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.
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, which lives in 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:
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:
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.
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
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.
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:
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.
Sep 20, 2009