Note
Prior to Django 1.3, generic views were implemented as functions. The function-based implementation has been deprecated in favor of the class-based approach described here.
For details on the previous generic views implementation, see the topic guide and detailed reference.
Writing Web applications can be monotonous, because we repeat certain patterns again and again. Django tries to take away some of that monotony at the model and template layers, but Web developers also experience this boredom at the view level.
Django’s generic views were developed to ease that pain. They take certain common idioms and patterns found in view development and abstract them so that you can quickly write common views of data without having to write too much code.
We can recognize certain common tasks, like displaying a list of objects, and write code that displays a list of any object. Then the model in question can be passed as an extra argument to the URLconf.
Django ships with generic views to do the following:
Taken together, these views provide easy interfaces to perform the most common tasks developers encounter.
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 - 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.defaults import *
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.defaults import *
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 RedirectView, another simple generic view.
TemplateView certainly is useful, but Django's generic views really shine when it comes to presenting views of your database content. Because it's such a common task, Django comes with a handful of built-in generic views that make generating list and detail views of objects incredibly easy.
Let's take a look at one of these generic views: the "object list" view. We'll be using these models:
# models.py
from django.db import models
class Publisher(models.Model):
name = models.CharField(max_length=30)
address = models.CharField(max_length=50)
city = models.CharField(max_length=60)
state_province = models.CharField(max_length=30)
country = models.CharField(max_length=50)
website = models.URLField()
class Meta:
ordering = ["-name"]
def __unicode__(self):
return self.name
class Book(models.Model):
title = models.CharField(max_length=100)
authors = models.ManyToManyField('Author')
publisher = models.ForeignKey(Publisher)
publication_date = models.DateField()
To build a list page of all publishers, we'd use a URLconf along these lines:
from django.conf.urls.defaults import *
from django.views.generic import ListView
from books.models import Publisher
urlpatterns = patterns('',
(r'^publishers/$', ListView.as_view(
model=Publisher,
)),
)
That's all the Python code we need to write. We still need to write a template, however. We could explicitly tell the view which template to use by including a template_name key in the arguments to as_view, but in the absence of an explicit template Django will infer one from the object's name. In this case, the inferred template will be "books/publisher_list.html" -- the "books" part comes from the name of the app that defines the model, while the "publisher" bit is just the lowercased version of the model's name.
Note
Thus, when (for example) the django.template.loaders.app_directories.Loader template loader is enabled in TEMPLATE_LOADERS, the template location would be:
/path/to/project/books/templates/books/publisher_list.html
This template will be rendered against a context containing a variable called object_list that contains all the publisher objects. A very simple template might look like the following:
{% extends "base.html" %}
{% block content %}
<h2>Publishers</h2>
<ul>
{% for publisher in object_list %}
<li>{{ publisher.name }}</li>
{% endfor %}
</ul>
{% endblock %}
That's really all there is to it. All the cool features of generic views come from changing the "info" dictionary passed to the generic view. The generic views reference documents all the generic views and their options in detail; the rest of this document will consider some of the common ways you might customize and extend generic views.
There's no question that using generic views can speed up development substantially. In most projects, however, there comes a moment when the generic views no longer suffice. Indeed, the most common question asked by new Django developers is how to make generic views handle a wider array of situations.
This is one of the reasons generic views were redesigned for the 1.3 release - previously, they were just view functions with a bewildering array of options; now, rather than passing in a large amount of configuration in the URLconf, the recommended way to extend generic views is to subclass them, and override their attributes or methods.
You might have noticed that our sample publisher list template stores all the publishers in a variable named object_list. While this works just fine, it isn't all that "friendly" to template authors: they have to "just know" that they're dealing with publishers here.
Well, if you're dealing with a model object, this is already done for you. When you are dealing with an object or queryset, Django is able to populate the context using the verbose name (or the plural verbose name, in the case of a list of objects) of the object being displayed. This is provided in addition to the default object_list entry, but contains exactly the same data.
