317 lines
13 KiB
Plaintext
317 lines
13 KiB
Plaintext
=================================
|
|
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 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 returns a
|
|
function that can be called when a request arrives for a URL matching the
|
|
associated pattern. The function creates an instance of the class, calls
|
|
:meth:`~django.views.generic.base.View.setup` to initialize its attributes, and
|
|
then 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.urls import path
|
|
from myapp.views import MyView
|
|
|
|
urlpatterns = [
|
|
path('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 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 = [
|
|
path('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.
|
|
|
|
|
|
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 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 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 = [
|
|
path('about/', login_required(TemplateView.as_view(template_name="secret.html"))),
|
|
path('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().dispatch(*args, **kwargs)
|
|
|
|
Or, more succinctly, you can decorate the class instead and pass the name
|
|
of the method to be decorated as the keyword argument ``name``::
|
|
|
|
@method_decorator(login_required, name='dispatch')
|
|
class ProtectedView(TemplateView):
|
|
template_name = 'secret.html'
|
|
|
|
If you have a set of common decorators used in several places, you can define
|
|
a list or tuple of decorators and use this instead of invoking
|
|
``method_decorator()`` multiple times. These two classes are equivalent::
|
|
|
|
decorators = [never_cache, login_required]
|
|
|
|
@method_decorator(decorators, name='dispatch')
|
|
class ProtectedView(TemplateView):
|
|
template_name = 'secret.html'
|
|
|
|
@method_decorator(never_cache, name='dispatch')
|
|
@method_decorator(login_required, name='dispatch')
|
|
class ProtectedView(TemplateView):
|
|
template_name = 'secret.html'
|
|
|
|
The decorators will process a request in the order they are passed to the
|
|
decorator. In the example, ``never_cache()`` will process the request before
|
|
``login_required()``.
|
|
|
|
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.
|