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.
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 HttpResponse from the
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 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 HttpResponse object that
contains the generated response. Each view function is responsible for
returning an HttpResponse object. (There are
exceptions, but we’ll get to those later.)
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.
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 URL dispatcher for instructions.
Returning HTTP error codes in Django is easy. There are subclasses of
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 request/response
documentation. Just return an instance of one of those subclasses instead of
a normal 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 HttpResponse documentation, you can also pass the
HTTP status code into the constructor for 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.
django.http.Http404¶When you return an error such as 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
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.
django.views.defaults.page_not_found(request, template_name='404.html')¶When you raise Http404 from within a view, Django loads a
special view devoted to handling 404 errors. By default, it’s the view
django.views.defaults.page_not_found(), which either produces a very
simple “Not Found” message or loads and renders the template 404.html if
you created it in your root template directory.
The default 404 view will pass one variable to the template: request_path,
which is the URL that resulted in the error.
The page_not_found view should suffice for 99% of Web applications, but if
you want to override it, you can specify handler404
in your root URLconf (setting handler404 anywhere else will have no
effect), like so:
handler404 = 'mysite.views.my_custom_404_view'
Behind the scenes, Django determines the 404 view by looking for
handler404 in your root URLconf, and falling back to
django.views.defaults.page_not_found if you did not define one.
Three things to note about 404 views:
RequestContext and
will have access to variables supplied by your
TEMPLATE_CONTEXT_PROCESSORS setting (e.g., MEDIA_URL).DEBUG is set to True (in your settings module), then
your 404 view will never be used, and your URLconf will be displayed
instead, with some debug information.django.views.defaults.server_error(request, template_name='500.html')¶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 either produces a very
simple “Server Error” message or loads and renders the template 500.html if
you created it in your root template directory.
The default 500 view passes no variables to the 500.html 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 root URLconf, like so:
handler500 = 'mysite.views.my_custom_error_view'
Behind the scenes, Django determines the 500 view by looking for
handler500 in your root URLconf, and falling back to
django.views.defaults.server_error if you did not define one.
One thing to note about 500 views:
DEBUG is set to True (in your settings module), then
your 500 view will never be used, and the traceback will be displayed
instead, with some debug information.django.views.defaults.permission_denied(request, template_name='403.html')¶In the same vein as the 404 and 500 views, Django has a view to handle 403
Forbidden errors. If a view results in a 403 exception then Django will, by
default, call the view django.views.defaults.permission_denied.
This view loads and renders the template 403.html in your root template
directory, or if this file does not exist, instead serves the text
“403 Forbidden”, as per RFC 2616 (the HTTP 1.1 Specification).
django.views.defaults.permission_denied is triggered by a
PermissionDenied exception. To deny access in a
view you can use code like this:
from django.core.exceptions import PermissionDenied
def edit(request, pk):
if not request.user.is_staff:
raise PermissionDenied
# ...
It is possible to override django.views.defaults.permission_denied in the
same way you can for the 404 and 500 views by specifying a
handler403 in your root URLconf:
handler403 = 'mysite.views.my_custom_permission_denied_view'
django.views.defaults.bad_request(request, template_name='400.html')¶When a SuspiciousOperation is raised in Django,
it may be handled by a component of Django (for example resetting the session
data). If not specifically handled, Django will consider the current request a
‘bad request’ instead of a server error.
django.views.defaults.bad_request, is otherwise very similar to the
server_error view, but returns with the status code 400 indicating that
the error condition was the result of a client operation.
Like server_error, the default bad_request should suffice for
99% of Web applications, but if you want to override the view, you can specify
handler400 in your root URLconf, like so:
handler400 = 'mysite.views.my_custom_bad_request_view'
bad_request views are also only used when DEBUG is False.
Feb 08, 2017