Views genéricas¶
Escrever aplicações web pode ser monótono, porque nós repetimos certos padrões várias e várias vezes. O Django tenta tirar um pouco dessa monotonia nas camadas de model e template, mas os desenvolvedores web também experimentam esse tédio na camada da view.
As views genéricas do Django foram criadas para diminuir esse sofrimento. Elas pegam padrões comuns encontrados no desenvolvimento web e abstraem eles, assim você pode escrever rapidamente views comuns sem ter que escrever muito código.
Podemos identificar algumas tarefas comuns, como mostrar uma lista de objetos, e escrever código que mostre um lista de qualquer objeto. Então, o model em questão pode ser passado como um argumento extra ao URLconf.
O Django vem com views genéricas que fazem o seguinte:
- Efetuam “simples” tarefas comuns: redirecionar para uma página diferente e renderizar determinado template.
- Mostrar uma lista e página com detalhes de um único objeto. Se estamos
criando uma aplicação para gerenciar conferências, então uma view
talk_list
e uma viewregistered_user_list
seriam exemplo de views de listas. Uma única página de conversa seria o que chamamos de view de “detalhe”. - Apresentar objetos baseados em data em páginas categorizadas por ano/mês/dia, os detalhes associados, e as “últimas” páginas. Os arquivos por ano, mês e dia do Blog da Django Brasil (http://www.djangobrasil.org/weblog/) foram construídos com nessas views, como exemplo de um típico arquivo de jornal.
- Permitir usuários a criar, atualizar e deletar objetos – com ou sem autorização.
Juntas, essas views oferecem uma interface fácil para realizar as tarefas mais comuns que os desenvolvedores encontram.
Usando views genéricas¶
Todas essas views são usadas através da criação de dicionários de configurações nos seus arquivos URLconf e passando esses dicionários como o terceiro membro da tupla URLconf para um determinado padrão.
Por exemplo, aqui está uma simples URLconf que poderíamos usar para apresentar uma página estática “sobre”:
from django.conf.urls.defaults import *
from django.views.generic.simple import direct_to_template
urlpatterns = patterns('',
('^about/$', direct_to_template, {
'template': 'about.html'
})
)
Isso pode parecer um pouco “mágico” à primeira vista – olhe, uma view sem
código! –, atualmente a view direct_to_template
simplesmente pega a
informação do dicionário de parâmetros extras e usa essa informação ao
renderizar a view.
Como essa view genérica – e todas as outras – é uma view normal como
qualquer outra, podemos reusá-la dentro de nossas próprias views. Como
exemplo, vamos extender nossa página “sobre” para mapear as URLs na forma
/about/<qualquer coisa>/
para arquivos about/<qualquer coisa>.html
.
Nós faremos isso modificando primeiramente a URLconf, para apontar para uma
função de view:
from django.conf.urls.defaults import * from django.views.generic.simple import direct_to_template from mysite.books.views import about_pages urlpatterns = patterns('', ('^about/$', direct_to_template, { 'template': 'about.html' }), ('^about/(w+)/$', about_pages), )
Depois, vamos escrever a view about_pages
:
from django.http import Http404
from django.template import TemplateDoesNotExist
from django.views.generic.simple import direct_to_template
def about_pages(request, page):
try:
return direct_to_template(request, template="about/%s.html" % page)
except TemplateDoesNotExist:
raise Http404()
Aqui tratamos o direct_to_template
como qualquer outra função. Uma vez que
ela retorna um HttpResponse
, podemos fazer como o exemplo. O único
pequeno truque aqui é lidar com a falta de templates. Não queremos que
um template inexistente cause um erro do servidor, então, capturamos as
exceções TemplateDoesNotExist
e retornamos um erro 404.
Existe uma falha de segurança aqui?
Os leitores mais atendos podem ter percebido uma possível falha de
segurança: estamos contruindo o nome do template usando conteúdo
do navegador (template="about/%s.html" % page
). A primeira vista,
isso parece uma clássica vulnerabilidade directory traversal. Mas
realmente é?
