621 lines
24 KiB
Plaintext
621 lines
24 KiB
Plaintext
=====================================
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Writing your first Django app, part 3
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=====================================
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This tutorial begins where :doc:`Tutorial 2 </intro/tutorial02>` left off. We're
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continuing the Web-poll application and will focus on creating the public
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interface -- "views."
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Philosophy
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==========
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A view is a "type" of Web page in your Django application that generally serves
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a specific function and has a specific template. For example, in a blog
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application, you might have the following views:
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* Blog homepage -- displays the latest few entries.
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* Entry "detail" page -- permalink page for a single entry.
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* Year-based archive page -- displays all months with entries in the
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given year.
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* Month-based archive page -- displays all days with entries in the
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given month.
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* Day-based archive page -- displays all entries in the given day.
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* Comment action -- handles posting comments to a given entry.
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In our poll application, we'll have the following four views:
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* Question "index" page -- displays the latest few questions.
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* Question "detail" page -- displays a question text, with no results but
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with a form to vote.
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* Question "results" page -- displays results for a particular question.
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* Vote action -- handles voting for a particular choice in a particular
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question.
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In Django, web pages and other content are delivered by views. Each view is
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represented by a simple Python function (or method, in the case of class-based
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views). Django will choose a view by examining the URL that's requested (to be
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precise, the part of the URL after the domain name).
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Now in your time on the web you may have come across such beauties as
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"ME2/Sites/dirmod.asp?sid=&type=gen&mod=Core+Pages&gid=A6CD4967199A42D9B65B1B".
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You will be pleased to know that Django allows us much more elegant
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*URL patterns* than that.
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A URL pattern is simply the general form of a URL - for example:
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``/newsarchive/<year>/<month>/``.
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To get from a URL to a view, Django uses what are known as 'URLconfs'. A
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URLconf maps URL patterns (described as regular expressions) to views.
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This tutorial provides basic instruction in the use of URLconfs, and you can
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refer to :mod:`django.core.urlresolvers` for more information.
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Write your first view
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=====================
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Let's write the first view. Open the file ``polls/views.py``
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and put the following Python code in it:
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.. snippet::
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:filename: polls/views.py
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from django.http import HttpResponse
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def index(request):
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return HttpResponse("Hello, world. You're at the polls index.")
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This is the simplest view possible in Django. To call the view, we need to map
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it to a URL - and for this we need a URLconf.
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To create a URLconf in the polls directory, create a file called ``urls.py``.
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Your app directory should now look like::
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polls/
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__init__.py
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admin.py
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models.py
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tests.py
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urls.py
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views.py
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In the ``polls/urls.py`` file include the following code:
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.. snippet::
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:filename: polls/urls.py
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from django.conf.urls import patterns, url
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from polls import views
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urlpatterns = patterns('',
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url(r'^$', views.index, name='index')
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)
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The next step is to point the root URLconf at the ``polls.urls`` module. In
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``mysite/urls.py`` insert an :func:`~django.conf.urls.include`, leaving you
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with:
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.. snippet::
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:filename: mysite/urls.py
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from django.conf.urls import patterns, include, url
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from django.contrib import admin
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urlpatterns = patterns('',
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url(r'^polls/', include('polls.urls')),
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url(r'^admin/', include(admin.site.urls)),
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)
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.. admonition:: Doesn't match what you see?
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If you're seeing ``admin.autodiscover()`` before the definition of
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``urlpatterns``, you're probably using a version of Django that doesn't
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match this tutorial version. You'll want to either switch to the older
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tutorial or the newer Django version.
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You have now wired an ``index`` view into the URLconf. Go to
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http://localhost:8000/polls/ in your browser, and you should see the text
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"*Hello, world. You're at the polls index.*", which you defined in the
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``index`` view.
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The :func:`~django.conf.urls.url` function is passed four arguments, two
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required: ``regex`` and ``view``, and two optional: ``kwargs``, and ``name``.
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At this point, it's worth reviewing what these arguments are for.
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:func:`~django.conf.urls.url` argument: regex
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---------------------------------------------
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The term "regex" is a commonly used short form meaning "regular expression",
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which is a syntax for matching patterns in strings, or in this case, url
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patterns. Django starts at the first regular expression and makes its way down
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the list, comparing the requested URL against each regular expression until it
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finds one that matches.
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Note that these regular expressions do not search GET and POST parameters, or
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the domain name. For example, in a request to
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``http://www.example.com/myapp/``, the URLconf will look for ``myapp/``. In a
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request to ``http://www.example.com/myapp/?page=3``, the URLconf will also
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look for ``myapp/``.
