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234 lines
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234 lines
8.6 KiB
Plaintext
=================
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Class-based views
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=================
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.. versionadded:: 1.3
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A view is a callable which takes a request and returns a
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response. This can be more than just a function, and Django provides
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an example of some classes which can be used as views. These allow you
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to structure your views and reuse code by harnessing inheritance and
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mixins. There are also some generic views for simple tasks which we'll
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get to later, but you may want to design your own structure of
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reusable views which suits your use case. For full details, see the
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:doc:`class-based views reference
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documentation</ref/class-based-views/index>`.
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.. toctree::
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:maxdepth: 1
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generic-display
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generic-editing
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mixins
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Basic examples
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==============
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Django provides base view classes which will suit a wide range of applications.
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All views inherit from the :class:`~django.views.generic.base.View` class, which
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handles linking the view in to the URLs, HTTP method dispatching and other
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simple features. :class:`~django.views.generic.base.RedirectView` is for a simple HTTP
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redirect, and :class:`~django.views.generic.base.TemplateView` extends the base class
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to make it also render a template.
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Simple usage
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============
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Class-based generic views (and any class-based views that inherit from
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the base classes Django provides) can be configured in two
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ways: subclassing, or passing in arguments directly in the URLconf.
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When you subclass a class-based view, you can override attributes
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(such as the ``template_name``) or methods (such as ``get_context_data``)
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in your subclass to provide new values or methods. Consider, for example,
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a view that just displays one template, ``about.html``. Django has a
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generic view to do this - :class:`~django.views.generic.base.TemplateView` -
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so we can just subclass it, and override the template name::
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# some_app/views.py
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from django.views.generic import TemplateView
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class AboutView(TemplateView):
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template_name = "about.html"
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Then, we just need to add this new view into our URLconf. As the class-based
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views themselves are classes, we point the URL to the ``as_view`` class method
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instead, which is the entry point for class-based views::
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# urls.py
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from django.conf.urls import patterns, url, include
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from some_app.views import AboutView
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urlpatterns = patterns('',
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(r'^about/', AboutView.as_view()),
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)
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Alternatively, if you're only changing a few simple attributes on a
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class-based view, you can simply pass the new attributes into the ``as_view``
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method call itself::
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from django.conf.urls import patterns, url, include
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from django.views.generic import TemplateView
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urlpatterns = patterns('',
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(r'^about/', TemplateView.as_view(template_name="about.html")),
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)
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A similar overriding pattern can be used for the ``url`` attribute on
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:class:`~django.views.generic.base.RedirectView`.
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.. _jsonresponsemixin-example:
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More than just HTML
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-------------------
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Where class based views shine is when you want to do the same thing many times.
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Suppose you're writing an API, and every view should return JSON instead of
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rendered HTML.
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We can use create a mixin class to use in all of our views, handling the
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conversion to JSON once.
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For example, a simple JSON mixin might look something like this::
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import json
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from django.http import HttpResponse
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class JSONResponseMixin(object):
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"""
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A mixin that can be used to render a JSON response.
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"""
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response_class = HttpResponse
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def render_to_response(self, context, **response_kwargs):
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"""
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Returns a JSON response, transforming 'context' to make the payload.
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"""
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response_kwargs['content_type'] = 'application/json'
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return self.response_class(
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self.convert_context_to_json(context),
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**response_kwargs
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)
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def convert_context_to_json(self, context):
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"Convert the context dictionary into a JSON object"
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# Note: This is *EXTREMELY* naive; in reality, you'll need
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# to do much more complex handling to ensure that arbitrary
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# objects -- such as Django model instances or querysets
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# -- can be serialized as JSON.
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return json.dumps(context)
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Now we mix this into the base view::
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from django.views.generic import View
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class JSONView(JSONResponseMixin, View):
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pass
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Equally we could use our mixin with one of the generic views. We can make our
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own version of :class:`~django.views.generic.detail.DetailView` by mixing
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:class:`JSONResponseMixin` with the
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:class:`~django.views.generic.detail.BaseDetailView` -- (the
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:class:`~django.views.generic.detail.DetailView` before template
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rendering behavior has been mixed in)::
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class JSONDetailView(JSONResponseMixin, BaseDetailView):
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pass
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This view can then be deployed in the same way as any other
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:class:`~django.views.generic.detail.DetailView`, with exactly the
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same behavior -- except for the format of the response.
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If you want to be really adventurous, you could even mix a
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:class:`~django.views.generic.detail.DetailView` subclass that is able
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to return *both* HTML and JSON content, depending on some property of
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the HTTP request, such as a query argument or a HTTP header. Just mix
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in both the :class:`JSONResponseMixin` and a
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:class:`~django.views.generic.detail.SingleObjectTemplateResponseMixin`,
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and override the implementation of :func:`render_to_response()` to defer
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to the appropriate subclass depending on the type of response that the user
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requested::
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class HybridDetailView(JSONResponseMixin, SingleObjectTemplateResponseMixin, BaseDetailView):
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def render_to_response(self, context):
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# Look for a 'format=json' GET argument
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if self.request.GET.get('format','html') == 'json':
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return JSONResponseMixin.render_to_response(self, context)
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else:
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return SingleObjectTemplateResponseMixin.render_to_response(self, context)
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Because of the way that Python resolves method overloading, the local
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``render_to_response()`` implementation will override the versions provided by
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:class:`JSONResponseMixin` and
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:class:`~django.views.generic.detail.SingleObjectTemplateResponseMixin`.
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For more information on how to use the built in generic views, consult the next
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topic on :doc:`generic class based views</topics/class-based-views/generic-display>`.
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Decorating class-based views
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============================
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.. highlightlang:: python
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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.
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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
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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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