Não exatamente. Sim, um valor malicioso para page
poderia causar
directory traversal, porém, a variável page
é advinda da
URL, e não é qualquer valor que será aceito. A chave para isso está
na URLconf: estamos usando a expressão regular \w+
para obter
parte da URL, e \w
aceita somente letras e números. Portanto,
qualquer caracter malicioso (pontos e barras, aqui) serão rejeitados
pelo resolvedor de URL antes de chegar à view.
Views genéricas de objetos¶
The direct_to_template
certainly is useful, but Django’s generic views
really shine when it comes to presenting views on 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()
def __unicode__(self):
return self.name
class Meta:
ordering = ["-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 books, we’d use a URLconf along these lines:
from django.conf.urls.defaults import *
from django.views.generic import list_detail
from mysite.books.models import Publisher
publisher_info = {
"queryset" : Publisher.objects.all(),
}
urlpatterns = patterns('',
(r'^publishers/$', list_detail.object_list, publisher_info)
)
That’s all the Python code we need to write. We still need to write a template,
however. We could explicitly tell the object_list
view which template to use
by including a template_name
key in the extra arguments dictionary, 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.
This template will be rendered against a context containing a variable called
object_list
that contains all the book 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 all their options in detail; the rest of this document will consider some of the common ways you might customize and extend generic views.
Extending 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.
Luckily, in nearly every one of these cases, there are ways to simply extend generic views to handle a larger array of use cases. These situations usually fall into a handful of patterns dealt with in the sections that follow.
Making “friendly” template contexts¶
You might have noticed that our sample publisher list template stores all the
books 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 books here. A better name for that variable would be
publisher_list
; that variable’s content is pretty obvious.
We can change the name of that variable easily with the template_object_name
argument:
publisher_info = { "queryset" : Publisher.objects.all(), "template_object_name" : "publisher", } urlpatterns = patterns('', (r'^publishers/$', list_detail.object_list, publisher_info) )
Providing a useful template_object_name
is always a good idea. Your
coworkers who design templates will thank you.
Adding extra context¶
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 other
publishers on each publisher detail page. The object_detail
generic view
provides the publisher to the context, but it seems there’s no way to get a list
of all publishers in that template.
But there is: all generic views take an extra optional parameter,
extra_context
. This is a dictionary of extra objects that will be added to
the template’s context. So, to provide the list of all publishers on the detail
detail view, we’d use an info dict like this:
from mysite.books.models import Publisher, Book publisher_info = { "queryset" : Publisher.objects.all(), "template_object_name" : "publisher", "extra_context" : {"book_list" : Book.objects.all()} }
This would populate a {{ book_list }}
variable in the template context.
This pattern can be used to pass any information down into the template for the
generic view. It’s very handy.
However, there’s actually a subtle bug here – can you spot it?
The problem has to do with when the queries in extra_context
are evaluated.
Because this example puts Publisher.objects.all()
in the URLconf, it will
be evaluated only once (when the URLconf is first loaded). Once you add or
remove publishers, you’ll notice that the generic view doesn’t reflect those
changes until you reload the Web server (see Cacheamento e QuerySets
for more information about when QuerySets are cached and evaluated).
Note
This problem doesn’t apply to the queryset
generic view argument. Since
Django knows that particular QuerySet should never be cached, the generic
view takes care of clearing the cache when each view is rendered.
The solution is to use a callback in extra_context
instead of a value. Any
callable (i.e., a function) that’s passed to extra_context
will be evaluated
when the view is rendered (instead of only once). You could do this with an
explicitly defined function:
def get_books(): return Book.objects.all() publisher_info = { "queryset" : Publisher.objects.all(), "template_object_name" : "publisher", "extra_context" : {"book_list" : get_books} }
or you could use a less obvious but shorter version that relies on the fact that
Book.objects.all
is itself a callable:
publisher_info = { "queryset" : Publisher.objects.all(), "template_object_name" : "publisher", "extra_context" : {"book_list" : Book.objects.all} }
Notice the lack of parentheses after Book.objects.all
; this references
the function without actually calling it (which the generic view will do later).