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If you need help with regular expressions, see `Wikipedia's entry`_ and the
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documentation of the :mod:`re` module. Also, the O'Reilly book "Mastering
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Regular Expressions" by Jeffrey Friedl is fantastic. In practice, however,
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you don't need to be an expert on regular expressions, as you really only need
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to know how to capture simple patterns. In fact, complex regexes can have poor
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lookup performance, so you probably shouldn't rely on the full power of regexes.
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Finally, a performance note: these regular expressions are compiled the first
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time the URLconf module is loaded. They're super fast (as long as the lookups
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aren't too complex as noted above).
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.. _Wikipedia's entry: http://en.wikipedia.org/wiki/Regular_expression
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:func:`~django.conf.urls.url` argument: view
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--------------------------------------------
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When Django finds a regular expression match, Django calls the specified view
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function, with an :class:`~django.http.HttpRequest` object as the first
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argument and any “captured” values from the regular expression as other
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arguments. If the regex uses simple captures, values are passed as positional
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arguments; if it uses named captures, values are passed as keyword arguments.
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We'll give an example of this in a bit.
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:func:`~django.conf.urls.url` argument: kwargs
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----------------------------------------------
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Arbitrary keyword arguments can be passed in a dictionary to the target view. We
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aren't going to use this feature of Django in the tutorial.
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:func:`~django.conf.urls.url` argument: name
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---------------------------------------------
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Naming your URL lets you refer to it unambiguously from elsewhere in Django
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especially templates. This powerful feature allows you to make global changes
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to the url patterns of your project while only touching a single file.
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Writing more views
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==================
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Now let's add a few more views to ``polls/views.py``. These views are
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slightly different, because they take an argument:
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.. snippet::
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:filename: polls/views.py
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def detail(request, question_id):
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return HttpResponse("You're looking at question %s." % question_id)
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def results(request, question_id):
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response = "You're looking at the results of question %s."
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return HttpResponse(response % question_id)
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def vote(request, question_id):
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return HttpResponse("You're voting on question %s." % question_id)
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Wire these new views into the ``polls.urls`` module by adding the following
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:func:`~django.conf.urls.url` calls:
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.. snippet::
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:filename: polls/urls.py
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from django.conf.urls import patterns, url
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from polls import views
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urlpatterns = patterns('',
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# ex: /polls/
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url(r'^$', views.index, name='index'),
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# ex: /polls/5/
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url(r'^(?P<question_id>\d+)/$', views.detail, name='detail'),
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# ex: /polls/5/results/
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url(r'^(?P<question_id>\d+)/results/$', views.results, name='results'),
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# ex: /polls/5/vote/
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url(r'^(?P<question_id>\d+)/vote/$', views.vote, name='vote'),
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)
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Take a look in your browser, at "/polls/34/". It'll run the ``detail()``
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method and display whatever ID you provide in the URL. Try
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"/polls/34/results/" and "/polls/34/vote/" too -- these will display the
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placeholder results and voting pages.
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When somebody requests a page from your Web site -- say, "/polls/34/", Django
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will load the ``mysite.urls`` Python module because it's pointed to by the
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:setting:`ROOT_URLCONF` setting. It finds the variable named ``urlpatterns``
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and traverses the regular expressions in order. The
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:func:`~django.conf.urls.include` functions we are using simply reference
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other URLconfs. Note that the regular expressions for the
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:func:`~django.conf.urls.include` functions don't have a ``$`` (end-of-string
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match character) but rather a trailing slash. Whenever Django encounters
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:func:`~django.conf.urls.include`, it chops off whatever part of the URL
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matched up to that point and sends the remaining string to the included
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URLconf for further processing.
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The idea behind :func:`~django.conf.urls.include` is to make it easy to
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plug-and-play URLs. Since polls are in their own URLconf
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(``polls/urls.py``), they can be placed under "/polls/", or under
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"/fun_polls/", or under "/content/polls/", or any other path root, and the
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app will still work.
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Here's what happens if a user goes to "/polls/34/" in this system:
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* Django will find the match at ``'^polls/'``
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* Then, Django will strip off the matching text (``"polls/"``) and send the
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remaining text -- ``"34/"`` -- to the 'polls.urls' URLconf for
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further processing which matches ``r'^(?P<question_id>\d+)/$'`` resulting in a
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call to the ``detail()`` view like so::
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detail(request=<HttpRequest object>, question_id='34')
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The ``question_id='34'`` part comes from ``(?P<question_id>\d+)``. Using parentheses
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around a pattern "captures" the text matched by that pattern and sends it as an
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argument to the view function; ``?P<question_id>`` defines the name that will
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be used to identify the matched pattern; and ``\d+`` is a regular expression to
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match a sequence of digits (i.e., a number).