If the verbose name (or plural verbose name) still isn't a good match, you can manually set the name of the context variable. The context_object_name attribute on a generic view specifies the context variable to use. In this example, we'll override it in the URLconf, since it's a simple change:
urlpatterns = patterns('',
(r'^publishers/$', ListView.as_view(
model=Publisher,
context_object_name="publisher_list",
)),
)
Providing a useful context_object_name is always a good idea. Your coworkers who design templates will thank you.
Often you simply need to present some extra information beyond that provided by the generic view. For example, think of showing a list of all the books on each publisher detail page. The DetailView generic view provides the publisher to the context, but it seems there's no way to get additional information in that template.
However, there is; you can subclass DetailView and provide your own implementation of the get_context_data method. The default implementation of this that comes with DetailView simply adds in the object being displayed to the template, but you can override it to show more:
from django.views.generic import DetailView
from books.models import Publisher, Book
class PublisherDetailView(DetailView):
context_object_name = "publisher"
model = Publisher
def get_context_data(self, **kwargs):
# Call the base implementation first to get a context
context = super(PublisherDetailView, self).get_context_data(**kwargs)
# Add in a QuerySet of all the books
context['book_list'] = Book.objects.all()
return context
Now let's take a closer look at the model argument we've been using all along. The model argument, which specifies the database model that the view will operate upon, is available on all the generic views that operate on a single object or a collection of objects. However, the model argument is not the only way to specify the objects that the view will operate upon -- you can also specify the list of objects using the queryset argument:
from django.views.generic import DetailView
from books.models import Publisher, Book
class PublisherDetailView(DetailView):
context_object_name = "publisher"
queryset = Publisher.objects.all()
Specifying model = Publisher is really just shorthand for saying queryset = Publisher.objects.all(). However, by using queryset to define a filtered list of objects you can be more specific about the objects that will be visible in the view (see Fazendo consultas for more information about QuerySet objects, and see the class-based views reference for the complete details).
To pick a simple example, we might want to order a list of books by publication date, with the most recent first:
urlpatterns = patterns('',
(r'^publishers/$', ListView.as_view(
queryset=Publisher.objects.all(),
context_object_name="publisher_list",
)),
(r'^books/$', ListView.as_view(
queryset=Book.objects.order_by("-publication_date"),
context_object_name="book_list",
)),
)
That's a pretty simple example, but it illustrates the idea nicely. Of course, you'll usually want to do more than just reorder objects. If you want to present a list of books by a particular publisher, you can use the same technique (here, illustrated using subclassing rather than by passing arguments in the URLconf):
from django.views.generic import ListView
from books.models import Book
class AcmeBookListView(ListView):
context_object_name = "book_list"
queryset = Book.objects.filter(publisher__name="Acme Publishing")
template_name = "books/acme_list.html"
Notice that along with a filtered queryset, we're also using a custom template name. If we didn't, the generic view would use the same template as the "vanilla" object list, which might not be what we want.
Also notice that this isn't a very elegant way of doing publisher-specific books. If we want to add another publisher page, we'd need another handful of lines in the URLconf, and more than a few publishers would get unreasonable. We'll deal with this problem in the next section.
Note
If you get a 404 when requesting /books/acme/, check to ensure you actually have a Publisher with the name 'ACME Publishing'. Generic views have an allow_empty parameter for this case. See the class-based-views reference for more details.
Another common need is to filter down the objects given in a list page by some key in the URL. Earlier we hard-coded the publisher's name in the URLconf, but what if we wanted to write a view that displayed all the books by some arbitrary publisher?
Handily, the ListView has a get_queryset() method we can override. Previously, it has just been returning the value of the queryset attribute, but now we can add more logic.
The key part to making this work is that when class-based views are called, various useful things are stored on self; as well as the request (self.request) this includes the positional (self.args) and name-based (self.kwargs) arguments captured according to the URLconf.
