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290 lines
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290 lines
12 KiB
Plaintext
=================================
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Introduction to Class-based views
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=================================
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Class-based views provide an alternative way to implement views as Python
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objects instead of functions. They do not replace function-based views, but
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have certain differences and advantages when compared to function-based views:
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* Organization of code related to specific HTTP methods (``GET``, ``POST``,
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etc) can be addressed by separate methods instead of conditional branching.
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* Object oriented techniques such as mixins (multiple inheritance) can be
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used to factor code into reusable components.
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The relationship and history of generic views, class-based views, and class-based generic views
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===============================================================================================
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In the beginning there was only the view function contract, Django passed your
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function an :class:`~django.http.HttpRequest` and expected back an
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:class:`~django.http.HttpResponse`. This was the extent of what Django provided.
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Early on it was recognized that there were common idioms and patterns found in
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view development. Function-based generic views were introduced to abstract
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these patterns and ease view development for the common cases.
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The problem with function-based generic views is that while they covered the
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simple cases well, there was no way to extend or customize them beyond some
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simple configuration options, limiting their usefulness in many real-world
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applications.
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Class-based generic views were created with the same objective as
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function-based generic views, to make view development easier. However, the way
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the solution is implemented, through the use of mixins, provides a toolkit that
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results in class-based generic views being more extensible and flexible than
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their function-based counterparts.
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If you have tried function based generic views in the past and found them
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lacking, you should not think of class-based generic views as simply a
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class-based equivalent, but rather as a fresh approach to solving the original
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problems that generic views were meant to solve.
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The toolkit of base classes and mixins that Django uses to build class-based
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generic views are built for maximum flexibility, and as such have many hooks in
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the form of default method implementations and attributes that you are unlikely
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to be concerned with in the simplest use cases. For example, instead of
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limiting you to a class based attribute for ``form_class``, the implementation
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uses a ``get_form`` method, which calls a ``get_form_class`` method, which in
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its default implementation just returns the ``form_class`` attribute of the
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class. This gives you several options for specifying what form to use, from a
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simple attribute, to a fully dynamic, callable hook. These options seem to add
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hollow complexity for simple situations, but without them, more advanced
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designs would be limited.
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Using class-based views
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=======================
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At its core, a class-based view allows you to respond to different HTTP request
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methods with different class instance methods, instead of with conditionally
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branching code inside a single view function.
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So where the code to handle HTTP ``GET`` in a view function would look
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something like::
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from django.http import HttpResponse
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def my_view(request):
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if request.method == 'GET':
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# <view logic>
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return HttpResponse('result')
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In a class-based view, this would become::
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from django.http import HttpResponse
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from django.views.base import View
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class MyView(View):
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def get(self, request):
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# <view logic>
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return HttpResponse('result')
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Because Django's URL resolver expects to send the request and associated
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arguments to a callable function, not a class, class-based views have an
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:meth:`~django.views.generic.base.View.as_view` class method which serves as
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the callable entry point to your class. The ``as_view`` entry point creates an
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instance of your class and calls its
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:meth:`~django.views.generic.base.View.dispatch` method. ``dispatch`` looks at
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the request to determine whether it is a ``GET``, ``POST``, etc, and relays the
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request to a matching method if one is defined, or raises
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:class:`~django.http.HttpResponseNotAllowed` if not::
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# urls.py
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from django.conf.urls import patterns
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from myapp.views import MyView
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urlpatterns = patterns('',
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(r'^about/', MyView.as_view()),
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)
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It is worth noting that what your method returns is identical to what you
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return from a function-based view, namely some form of
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:class:`~django.http.HttpResponse`. This means that
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:doc:`http shortcuts </topics/http/shortcuts>` or
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:class:`~django.template.response.TemplateResponse` objects are valid to use
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inside a class-based view.
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While a minimal class-based view does not require any class attributes to
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perform its job, class attributes are useful in many class-based designs,
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and there are two ways to configure or set class attributes.
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The first is the standard Python way of subclassing and overriding attributes
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and methods in the subclass. So that if your parent class had an attribute
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``greeting`` like this::
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from django.http import HttpResponse
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from django.views.base import View
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class GreetingView(View):
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greeting = "Good Day"
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def get(self, request):
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return HttpResponse(self.greeting)
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You can override that in a subclass::
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class MorningGreetingView(GreetingView):
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greeting = "Morning to ya"
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Another option is to configure class attributes as keyword arguments to the
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:meth:`~django.views.generic.base.View.as_view` call in the URLconf::
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urlpatterns = patterns('',
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(r'^about/', MyView.as_view(greeting="G'day")),
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)
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.. note::
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While your class is instantiated for each request dispatched to it, class
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attributes set through the
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:meth:`~django.views.generic.base.View.as_view` entry point are
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configured only once at the time your URLs are imported.
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Using mixins
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============
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Mixins are a form of multiple inheritance where behaviors and attributes of
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multiple parent classes can be combined.
