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

.. versionadded:: 1.3

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 simple 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

   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 in to the URLs, HTTP method dispatching and other
simple features. :class:`~django.views.generic.base.RedirectView` is for a simple HTTP
redirect, and :class:`~django.views.generic.base.TemplateView` extends the base class
to make it also render a template.


Simple usage
============

Class-based generic views (and any class-based views that inherit from
the base classes Django provides) can be configured in two
ways: subclassing, or passing in arguments directly in the URLconf.

When you subclass a class-based view, you can 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 just 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 just need to add this new view into our URLconf. As the class-based
views themselves are classes, we point the URL to the ``as_view`` class method
instead, which is the entry point for class-based views::

    # urls.py
    from django.conf.urls import patterns, url, include
    from some_app.views import AboutView

    urlpatterns = patterns('',
        (r'^about/', AboutView.as_view()),
    )

Alternatively, if you're only changing a few simple attributes on a
class-based view, you can simply pass the new attributes into the ``as_view``
method call itself::

    from django.conf.urls import patterns, url, include
    from django.views.generic import TemplateView

    urlpatterns = patterns('',
        (r'^about/', TemplateView.as_view(template_name="about.html")),
    )

A similar overriding pattern can be used for the ``url`` attribute on
:class:`~django.views.generic.base.RedirectView`.

.. _jsonresponsemixin-example:

More than just HTML
-------------------

Where class based views shine is when you want to do the same thing many times.
Suppose you're writing an API, and every view should return JSON instead of
rendered HTML.

We can create a mixin class to use in all of our views, handling the
conversion to JSON once.

For example, a simple JSON mixin might look something like this::

    import json
    from django.http import HttpResponse

    class JSONResponseMixin(object):
        """
        A mixin that can be used to render a JSON response.
        """
        response_class = HttpResponse

        def render_to_response(self, context, **response_kwargs):
            """
            Returns a JSON response, transforming 'context' to make the payload.
            """
            response_kwargs['content_type'] = 'application/json'
            return self.response_class(
                self.convert_context_to_json(context),
                **response_kwargs
            )

        def convert_context_to_json(self, context):
            "Convert the context dictionary into a JSON object"
            # Note: This is *EXTREMELY* naive; in reality, you'll need
            # to do much more complex handling to ensure that arbitrary
            # objects -- such as Django model instances or querysets
            # -- can be serialized as JSON.
            return json.dumps(context)

Now we mix this into the base TemplateView::

    from django.views.generic import TemplateView

    class JSONView(JSONResponseMixin, TemplateView):
        pass

Equally we could use our mixin with one of the generic views. We can make our
own version of :class:`~django.views.generic.detail.DetailView` by mixing
:class:`JSONResponseMixin` with the
:class:`~django.views.generic.detail.BaseDetailView` -- (the
:class:`~django.views.generic.detail.DetailView` before template
rendering behavior has been mixed in)::

    class JSONDetailView(JSONResponseMixin, BaseDetailView):
        pass

This view can then be deployed in the same way as any other
:class:`~django.views.generic.detail.DetailView`, with exactly the
same behavior -- except for the format of the response.

If you want to be really adventurous, you could even mix a
:class:`~django.views.generic.detail.DetailView` subclass that is able
to return *both* HTML and JSON content, depending on some property of
the HTTP request, such as a query argument or a HTTP header. Just mix
in both the :class:`JSONResponseMixin` and a
:class:`~django.views.generic.detail.SingleObjectTemplateResponseMixin`,
and override the implementation of :func:`render_to_response()` to defer
to the appropriate subclass depending on the type of response that the user
requested::

    class HybridDetailView(JSONResponseMixin, SingleObjectTemplateResponseMixin, BaseDetailView):
        def render_to_response(self, context):
            # Look for a 'format=json' GET argument
            if self.request.GET.get('format','html') == 'json':
                return JSONResponseMixin.render_to_response(self, context)
            else:
                return SingleObjectTemplateResponseMixin.render_to_response(self, context)

Because of the way that Python resolves method overloading, the local
``render_to_response()`` implementation will override the versions provided by
:class:`JSONResponseMixin` and
:class:`~django.views.generic.detail.SingleObjectTemplateResponseMixin`.

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>`.

Decorating class-based views
============================

.. highlightlang:: python

The extension of class-based views isn't limited to using mixins. You
can use also use decorators.

Decorating in URLconf
---------------------

The simplest way of decorating class-based views is to decorate the
result of the :meth:`~django.views.generic.base.View.as_view` method.
The easiest place to do this is in the URLconf where you deploy your
view::

    from django.contrib.auth.decorators import login_required, permission_required
    from django.views.generic import TemplateView

    from .views import VoteView

    urlpatterns = patterns('',
        (r'^about/', login_required(TemplateView.as_view(template_name="secret.html"))),
        (r'^vote/', permission_required('polls.can_vote')(VoteView.as_view())),
    )

This approach applies the decorator on a per-instance basis. If you
want every instance of a view to be decorated, you need to take a
different approach.

.. _decorating-class-based-views:

Decorating the class
--------------------

To decorate every instance of a class-based view, you need to decorate
the class definition itself. To do this you apply the decorator to the
:meth:`~django.views.generic.base.View.dispatch` method of the class.

A method on a class isn't quite the same as a standalone function, so
you can't just apply a function decorator to the method -- you need to
transform it into a method decorator first. The ``method_decorator``
decorator transforms a function decorator into a method decorator so
that it can be used on an instance method. For example::

    from django.contrib.auth.decorators import login_required
    from django.utils.decorators import method_decorator
    from django.views.generic import TemplateView

    class ProtectedView(TemplateView):
        template_name = 'secret.html'

        @method_decorator(login_required)
        def dispatch(self, *args, **kwargs):
            return super(ProtectedView, self).dispatch(*args, **kwargs)

In this example, every instance of ``ProtectedView`` will have
login protection.

.. note::

    ``method_decorator`` passes ``*args`` and ``**kwargs``
    as parameters to the decorated method on the class. If your method
    does not accept a compatible set of parameters it will raise a
    ``TypeError`` exception.