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Because the URL patterns are regular expressions, there really is no limit on
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what you can do with them. And there's no need to add URL cruft such as
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``.html`` -- unless you want to, in which case you can do something like
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this::
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(r'^polls/latest\.html$', 'polls.views.index'),
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But, don't do that. It's silly.
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Write views that actually do something
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======================================
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Each view is responsible for doing one of two things: returning an
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:class:`~django.http.HttpResponse` object containing the content for the
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requested page, or raising an exception such as :exc:`~django.http.Http404`. The
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rest is up to you.
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Your view can read records from a database, or not. It can use a template
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system such as Django's -- or a third-party Python template system -- or not.
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It can generate a PDF file, output XML, create a ZIP file on the fly, anything
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you want, using whatever Python libraries you want.
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All Django wants is that :class:`~django.http.HttpResponse`. Or an exception.
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Because it's convenient, let's use Django's own database API, which we covered
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in :doc:`Tutorial 1 </intro/tutorial01>`. Here's one stab at the ``index()``
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view, which displays the latest 5 poll questions in the system, separated by
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commas, according to publication date:
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.. snippet::
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:filename: polls/views.py
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from django.http import HttpResponse
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from polls.models import Question
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def index(request):
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latest_question_list = Question.objects.order_by('-pub_date')[:5]
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output = ', '.join([p.question_text for p in latest_question_list])
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return HttpResponse(output)
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There's a problem here, though: the page's design is hard-coded in the view. If
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you want to change the way the page looks, you'll have to edit this Python code.
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So let's use Django's template system to separate the design from Python by
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creating a template that the view can use.
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First, create a directory called ``templates`` in your ``polls`` directory.
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Django will look for templates in there.
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Django's :setting:`TEMPLATE_LOADERS` setting contains a list of callables that
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know how to import templates from various sources. One of the defaults is
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:class:`django.template.loaders.app_directories.Loader` which looks for a
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"templates" subdirectory in each of the :setting:`INSTALLED_APPS` - this is how
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Django knows to find the polls templates even though we didn't modify
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:setting:`TEMPLATE_DIRS`, as we did in :ref:`Tutorial 2
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<ref-customizing-your-projects-templates>`.
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.. admonition:: Organizing templates
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We *could* have all our templates together, in one big templates directory,
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and it would work perfectly well. However, this template belongs to the
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polls application, so unlike the admin template we created in the previous
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tutorial, we'll put this one in the application's template directory
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(``polls/templates``) rather than the project's (``templates``). We'll
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discuss in more detail in the :doc:`reusable apps tutorial
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</intro/reusable-apps>` *why* we do this.
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Within the ``templates`` directory you have just created, create another
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directory called ``polls``, and within that create a file called
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``index.html``. In other words, your template should be at
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``polls/templates/polls/index.html``. Because of how the ``app_directories``
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template loader works as described above, you can refer to this template within
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Django simply as ``polls/index.html``.
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.. admonition:: Template namespacing
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Now we *might* be able to get away with putting our templates directly in
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``polls/templates`` (rather than creating another ``polls`` subdirectory),
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but it would actually be a bad idea. Django will choose the first template
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it finds whose name matches, and if you had a template with the same name
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in a *different* application, Django would be unable to distinguish between
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them. We need to be able to point Django at the right one, and the easiest
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way to ensure this is by *namespacing* them. That is, by putting those
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templates inside *another* directory named for the application itself.
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Put the following code in that template:
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.. snippet:: html+django
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:filename: polls/templates/polls/index.html
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{% if latest_question_list %}
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<ul>
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{% for question in latest_question_list %}
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<li><a href="/polls/{{ question.id }}/">{{ question.question_text }}</a></li>
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{% endfor %}
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</ul>
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{% else %}
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<p>No polls are available.</p>
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{% endif %}
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Now let's update our ``index`` view in ``polls/views.py`` to use the template:
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.. snippet::
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:filename: polls/views.py
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from django.http import HttpResponse
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from django.template import RequestContext, loader
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from polls.models import Question
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def index(request):
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latest_question_list = Question.objects.order_by('-pub_date')[:5]
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template = loader.get_template('polls/index.html')
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context = RequestContext(request, {
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'latest_question_list': latest_question_list,
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})
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return HttpResponse(template.render(context))
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That code loads the template called ``polls/index.html`` and passes it a
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context. The context is a dictionary mapping template variable names to Python
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objects.
