Writing views

Writing views

A view function, or view for short, is a Python function that takes aWeb request and returns a Web response. This response can be the HTML contentsof 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 isnecessary to return that response. This code can live anywhere you want, as longas it’s on your Python path. There’s no other requirement–no “magic,” so tospeak. For the sake of putting the code somewhere, the convention is toput views in a file called views.py, placed in your project orapplication 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 HttpResponse from thedjango.http module, along with Python’s datetime library.
Next, we define a function called current_datetime. This is the viewfunction. Each view function takes an HttpRequestobject as its first parameter, which is typically named request.

Note that the name of the view function doesn’t matter; it doesn’t have tobe named in a certain way in order for Django to recognize it. We’recalling it current_datetime here, because that name clearly indicateswhat it does.

The view returns an HttpResponse object thatcontains the generated response. Each view function is responsible forreturning an HttpResponse object. (There areexceptions, but we’ll get to those later.)

Django’s Time Zone

Django includes a TIME_ZONE setting that defaults toAmerica/Chicago. This probably isn’t where you live, so you might wantto change it in your settings file.

Mapping URLs to views

So, to recap, this view function returns an HTML page that includes the currentdate and time. To display this view at a particular URL, you’ll need to create aURLconf; see URL dispatcher for instructions.

Returning errors

Django provides help for returning HTTP error codes. There are subclasses ofHttpResponse for a number of common HTTP status codesother than 200 (which means “OK”). You can find the full list of availablesubclasses in the request/responsedocumentation. Return an instance of one of those subclasses instead of anormal HttpResponse in order to signify an error. Forexample:

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 inthe HttpResponse documentation, you can also pass theHTTP status code into the constructor for HttpResponseto 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 wayto handle those errors.

The Http404 exception

class 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 pageacross your site, Django provides an Http404 exception. If you raiseHttp404 at any point in a view function, Django will catch it and return thestandard error page for your application, along with an HTTP error code 404.

Example usage:

from django.http import Http404
from django.shortcuts import render
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(request, 'polls/detail.html', {'poll': p})

In order to show customized HTML when Django returns a 404, you can create anHTML template named 404.html and place it in the top level of yourtemplate tree. This template will then be served when DEBUG is setto False.

When DEBUG is True, you can provide a message to Http404 andit will appear in the standard 404 debug template. Use these messages fordebugging purposes; they generally aren’t suitable for use in a production 404template.

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. Specify thehandlers as seen below in your URLconf (setting them anywhere else will have noeffect).

The page_not_found() view is overridden byhandler404:

handler404 = 'mysite.views.my_custom_page_not_found_view'

The server_error() view is overridden byhandler500:

handler500 = 'mysite.views.my_custom_error_view'

The permission_denied() view is overridden byhandler403:

handler403 = 'mysite.views.my_custom_permission_denied_view'

The bad_request() view is overridden byhandler400:

handler400 = 'mysite.views.my_custom_bad_request_view'

Testing custom error views

To test the response of a custom error handler, raise the appropriate exceptionin a test view. For example:

from django.core.exceptions import PermissionDenied
from django.http import HttpResponse
from django.test import SimpleTestCase, override_settings
from django.urls import path
 
 
def response_error_handler(request, exception=None):
    return HttpResponse('Error handler content', status=403)
 
 
def permission_denied_view(request):
    raise PermissionDenied
 
 
urlpatterns = [
    path('403/', permission_denied_view),
]
 
handler403 = response_error_handler
 
 
# ROOT_URLCONF must specify the module that contains handler403 = ...
@override_settings(ROOT_URLCONF=__name__)
class CustomErrorHandlerTests(SimpleTestCase):
 
    def test_handler_renders_template_response(self):
        response = self.client.get('/403/')
        # Make assertions on the response here. For example:
        self.assertContains(response, 'Error handler content', status_code=403)