Django2.0手册: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 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.

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 URL调度器 for instructions.

Returning errors¶

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.

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 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
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 an
HTML template named 404.html and place it in the top level of your
template tree. This template will then be served when DEBUG is set
to False.

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

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 page_not_found() view is overridden by
handler404:

handler404 = 'mysite.views.my_custom_page_not_found_view'

The server_error() view is overridden by
handler500:

handler500 = 'mysite.views.my_custom_error_view'

The permission_denied() view is overridden by
handler403:

handler403 = 'mysite.views.my_custom_permission_denied_view'

The bad_request() view is overridden by
handler400:

handler400 = 'mysite.views.my_custom_bad_request_view'

See also

Use the CSRF_FAILURE_VIEW setting to override the CSRF error
view.