=================
Class-based views
=================

A view is a callable which takes a request and returns a
response. This can be more than just a function, and Django provides
an example of some classes which can be used as views. These allow you
to structure your views and reuse code by harnessing inheritance and
mixins. There are also some generic views for tasks which we'll get to later,
but you may want to design your own structure of reusable views which suits
your use case. For full details, see the :doc:`class-based views reference
documentation</ref/class-based-views/index>`.

.. toctree::
   :maxdepth: 1

   intro
   generic-display
   generic-editing
   mixins

Basic examples
==============

Django provides base view classes which will suit a wide range of applications.
All views inherit from the :class:`~django.views.generic.base.View` class,
which handles linking the view into the URLs, HTTP method dispatching and other
common features. :class:`~django.views.generic.base.RedirectView` provides a
HTTP redirect, and :class:`~django.views.generic.base.TemplateView` extends the
base class to make it also render a template.


Usage in your URLconf
=====================

The most direct way to use generic views is to create them directly in your
URLconf. If you're only changing a few attributes on a class-based view, you
can pass them into the :meth:`~django.views.generic.base.View.as_view` method
call itself::

    from django.urls import path
    from django.views.generic import TemplateView

    urlpatterns = [
        path("about/", TemplateView.as_view(template_name="about.html")),
    ]

Any arguments passed to :meth:`~django.views.generic.base.View.as_view` will
override attributes set on the class. In this example, we set ``template_name``
on the ``TemplateView``. A similar overriding pattern can be used for the
``url`` attribute on :class:`~django.views.generic.base.RedirectView`.


Subclassing generic views
=========================

The second, more powerful way to use generic views is to inherit from an
existing view and override attributes (such as the ``template_name``) or
methods (such as ``get_context_data``) in your subclass to provide new values
or methods. Consider, for example, a view that just displays one template,
``about.html``. Django has a generic view to do this -
:class:`~django.views.generic.base.TemplateView` - so we can subclass it, and
override the template name::

    # some_app/views.py
    from django.views.generic import TemplateView


    class AboutView(TemplateView):
        template_name = "about.html"

Then we need to add this new view into our URLconf.
:class:`~django.views.generic.base.TemplateView` is a class, not a function, so
we point the URL to the :meth:`~django.views.generic.base.View.as_view` class
method instead, which provides a function-like entry to class-based views::

    # urls.py
    from django.urls import path
    from some_app.views import AboutView

    urlpatterns = [
        path("about/", AboutView.as_view()),
    ]


For more information on how to use the built in generic views, consult the next
topic on
:doc:`generic class-based views</topics/class-based-views/generic-display>`.

.. _supporting-other-http-methods:

Supporting other HTTP methods
-----------------------------

Suppose somebody wants to access our book library over HTTP using the views
as an API. The API client would connect every now and then and download book
data for the books published since last visit. But if no new books appeared
since then, it is a waste of CPU time and bandwidth to fetch the books from the
database, render a full response and send it to the client. It might be
preferable to ask the API when the most recent book was published.

We map the URL to book list view in the URLconf::

    from django.urls import path
    from books.views import BookListView

    urlpatterns = [
        path("books/", BookListView.as_view()),
    ]

And the view::

    from django.http import HttpResponse
    from django.views.generic import ListView
    from books.models import Book


    class BookListView(ListView):
        model = Book

        def head(self, *args, **kwargs):
            last_book = self.get_queryset().latest("publication_date")
            response = HttpResponse(
                # RFC 1123 date format.
                headers={
                    "Last-Modified": last_book.publication_date.strftime(
                        "%a, %d %b %Y %H:%M:%S GMT"
                    )
                },
            )
            return response

If the view is accessed from a ``GET`` request, an object list is returned in
the response (using the ``book_list.html`` template). But if the client issues
a ``HEAD`` request, the response has an empty body and the ``Last-Modified``
header indicates when the most recent book was published. Based on this
information, the client may or may not download the full object list.

.. _async-class-based-views:

Asynchronous class-based views
==============================

As well as the synchronous (``def``) method handlers already shown, ``View``
subclasses may define asynchronous (``async def``) method handlers to leverage
asynchronous code using ``await``::

    import asyncio
    from django.http import HttpResponse
    from django.views import View


    class AsyncView(View):
        async def get(self, request, *args, **kwargs):
            # Perform io-blocking view logic using await, sleep for example.
            await asyncio.sleep(1)
            return HttpResponse("Hello async world!")

Within a single view-class, all user-defined method handlers must be either
synchronous, using ``def``, or all asynchronous, using ``async def``. An
``ImproperlyConfigured`` exception will be raised in ``as_view()`` if ``def``
and ``async def`` declarations are mixed.

Django will automatically detect asynchronous views and run them in an
asynchronous context. You can read more about Django's asynchronous support,
and how to best use async views, in :doc:`/topics/async`.
