request' instead of a server error.
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The view ``django.views.defaults.bad_request``, is otherwise very similar to
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the ``server_error`` view, but returns with the status code 400 indicating that
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the error condition was the result of a client operation.
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Like the ``server_error`` view, the default ``bad_request`` should suffice for
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99% of Web applications, but if you want to override the view, you can specify
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``handler400`` in your URLconf, like so::
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handler400 = 'mysite.views.my_custom_bad_request_view'
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``bad_request`` views are also only used when :setting:`DEBUG` is ``False``.
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