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For example, in the generic class-based views there is a mixin called
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:class:`~django.views.generic.base.TemplateResponseMixin` whose primary purpose
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is to define the method
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:meth:`~django.views.generic.base.TemplateResponseMixin.render_to_response`.
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When combined with the behavior of the :class:`~django.views.generic.base.View`
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base class, the result is a :class:`~django.views.generic.base.TemplateView`
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class that will dispatch requests to the appropriate matching methods (a
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behavior defined in the ``View`` base class), and that has a
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:meth:`~django.views.generic.base.TemplateResponseMixin.render_to_response`
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method that uses a
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:attr:`~django.views.generic.base.TemplateResponseMixin.template_name`
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attribute to return a :class:`~django.template.response.TemplateResponse`
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object (a behavior defined in the ``TemplateResponseMixin``).
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Mixins are an excellent way of reusing code across multiple classes, but they
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come with some cost. The more your code is scattered among mixins, the harder
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it will be to read a child class and know what exactly it is doing, and the
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harder it will be to know which methods from which mixins to override if you
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are subclassing something that has a deep inheritance tree.
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Note also that you can only inherit from one generic view - that is, only one
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parent class may inherit from :class:`~django.views.generic.base.View` and
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the rest (if any) should be mixins. Trying to inherit from more than one class
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that inherits from ``View`` - for example, trying to use a form at the top of a
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list and combining :class:`~django.views.generic.edit.ProcessFormView` and
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:class:`~django.views.generic.list.ListView` - won't work as expected.
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Handling forms with class-based views
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=====================================
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A basic function-based view that handles forms may look something like this::
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from django.http import HttpResponseRedirect
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from django.shortcuts import render
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from .forms import MyForm
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def myview(request):
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if request.method == "POST":
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form = MyForm(request.POST)
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if form.is_valid():
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# <process form cleaned data>
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return HttpResponseRedirect('/success/')
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else:
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form = MyForm(initial={'key': 'value'})
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return render(request, 'form_template.html', {'form': form})
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A similar class-based view might look like::
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from django.http import HttpResponseRedirect
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from django.shortcuts import render
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from .forms import MyForm
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class MyFormView(View):
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form_class = MyForm
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initial = {'key': 'value'}
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template_name = 'form_template.html'
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def get(self, request, *args, **kwargs):
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form = self.form_class(initial=self.initial)
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return render(request, self.template_name, {'form': form})
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def post(self, request, *args, **kwargs):
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form = self.form_class(request.POST)
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if form.is_valid():
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# <process form cleaned data>
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return HttpResponseRedirect('/success/')
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return render(request, self.template_name, {'form': form})
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This is a very simple case, but you can see that you would then have the option
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of customizing this view by overriding any of the class attributes, e.g.
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``form_class``, via URLconf configuration, or subclassing and overriding one or
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more of the methods (or both!).
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Decorating class-based views
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============================
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The extension of class-based views isn't limited to using mixins. You
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can use also use decorators. Since class-based views aren't functions,
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decorating them works differently depending on if you're using ``as_view`` or
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creating a subclass.
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Decorating in URLconf
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---------------------
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The simplest way of decorating class-based views is to decorate the
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result of the :meth:`~django.views.generic.base.View.as_view` method.
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The easiest place to do this is in the URLconf where you deploy your view::
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from django.contrib.auth.decorators import login_required, permission_required
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from django.views.generic import TemplateView
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from .views import VoteView
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urlpatterns = patterns('',
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(r'^about/', login_required(TemplateView.as_view(template_name="secret.html"))),
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(r'^vote/', permission_required('polls.can_vote')(VoteView.as_view())),
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)
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This approach applies the decorator on a per-instance basis. If you
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want every instance of a view to be decorated, you need to take a
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different approach.
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.. _decorating-class-based-views:
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Decorating the class
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--------------------
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To decorate every instance of a class-based view, you need to decorate
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the class definition itself. To do this you apply the decorator to the
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:meth:`~django.views.generic.base.View.dispatch` method of the class.
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A method on a class isn't quite the same as a standalone function, so
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you can't just apply a function decorator to the method -- you need to
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transform it into a method decorator first. The ``method_decorator``
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decorator transforms a function decorator into a method decorator so
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that it can be used on an instance method. For example::
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from django.contrib.auth.decorators import login_required
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from django.utils.decorators import method_decorator
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from django.views.generic import TemplateView
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class ProtectedView(TemplateView):
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template_name = 'secret.html'
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@method_decorator(login_required)
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def dispatch(self, *args, **kwargs):
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return super(ProtectedView, self).dispatch(*args, **kwargs)
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In this example, every instance of ``ProtectedView`` will have
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login protection.
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.. note::
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``method_decorator`` passes ``*args`` and ``**kwargs``
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as parameters to the decorated method on the class. If your method
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does not accept a compatible set of parameters it will raise a
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``TypeError`` exception.
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