=============
Writing views
=============
A view function, or *view* for short, is simply a Python function that takes a
Web request and returns a Web response. This response can be the HTML contents
of a Web page, or a redirect, or a 404 error, or an XML document, or an image .
. . or anything, really. The view itself contains whatever arbitrary logic is
necessary to return that response. This code can live anywhere you want, as long
as it's on your Python path. There's no other requirement--no "magic," so to
speak. For the sake of putting the code *somewhere*, the convention is to
put views in a file called ``views.py``, placed in your project or
application directory.
A simple view
=============
Here's a view that returns the current date and time, as an HTML document::
from django.http import HttpResponse
import datetime
def current_datetime(request):
now = datetime.datetime.now()
html = "<html><body>It is now %s.</body></html>" % now
return HttpResponse(html)
Let's step through this code one line at a time:
* First, we import the class :class:`~django.http.HttpResponse` from the
:mod:`django.http` module, along with Python's ``datetime`` library.
* Next, we define a function called ``current_datetime``. This is the view
function. Each view function takes an :class:`~django.http.HttpRequest`
object as its first parameter, which is typically named ``request``.
Note that the name of the view function doesn't matter; it doesn't have to
be named in a certain way in order for Django to recognize it. We're
calling it ``current_datetime`` here, because that name clearly indicates
what it does.
* The view returns an :class:`~django.http.HttpResponse` object that
contains the generated response. Each view function is responsible for
returning an :class:`~django.http.HttpResponse` object. (There are
exceptions, but we'll get to those later.)
.. admonition:: Django's Time Zone
Django includes a :setting:`TIME_ZONE` setting that defaults to
``America/Chicago``. This probably isn't where you live, so you might want
to change it in your settings file.
Mapping URLs to views
=====================
So, to recap, this view function returns an HTML page that includes the current
date and time. To display this view at a particular URL, you'll need to create a
*URLconf*; see :doc:`/topics/http/urls` for instructions.
Returning errors
================
Returning HTTP error codes in Django is easy. There are subclasses of
:class:`~django.http.HttpResponse` for a number of common HTTP status codes
other than 200 (which means *"OK"*). You can find the full list of available
subclasses in the :ref:`request/response <ref-httpresponse-subclasses>`
documentation. Just return an instance of one of those subclasses instead of
a normal :class:`~django.http.HttpResponse` in order to signify an error. For
example::
from django.http import HttpResponse, HttpResponseNotFound
def my_view(request):
# ...
if foo:
return HttpResponseNotFound('<h1>Page not found</h1>')
else:
return HttpResponse('<h1>Page was found</h1>')
There isn't a specialized subclass for every possible HTTP response code,
since many of them aren't going to be that common. However, as documented in
the :class:`~django.http.HttpResponse` documentation, you can also pass the
HTTP status code into the constructor for :class:`~django.http.HttpResponse`
to create a return class for any status code you like. For example::
from django.http import HttpResponse
def my_view(request):
# ...
# Return a "created" (201) response code.
return HttpResponse(status=201)
Because 404 errors are by far the most common HTTP error, there's an easier way
to handle those errors.
The Http404 exception
---------------------
.. class:: django.http.Http404()
When you return an error such as :class:`~django.http.HttpResponseNotFound`,
you're responsible for defining the HTML of the resulting error page::
return HttpResponseNotFound('<h1>Page not found</h1>')
For convenience, and because it's a good idea to have a consistent 404 error page
across your site, Django provides an ``Http404`` exception. If you raise
``Http404`` at any point in a view function, Django will catch it and return the
standard error page for your application, along with an HTTP error code 404.
Example usage::
from django.http import Http404
from django.shortcuts import render_to_response
from polls.models import Poll
def detail(request, poll_id):
try:
p = Poll.objects.get(pk=poll_id)
except Poll.DoesNotExist:
raise Http404("Poll does not exist")
return render_to_response('polls/detail.html', {'poll': p})
In order to use the ``Http404`` exception to its fullest, you should create a
template that is displayed when a 404 error is raised. This template should be
called ``404.html`` and located in the top level of your template tree.
If you provide a message when raising an ``Http404`` exception, it will appear
in the standard 404 template displayed when :setting:`DEBUG` is ``True``. Use
these messages for debugging purposes; they generally aren't suitable for use
in a production 404 template.
.. _customizing-error-views:
Customizing error views
=======================
The default error views in Django should suffice for most Web applications,
but can easily be overridden if you need any custom behavior. Simply specify
the handlers as seen below in your URLconf (setting them anywhere else will
have no effect).
The :func:`~django.views.defaults.page_not_found` view is overridden by
:data:`~django.conf.urls.handler404`::
handler404 = 'mysite.views.my_custom_page_not_found_view'
The :func:`~django.views.defaults.server_error` view is overridden by
:data:`~django.conf.urls.handler500`::
handler500 = 'mysite.views.my_custom_error_view'
The :func:`~django.views.defaults.permission_denied` view is overridden by
:data:`~django.conf.urls.handler403`::
handler403 = 'mysite.views.my_custom_permission_denied_view'
The :func:`~django.views.defaults.bad_request` view is overridden by
:data:`~django.conf.urls.handler400`::
handler400 = 'mysite.views.my_custom_bad_request_view'
