2008-08-24 06:25:40 +08:00
|
|
|
=============
|
|
|
|
|
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*, let's create a file called
|
|
|
|
|
``views.py`` in the ``mysite`` directory, which you created in the previous
|
|
|
|
|
chapter.
|
|
|
|
|
|
|
|
|
|
A simple view
|
|
|
|
|
=============
|
|
|
|
|
|
|
|
|
|
Here's a view that returns the current date and time, as an HTML document:
|
|
|
|
|
|
|
|
|
|
.. code-block:: python
|
|
|
|
|
|
|
|
|
|
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:
|
|
|
|
|
|
2010-11-27 00:53:38 +08:00
|
|
|
* First, we import the class :class:`~django.http.HttpResponse` from the
|
|
|
|
|
:mod:`django.http` module, along with Python's ``datetime`` library.
|
2008-08-24 06:25:40 +08:00
|
|
|
|
|
|
|
|
* Next, we define a function called ``current_datetime``. This is the view
|
2010-11-27 00:53:38 +08:00
|
|
|
function. Each view function takes an :class:`~django.http.HttpRequest`
|
|
|
|
|
object as its first parameter, which is typically named ``request``.
|
2008-08-24 06:25:40 +08:00
|
|
|
|
|
|
|
|
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.
|
|
|
|
|
|
2010-11-27 00:53:38 +08:00
|
|
|
* 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.)
|
2008-08-24 06:25:40 +08:00
|
|
|
|
|
|
|
|
.. admonition:: Django's Time Zone
|
|
|
|
|
|
|
|
|
|
Django includes a ``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
|
2010-08-20 03:27:44 +08:00
|
|
|
*URLconf*; see :doc:`/topics/http/urls` for instructions.
|
2008-08-24 06:25:40 +08:00
|
|
|
|
|
|
|
|
Returning errors
|
|
|
|
|
================
|
|
|
|
|
|
2008-10-24 17:24:42 +08:00
|
|
|
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::
|
2008-08-24 06:25:40 +08:00
|
|
|
|
|
|
|
|
def my_view(request):
|
|
|
|
|
# ...
|
|
|
|
|
if foo:
|
|
|
|
|
return HttpResponseNotFound('<h1>Page not found</h1>')
|
|
|
|
|
else:
|
|
|
|
|
return HttpResponse('<h1>Page was found</h1>')
|
|
|
|
|
|
2008-10-24 17:24:42 +08:00
|
|
|
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::
|
|
|
|
|
|
|
|
|
|
def my_view(request):
|
|
|
|
|
# ...
|
|
|
|
|
|
|
|
|
|
# Return a "created" (201) response code.
|
|
|
|
|
return HttpResponse(status=201)
|
|
|
|
|
|
2008-08-24 06:25:40 +08:00
|
|
|
Because 404 errors are by far the most common HTTP error, there's an easier way
|
|
|
|
|
to handle those errors.
|
|
|
|
|
|
|
|
|
|
The Http404 exception
|
|
|
|
|
---------------------
|
|
|
|
|
|
2010-11-18 04:03:15 +08:00
|
|
|
.. class:: django.http.Http404()
|
|
|
|
|
|
2010-11-27 00:53:38 +08:00
|
|
|
When you return an error such as :class:`~django.http.HttpResponseNotFound`,
|
|
|
|
|
you're responsible for defining the HTML of the resulting error page::
|
2008-08-24 06:25:40 +08:00
|
|
|
|
|
|
|
|
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
|
|
|
|
|
|
|
|
|
|
def detail(request, poll_id):
|
|
|
|
|
try:
|
|
|
|
|
p = Poll.objects.get(pk=poll_id)
|
|
|
|
|
except Poll.DoesNotExist:
|
|
|
|
|
raise Http404
|
|
|
|
|
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.
|
|
|
|
|
|
|
|
|
|
Customizing error views
|
|
|
|
|
=======================
|
|
|
|
|
|
|
|
|
|
The 404 (page not found) view
|
|
|
|
|
-----------------------------
|
|
|
|
|
|
|
|
|
|
When you raise an ``Http404`` exception, Django loads a special view devoted
|
|
|
|
|
to handling 404 errors. By default, it's the view
|
|
|
|
|
``django.views.defaults.page_not_found``, which loads and renders the template
|
|
|
|
|
``404.html``.