Viewing subsets of objects¶
Now let’s take a closer look at this queryset
key we’ve been using all
along. Most generic views take one of these queryset
arguments – it’s how
the view knows which set of objects to display (see Fazendo consultas for
more information about QuerySet
objects, and see the
generic 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:
book_info = { "queryset" : Book.objects.all().order_by("-publication_date"), } urlpatterns = patterns('', (r'^publishers/$', list_detail.object_list, publisher_info), (r'^books/$', list_detail.object_list, book_info), )
That’s a pretty simple example, but it illustrates the idea nicely. Of course, you’ll usually want to do more than just reorder objects. If you want to present a list of books by a particular publisher, you can use the same technique:
acme_books = { "queryset": Book.objects.filter(publisher__name="Acme Publishing"), "template_name" : "books/acme_list.html" } urlpatterns = patterns('', (r'^publishers/$', list_detail.object_list, publisher_info), (r'^books/acme/$', list_detail.object_list, acme_books), )
Notice that along with a filtered queryset
, we’re also using a custom
template name. If we didn’t, the generic view would use the same template as the
“vanilla” object list, which might not be what we want.
Also notice that this isn’t a very elegant way of doing publisher-specific books. If we want to add another publisher page, we’d need another handful of lines in the URLconf, and more than a few publishers would get unreasonable. We’ll deal with this problem in the next section.
Note
If you get a 404 when requesting /books/acme/
, check to ensure you
actually have a Publisher with the name ‘ACME Publishing’. Generic
views have an allow_empty
parameter for this case. See the
generic views reference for more details.
Complex filtering with wrapper functions¶
Another common need is to filter down the objects given in a list page by some
key in the URL. Earlier we hard-coded the publisher’s name in the URLconf, but
what if we wanted to write a view that displayed all the books by some arbitrary
publisher? We can “wrap” the object_list
generic view to avoid writing a lot
of code by hand. As usual, we’ll start by writing a URLconf:
from mysite.books.views import books_by_publisher urlpatterns = patterns('', (r'^publishers/$', list_detail.object_list, publisher_info), (r'^books/(w+)/$', books_by_publisher), )
Next, we’ll write the books_by_publisher
view itself:
from django.http import Http404
from django.views.generic import list_detail
from mysite.books.models import Book, Publisher
def books_by_publisher(request, name):
# Look up the publisher (and raise a 404 if it can't be found).
try:
publisher = Publisher.objects.get(name__iexact=name)
except Publisher.DoesNotExist:
raise Http404
# Use the object_list view for the heavy lifting.
return list_detail.object_list(
request,
queryset = Book.objects.filter(publisher=publisher),
template_name = "books/books_by_publisher.html",
template_object_name = "books",
extra_context = {"publisher" : publisher}
)
This works because there’s really nothing special about generic views – they’re
just Python functions. Like any view function, generic views expect a certain
set of arguments and return HttpResponse
objects. Thus, it’s incredibly easy
to wrap a small function around a generic view that does additional work before
(or after; see the next section) handing things off to the generic view.
Note
Notice that in the preceding example we passed the current publisher being
displayed in the extra_context
. This is usually a good idea in wrappers
of this nature; it lets the template know which “parent” object is currently
being browsed.
Performing extra work¶
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 object_detail
view, 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 mysite.books.views import author_detail urlpatterns = patterns('', #... (r'^authors/(?P<author_id>d+)/$', author_detail), )
Then we’d write our wrapper function:
import datetime
from mysite.books.models import Author
from django.views.generic import list_detail
from django.shortcuts import get_object_or_404
def author_detail(request, author_id):
# Look up the Author (and raise a 404 if she's not found)
author = get_object_or_404(Author, pk=author_id)
# Record the last accessed date
author.last_accessed = datetime.datetime.now()
author.save()
# Show the detail page
return list_detail.object_detail(
request,
queryset = Author.objects.all(),
object_id = author_id,
)
Note
This code won’t actually work unless you create a
books/author_detail.html
template.
We can use a similar idiom to alter the response returned by the generic view. If we wanted to provide a downloadable plain-text version of the list of authors, we could use a view like this:
def author_list_plaintext(request):
response = list_detail.object_list(
request,
queryset = Author.objects.all(),
mimetype = "text/plain",
template_name = "books/author_list.txt"
)
response["Content-Disposition"] = "attachment; filename=authors.txt"
return response
This works because the generic views return simple HttpResponse
objects
that can be treated like dictionaries to set HTTP headers. This
Content-Disposition
business, by the way, instructs the browser to
download and save the page instead of displaying it in the browser.