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Load the page by pointing your browser at "/polls/", and you should see a
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bulleted-list containing the "What's up" question from Tutorial 1. The link points
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to the question's detail page.
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A shortcut: :func:`~django.shortcuts.render`
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--------------------------------------------
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It's a very common idiom to load a template, fill a context and return an
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:class:`~django.http.HttpResponse` object with the result of the rendered
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template. Django provides a shortcut. Here's the full ``index()`` view,
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rewritten:
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.. snippet::
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:filename: polls/views.py
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from django.shortcuts import render
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from polls.models import Question
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def index(request):
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latest_question_list = Question.objects.all().order_by('-pub_date')[:5]
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context = {'latest_question_list': latest_question_list}
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return render(request, 'polls/index.html', context)
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Note that once we've done this in all these views, we no longer need to import
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:mod:`~django.template.loader`, :class:`~django.template.RequestContext` and
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:class:`~django.http.HttpResponse` (you'll want to keep ``HttpResponse`` if you
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still have the stub methods for ``detail``, ``results``, and ``vote``).
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The :func:`~django.shortcuts.render` function takes the request object as its
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first argument, a template name as its second argument and a dictionary as its
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optional third argument. It returns an :class:`~django.http.HttpResponse`
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object of the given template rendered with the given context.
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Raising a 404 error
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===================
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Now, let's tackle the question detail view -- the page that displays the question text
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for a given poll. Here's the view:
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.. snippet::
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:filename: polls/views.py
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from django.http import Http404
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from django.shortcuts import render
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from polls.models import Question
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# ...
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def detail(request, question_id):
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try:
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question = Question.objects.get(pk=question_id)
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except Question.DoesNotExist:
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raise Http404
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return render(request, 'polls/detail.html', {'question': question})
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The new concept here: The view raises the :exc:`~django.http.Http404` exception
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if a question with the requested ID doesn't exist.
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We'll discuss what you could put in that ``polls/detail.html`` template a bit
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later, but if you'd like to quickly get the above example working, a file
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containing just:
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.. snippet:: html+django
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:filename: polls/templates/polls/detail.html
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{{ question }}
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will get you started for now.
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A shortcut: :func:`~django.shortcuts.get_object_or_404`
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-------------------------------------------------------
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It's a very common idiom to use :meth:`~django.db.models.query.QuerySet.get`
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and raise :exc:`~django.http.Http404` if the object doesn't exist. Django
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provides a shortcut. Here's the ``detail()`` view, rewritten:
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.. snippet::
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:filename: polls/views.py
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from django.shortcuts import render, get_object_or_404
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from polls.models import Question
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# ...
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def detail(request, question_id):
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question = get_object_or_404(Question, pk=question_id)
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return render(request, 'polls/detail.html', {'question': question})
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The :func:`~django.shortcuts.get_object_or_404` function takes a Django model
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as its first argument and an arbitrary number of keyword arguments, which it
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passes to the :meth:`~django.db.models.query.QuerySet.get` function of the
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model's manager. It raises :exc:`~django.http.Http404` if the object doesn't
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exist.
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.. admonition:: Philosophy
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Why do we use a helper function :func:`~django.shortcuts.get_object_or_404`
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instead of automatically catching the
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:exc:`~django.core.exceptions.ObjectDoesNotExist` exceptions at a higher
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level, or having the model API raise :exc:`~django.http.Http404` instead of
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|
:exc:`~django.core.exceptions.ObjectDoesNotExist`?
|
|
|
|
Because that would couple the model layer to the view layer. One of the
|
|
foremost design goals of Django is to maintain loose coupling. Some
|
|
controlled coupling is introduced in the :mod:`django.shortcuts` module.
|
|
|
|
There's also a :func:`~django.shortcuts.get_list_or_404` function, which works
|
|
just as :func:`~django.shortcuts.get_object_or_404` -- except using
|
|
:meth:`~django.db.models.query.QuerySet.filter` instead of
|
|
:meth:`~django.db.models.query.QuerySet.get`. It raises
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|
:exc:`~django.http.Http404` if the list is empty.
|
|
|
|
Use the template system
|
|
=======================
|
|
|
|
Back to the ``detail()`` view for our poll application. Given the context
|
|
variable ``question``, here's what the ``polls/detail.html`` template might look
|
|
like:
|
|
|
|
.. snippet:: html+django
|
|
:filename: polls/templates/polls/detail.html
|
|
|
|
<h1>{{ question.question_text }}</h1>
|
|
<ul>
|
|
{% for choice in question.choice_set.all %}
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|
<li>{{ choice.choice_text }}</li>
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|
{% endfor %}
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|
</ul>
|
|
|
|
The template system uses dot-lookup syntax to access variable attributes. In
|
|
the example of ``{{ question.question_text }}``, first Django does a dictionary lookup
|
|
on the object ``question``. Failing that, it tries an attribute lookup -- which
|
|
works, in this case. If attribute lookup had failed, it would've tried a
|
|
list-index lookup.
