intro.txt

=================================
Introduction to Class-based views
=================================

Class-based views provide an alternative way to implement views as Python
objects instead of functions. They do not replace function-based views, but
have certain differences and advantages when compared to function-based views:

* Organization of code related to specific HTTP methods (``GET``, ``POST``,
  etc) can be addressed by separate methods instead of conditional branching.

* Object oriented techniques such as mixins (multiple inheritance) can be
  used to factor code into reusable components.

The relationship and history of generic views, class-based views, and class-based generic views
===============================================================================================

In the beginning there was only the view function contract, Django passed your
function an :class:`~django.http.HttpRequest` and expected back an
:class:`~django.http.HttpResponse`. This was the extent of what Django provided.

Early on it was recognized that there were common idioms and patterns found in
view development. Function-based generic views were introduced to abstract
these patterns and ease view development for the common cases.

The problem with function-based generic views is that while they covered the
simple cases well, there was no way to extend or customize them beyond some
simple configuration options, limiting their usefulness in many real-world
applications.

Class-based generic views were created with the same objective as
function-based generic views, to make view development easier. However, the way
the solution is implemented, through the use of mixins, provides a toolkit that
results in class-based generic views being more extensible and flexible than
their function-based counterparts.

If you have tried function based generic views in the past and found them
lacking, you should not think of class-based generic views as simply a
class-based equivalent, but rather as a fresh approach to solving the original
problems that generic views were meant to solve.

The toolkit of base classes and mixins that Django uses to build class-based
generic views are built for maximum flexibility, and as such have many hooks in
the form of default method implementations and attributes that you are unlikely
to be concerned with in the simplest use cases. For example, instead of
limiting you to a class based attribute for ``form_class``, the implementation
uses a ``get_form`` method, which calls a ``get_form_class`` method, which in
its default implementation just returns the ``form_class`` attribute of the
class. This gives you several options for specifying what form to use, from a
simple attribute, to a fully dynamic, callable hook. These options seem to add
hollow complexity for simple situations, but without them, more advanced
designs would be limited.

Using class-based views
=======================

At its core, a class-based view allows you to respond to different HTTP request
methods with different class instance methods, instead of with conditionally
branching code inside a single view function.

So where the code to handle HTTP ``GET`` in a view function would look
something like::

    from django.http import HttpResponse

    def my_view(request):
        if request.method == 'GET':
            # <view logic>
            return HttpResponse('result')

In a class-based view, this would become::

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

    class MyView(View):
        def get(self, request):
            # <view logic>
            return HttpResponse('result')

Because Django's URL resolver expects to send the request and associated
arguments to a callable function, not a class, class-based views have an
:meth:`~django.views.generic.base.View.as_view` class method which serves as
the callable entry point to your class. The ``as_view`` entry point creates an
instance of your class and calls its
:meth:`~django.views.generic.base.View.dispatch` method. ``dispatch`` looks at
the request to determine whether it is a ``GET``, ``POST``, etc, and relays the
request to a matching method if one is defined, or raises
:class:`~django.http.HttpResponseNotAllowed` if not::

    # urls.py
    from django.conf.urls import patterns
    from myapp.views import MyView

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


It is worth noting that what your method returns is identical to what you
return from a function-based view, namely some form of
:class:`~django.http.HttpResponse`. This means that
:doc:`http shortcuts </topics/http/shortcuts>` or
:class:`~django.template.response.TemplateResponse` objects are valid to use
inside a class-based view.

While a minimal class-based view does not require any class attributes to
perform its job, class attributes are useful in many class-based designs,
and there are two ways to configure or set class attributes.

The first is the standard Python way of subclassing and overriding attributes
and methods in the subclass. So that if your parent class had an attribute
``greeting`` like this::

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

    class GreetingView(View):
        greeting = "Good Day"

        def get(self, request):
            return HttpResponse(self.greeting)

You can override that in a subclass::

    class MorningGreetingView(GreetingView):
        greeting = "Morning to ya"

Another option is to configure class attributes as keyword arguments to the
:meth:`~django.views.generic.base.View.as_view` call in the URLconf::

    urlpatterns = patterns('',
        (r'^about/', GreetingView.as_view(greeting="G'day")),
    )

.. note::

    While your class is instantiated for each request dispatched to it, class
    attributes set through the
    :meth:`~django.views.generic.base.View.as_view` entry point are
    configured only once at the time your URLs are imported.

