=============
Writing views
=============

A view function, or *view* for short, is 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 :class:`~django.http.HttpResponse` from the
  :mod:`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 :class:`~django.http.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 :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.)

.. admonition:: Django's Time Zone

    Django includes a :setting:`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 :doc:`/topics/http/urls` for instructions.

Returning errors
================

Django provides help for returning HTTP error codes. 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. Return an instance of one of those subclasses instead of a
normal :class:`~django.http.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 :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::

    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 :class:`~django.http.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 :setting:`DEBUG` is set
to ``False``.

When :setting:`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:

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 the
handlers as seen below in your URLconf (setting them anywhere else will have no
effect).

The :func:`~django.views.defaults.page_not_found` view is overridden by
:data:`~django.conf.urls.handler404`::

    handler404 = 'mysite.views.my_custom_page_not_found_view'

The :func:`~django.views.defaults.server_error` view is overridden by
:data:`~django.conf.urls.handler500`::

    handler500 = 'mysite.views.my_custom_error_view'

The :func:`~django.views.defaults.permission_denied` view is overridden by
:data:`~django.conf.urls.handler403`::

    handler403 = 'mysite.views.my_custom_permission_denied_view'

The :func:`~django.views.defaults.bad_request` view is overridden by
:data:`~django.conf.urls.handler400`::

    handler400 = 'mysite.views.my_custom_bad_request_view'

.. seealso::

    Use the :setting:`CSRF_FAILURE_VIEW` setting to override the CSRF error
    view.

Testing custom error views
--------------------------

To test the response of a custom error handler, raise the appropriate exception
in 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)

.. _async-views:

Async views
===========

As well as being synchronous functions, views can also be asynchronous
("async") functions, normally defined using Python's ``async def`` syntax.
Django will automatically detect these and run them in an async context.
However, you will need to use an async server based on ASGI to get their
performance benefits.

Here's an example of an async view::

    import datetime
    from django.http import HttpResponse

    async def current_datetime(request):
        now = datetime.datetime.now()
        html = '<html><body>It is now %s.</body></html>' % now
        return HttpResponse(html)

You can read more about Django's async support, and how to best use async
views, in :doc:`/topics/async`.