|
|
|
|
Method-calling happens in the :ttag:`{% for %}<for>` loop:
|
|
``question.choice_set.all`` is interpreted as the Python code
|
|
``question.choice_set.all()``, which returns an iterable of ``Choice`` objects and is
|
|
suitable for use in the :ttag:`{% for %}<for>` tag.
|
|
|
|
See the :doc:`template guide </topics/templates>` for more about templates.
|
|
|
|
Removing hardcoded URLs in templates
|
|
====================================
|
|
|
|
Remember, when we wrote the link to a question in the ``polls/index.html``
|
|
template, the link was partially hardcoded like this:
|
|
|
|
.. code-block:: html+django
|
|
|
|
<li><a href="/polls/{{ question.id }}/">{{ question.question_text }}</a></li>
|
|
|
|
The problem with this hardcoded, tightly-coupled approach is that it becomes
|
|
challenging to change URLs on projects with a lot of templates. However, since
|
|
you defined the name argument in the :func:`~django.conf.urls.url` functions in
|
|
the ``polls.urls`` module, you can remove a reliance on specific URL paths
|
|
defined in your url configurations by using the ``{% url %}`` template tag:
|
|
|
|
.. code-block:: html+django
|
|
|
|
<li><a href="{% url 'detail' question.id %}">{{ question.question_text }}</a></li>
|
|
|
|
.. note::
|
|
|
|
If ``{% url 'detail' question.id %}`` (with quotes) doesn't work, but
|
|
``{% url detail question.id %}`` (without quotes) does, that means you're
|
|
using a version of Django < 1.5. In this case, add the following
|
|
declaration at the top of your template:
|
|
|
|
.. code-block:: html+django
|
|
|
|
{% load url from future %}
|
|
|
|
The way this works is by looking up the URL definition as specified in the
|
|
``polls.urls`` module. You can see exactly where the URL name of 'detail' is
|
|
defined below::
|
|
|
|
...
|
|
# the 'name' value as called by the {% url %} template tag
|
|
url(r'^(?P<question_id>\d+)/$', views.detail, name='detail'),
|
|
...
|
|
|
|
If you want to change the URL of the polls detail view to something else,
|
|
perhaps to something like ``polls/specifics/12/`` instead of doing it in the
|
|
template (or templates) you would change it in ``polls/urls.py``::
|
|
|
|
...
|
|
# added the word 'specifics'
|
|
url(r'^specifics/(?P<question_id>\d+)/$', views.detail, name='detail'),
|
|
...
|
|
|
|
Namespacing URL names
|
|
======================
|
|
|
|
The tutorial project has just one app, ``polls``. In real Django projects,
|
|
there might be five, ten, twenty apps or more. How does Django differentiate
|
|
the URL names between them? For example, the ``polls`` app has a ``detail``
|
|
view, and so might an app on the same project that is for a blog. How does one
|
|
make it so that Django knows which app view to create for a url when using the
|
|
``{% url %}`` template tag?
|
|
|
|
The answer is to add namespaces to your root URLconf. In the ``mysite/urls.py``
|
|
file (the project's ``urls.py``, not the application's), go ahead and change
|
|
it to include namespacing:
|
|
|
|
.. snippet::
|
|
:filename: mysite/urls.py
|
|
|
|
from django.conf.urls import patterns, include, url
|
|
from django.contrib import admin
|
|
|
|
urlpatterns = patterns('',
|
|
url(r'^polls/', include('polls.urls', namespace="polls")),
|
|
url(r'^admin/', include(admin.site.urls)),
|
|
)
|
|
|
|
Now change your ``polls/index.html`` template from:
|
|
|
|
.. snippet:: html+django
|
|
:filename: polls/templates/polls/index.html
|
|
|
|
<li><a href="{% url 'detail' question.id %}">{{ question.question_text }}</a></li>
|
|
|
|
to point at the namespaced detail view:
|
|
|
|
.. snippet:: html+django
|
|
:filename: polls/templates/polls/index.html
|
|
|
|
<li><a href="{% url 'polls:detail' question.id %}">{{ question.question_text }}</a></li>
|
|
|
|
When you're comfortable with writing views, read :doc:`part 4 of this tutorial
|
|
</intro/tutorial04>` to learn about simple form processing and generic views.
|