Using mixins
============

Mixins are a form of multiple inheritance where behaviors and attributes of
multiple parent classes can be combined.

For example, in the generic class-based views there is a mixin called
:class:`~django.views.generic.base.TemplateResponseMixin` whose primary purpose
is to define the method
:meth:`~django.views.generic.base.TemplateResponseMixin.render_to_response`.
When combined with the behavior of the :class:`~django.views.generic.base.View`
base class, the result is a :class:`~django.views.generic.base.TemplateView`
class that will dispatch requests to the appropriate matching methods (a
behavior defined in the ``View`` base class), and that has a
:meth:`~django.views.generic.base.TemplateResponseMixin.render_to_response`
method that uses a
:attr:`~django.views.generic.base.TemplateResponseMixin.template_name`
attribute to return a :class:`~django.template.response.TemplateResponse`
object (a behavior defined in the ``TemplateResponseMixin``).

Mixins are an excellent way of reusing code across multiple classes, but they
come with some cost. The more your code is scattered among mixins, the harder
it will be to read a child class and know what exactly it is doing, and the
harder it will be to know which methods from which mixins to override if you
are subclassing something that has a deep inheritance tree.

Note also that you can only inherit from one generic view - that is, only one
parent class may inherit from :class:`~django.views.generic.base.View` and
the rest (if any) should be mixins. Trying to inherit from more than one class
that inherits from ``View`` - for example, trying to use a form at the top of a
list and combining :class:`~django.views.generic.edit.ProcessFormView` and
:class:`~django.views.generic.list.ListView` - won't work as expected.

.. _mixins_that_wrap_as_view:

Mixins that wrap ``as_view()``
------------------------------

One way to apply common behavior to many classes is to write a mixin that wraps
the :meth:`~django.views.generic.base.View.as_view()` method.

For example, if you have many generic views that should be decorated with
:func:`~django.contrib.auth.decorators.login_required` you could implement a
mixin like this::

    from django.contrib.auth.decorators import login_required

    class LoginRequiredMixin(object):
        @classmethod
        def as_view(cls, **initkwargs):
            view = super(LoginRequiredMixin, cls).as_view(**initkwargs)
            return login_required(view)

    class MyView(LoginRequiredMixin, ...):
        # this is a generic view
        ...

Handling forms with class-based views
=====================================

A basic function-based view that handles forms may look something like this::

    from django.http import HttpResponseRedirect
    from django.shortcuts import render

    from .forms import MyForm

    def myview(request):
        if request.method == "POST":
            form = MyForm(request.POST)
            if form.is_valid():
                # <process form cleaned data>
                return HttpResponseRedirect('/success/')
        else:
            form = MyForm(initial={'key': 'value'})

        return render(request, 'form_template.html', {'form': form})

A similar class-based view might look like::

    from django.http import HttpResponseRedirect
    from django.shortcuts import render
    from django.views.generic import View

    from .forms import MyForm

    class MyFormView(View):
        form_class = MyForm
        initial = {'key': 'value'}
        template_name = 'form_template.html'

        def get(self, request, *args, **kwargs):
            form = self.form_class(initial=self.initial)
            return render(request, self.template_name, {'form': form})

        def post(self, request, *args, **kwargs):
            form = self.form_class(request.POST)
            if form.is_valid():
                # <process form cleaned data>
                return HttpResponseRedirect('/success/')

            return render(request, self.template_name, {'form': form})

This is a very simple case, but you can see that you would then have the option
of customizing this view by overriding any of the class attributes, e.g.
``form_class``, via URLconf configuration, or subclassing and overriding one or
more of the methods (or both!).

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

The extension of class-based views isn't limited to using mixins. You
can use also use decorators. Since class-based views aren't functions,
decorating them works differently depending on if you're using ``as_view`` or
creating a subclass.

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.