Here, we have a URLconf with a single captured group:
from books.views import PublisherBookListView
urlpatterns = patterns('',
(r'^books/(\w+)/$', PublisherBookListView.as_view()),
)
Next, we'll write the PublisherBookListView view itself:
from django.shortcuts import get_object_or_404
from django.views.generic import ListView
from books.models import Book, Publisher
class PublisherBookListView(ListView):
context_object_name = "book_list"
template_name = "books/books_by_publisher.html",
def get_queryset(self):
publisher = get_object_or_404(Publisher, name__iexact=self.args[0])
return Book.objects.filter(publisher=publisher)
As you can see, it's quite easy to add more logic to the queryset selection; if we wanted, we could use self.request.user to filter using the current user, or other more complex logic.
We can also add the publisher into the context at the same time, so we can use it in the template:
class PublisherBookListView(ListView):
context_object_name = "book_list"
template_name = "books/books_by_publisher.html",
def get_queryset(self):
self.publisher = get_object_or_404(Publisher, name__iexact=self.args[0])
return Book.objects.filter(publisher=self.publisher)
def get_context_data(self, **kwargs):
# Call the base implementation first to get a context
context = super(PublisherBookListView, self).get_context_data(**kwargs)
# Add in the publisher
context['publisher'] = self.publisher
return context
The last common pattern we'll look at involves doing some extra work before or after calling the generic view.
Imagine we had a last_accessed field on our Author object that we were using to keep track of the last time anybody looked at that author:
# models.py
class Author(models.Model):
salutation = models.CharField(max_length=10)
first_name = models.CharField(max_length=30)
last_name = models.CharField(max_length=40)
email = models.EmailField()
headshot = models.ImageField(upload_to='/tmp')
last_accessed = models.DateTimeField()
The generic DetailView class, of course, wouldn't know anything about this field, but once again we could easily write a custom view to keep that field updated.
First, we'd need to add an author detail bit in the URLconf to point to a custom view:
from books.views import AuthorDetailView
urlpatterns = patterns('',
#...
(r'^authors/(?P<pk>\d+)/$', AuthorDetailView.as_view()),
)
Then we'd write our new view -- get_object is the method that retrieves the object -- so we simply override it and wrap the call:
import datetime
from books.models import Author
from django.views.generic import DetailView
from django.shortcuts import get_object_or_404
class AuthorDetailView(DetailView):
queryset = Author.objects.all()
def get_object(self):
# Call the superclass
object = super(AuthorDetailView, self).get_object()
# Record the last accessed date
object.last_accessed = datetime.datetime.now()
object.save()
# Return the object
return object
Note
This code won't actually work unless you create a books/author_detail.html template.
Note
The URLconf here uses the named group pk - this name is the default name that DetailView uses to find the value of the primary key used to filter the queryset.
If you want to change it, you'll need to do your own get() call on self.queryset using the new named parameter from self.kwargs.
So far, we've been focusing on rendering templates to generate responses. However, that's not all generic views can do.
Each generic view is composed out of a series of mixins, and each mixin contributes a little piece of the entire view. Some of these mixins -- such as TemplateResponseMixin -- are specifically designed for rendering content to an HTML response using a template. However, you can write your own mixins that perform different rendering behavior.
For example, a simple JSON mixin might look something like this:
from django import http
from django.utils import simplejson as json
class JSONResponseMixin(object):
def render_to_response(self, context):
"Returns a JSON response containing 'context' as payload"
return self.get_json_response(self.convert_context_to_json(context))
def get_json_response(self, content, **httpresponse_kwargs):
"Construct an `HttpResponse` object."
return http.HttpResponse(content,
content_type='application/json',
**httpresponse_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)
Then, you could build a JSON-returning DetailView by mixing your JSONResponseMixin with the BaseDetailView -- (the 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 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 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 JSONResponseMixin and a SingleObjectTemplateResponseMixin, and override the implementation of 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 JSONResponseMixin and SingleObjectTemplateResponseMixin.
The extension of class-based views isn't limited to using mixins. You can use also use decorators.
The simplest way of decorating class-based views is to decorate the result of the 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.
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 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.
Dec 26, 2011