|
|
|
|
|
|
|
|
|
|
This means you need to define a ``404.html`` template in your root template
|
|
|
|
|
directory. This template will be used for all 404 errors.
|
|
|
|
|
|
|
|
|
|
This ``page_not_found`` view should suffice for 99% of Web applications, but if
|
|
|
|
|
you want to override the 404 view, you can specify ``handler404`` in your
|
|
|
|
|
URLconf, like so::
|
|
|
|
|
|
|
|
|
|
handler404 = 'mysite.views.my_custom_404_view'
|
|
|
|
|
|
|
|
|
|
Behind the scenes, Django determines the 404 view by looking for ``handler404``.
|
|
|
|
|
By default, URLconfs contain the following line::
|
|
|
|
|
|
|
|
|
|
from django.conf.urls.defaults import *
|
|
|
|
|
|
|
|
|
|
That takes care of setting ``handler404`` in the current module. As you can see
|
|
|
|
|
in ``django/conf/urls/defaults.py``, ``handler404`` is set to
|
|
|
|
|
``'django.views.defaults.page_not_found'`` by default.
|
|
|
|
|
|
|
|
|
|
Three things to note about 404 views:
|
|
|
|
|
|
|
|
|
|
* The 404 view is also called if Django doesn't find a match after checking
|
|
|
|
|
every regular expression in the URLconf.
|
|
|
|
|
|
|
|
|
|
* If you don't define your own 404 view -- and simply use the
|
|
|
|
|
default, which is recommended -- you still have one obligation:
|
|
|
|
|
you must create a ``404.html`` template in the root of your
|
|
|
|
|
template directory. The default 404 view will use that template
|
|
|
|
|
for all 404 errors. The default 404 view will pass one variable
|
|
|
|
|
to the template: ``request_path``, which is the URL that resulted
|
|
|
|
|
in the 404.
|
|
|
|
|
|
2010-11-27 00:53:38 +08:00
|
|
|
* The 404 view is passed a :class:`~django.template.RequestContext` and
|
|
|
|
|
will have access to variables supplied by your
|
|
|
|
|
:setting:`TEMPLATE_CONTEXT_PROCESSORS` setting (e.g.,
|
|
|
|
|
:setting:`MEDIA_URL`).
|
2008-08-24 06:25:40 +08:00
|
|
|
|
2010-11-27 00:53:38 +08:00
|
|
|
* If :setting:`DEBUG` is set to ``True`` (in your settings module), then
|
|
|
|
|
your 404 view will never be used, and the traceback will be displayed
|
|
|
|
|
instead.
|
2008-08-24 06:25:40 +08:00
|
|
|
|
|
|
|
|
The 500 (server error) view
|
|
|
|
|
----------------------------
|
|
|
|
|
|
|
|
|
|
Similarly, Django executes special-case behavior in the case of runtime errors
|
|
|
|
|
in view code. If a view results in an exception, Django will, by default, call
|
|
|
|
|
the view ``django.views.defaults.server_error``, which loads and renders the
|
|
|
|
|
template ``500.html``.
|
|
|
|
|
|
|
|
|
|
This means you need to define a ``500.html`` template in your root template
|
|
|
|
|
directory. This template will be used for all server errors. The default 500
|
|
|
|
|
view passes no variables to this template and is rendered with an empty
|
|
|
|
|
``Context`` to lessen the chance of additional errors.
|
|
|
|
|
|
|
|
|
|
This ``server_error`` view should suffice for 99% of Web applications, but if
|
|
|
|
|
you want to override the view, you can specify ``handler500`` in your
|
|
|
|
|
URLconf, like so::
|
|
|
|
|
|
|
|
|
|
handler500 = 'mysite.views.my_custom_error_view'
|
|
|
|
|
|
|
|
|
|
Behind the scenes, Django determines the error view by looking for ``handler500``.
|
|
|
|
|
By default, URLconfs contain the following line::
|
|
|
|
|
|
|
|
|
|
from django.conf.urls.defaults import *
|
|
|
|
|
|
|
|
|
|
That takes care of setting ``handler500`` in the current module. As you can see
|
|
|
|
|
in ``django/conf/urls/defaults.py``, ``handler500`` is set to
|
|
|
|
|
``'django.views.defaults.server_error'`` by default.
|
