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Many thanks to Daniel Greenfeld, James Aylett, Marc Tamlyn, Simon Williams, Danilo Bargen and Luke Plant for their work on this.
340 lines
14 KiB
Plaintext
340 lines
14 KiB
Plaintext
=====================================
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Writing your first Django app, part 4
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=====================================
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This tutorial begins where :doc:`Tutorial 3 </intro/tutorial03>` left off. We're
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continuing the Web-poll application and will focus on simple form processing and
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cutting down our code.
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Write a simple form
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===================
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Let's update our poll detail template ("polls/detail.html") from the last
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tutorial, so that the template contains an HTML ``<form>`` element:
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.. code-block:: html+django
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<h1>{{ poll.question }}</h1>
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{% if error_message %}<p><strong>{{ error_message }}</strong></p>{% endif %}
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<form action="/polls/{{ poll.id }}/vote/" method="post">
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{% csrf_token %}
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{% for choice in poll.choice_set.all %}
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<input type="radio" name="choice" id="choice{{ forloop.counter }}" value="{{ choice.id }}" />
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<label for="choice{{ forloop.counter }}">{{ choice.choice_text }}</label><br />
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{% endfor %}
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<input type="submit" value="Vote" />
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</form>
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A quick rundown:
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* The above template displays a radio button for each poll choice. The
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``value`` of each radio button is the associated poll choice's ID. The
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``name`` of each radio button is ``"choice"``. That means, when somebody
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selects one of the radio buttons and submits the form, it'll send the
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POST data ``choice=3``. This is HTML Forms 101.
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* We set the form's ``action`` to ``/polls/{{ poll.id }}/vote/``, and we
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set ``method="post"``. Using ``method="post"`` (as opposed to
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``method="get"``) is very important, because the act of submitting this
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form will alter data server-side. Whenever you create a form that alters
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data server-side, use ``method="post"``. This tip isn't specific to
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Django; it's just good Web development practice.
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* ``forloop.counter`` indicates how many times the :ttag:`for` tag has gone
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through its loop
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* Since we're creating a POST form (which can have the effect of modifying
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data), we need to worry about Cross Site Request Forgeries.
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Thankfully, you don't have to worry too hard, because Django comes with
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a very easy-to-use system for protecting against it. In short, all POST
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forms that are targeted at internal URLs should use the
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:ttag:`{% csrf_token %}<csrf_token>` template tag.
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The :ttag:`{% csrf_token %}<csrf_token>` tag requires information from the
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request object, which is not normally accessible from within the template
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context. To fix this, a small adjustment needs to be made to the ``detail``
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view, so that it looks like the following::
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from django.template import RequestContext
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# ...
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def detail(request, poll_id):
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p = get_object_or_404(Poll, pk=poll_id)
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return render_to_response('polls/detail.html', {'poll': p},
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context_instance=RequestContext(request))
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The details of how this works are explained in the documentation for
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:ref:`RequestContext <subclassing-context-requestcontext>`.
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Now, let's create a Django view that handles the submitted data and does
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something with it. Remember, in :doc:`Tutorial 3 </intro/tutorial03>`, we
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created a URLconf for the polls application that includes this line::
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(r'^(?P<poll_id>\d+)/vote/$', 'vote'),
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We also created a dummy implementation of the ``vote()`` function. Let's
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create a real version. Add the following to ``polls/views.py``::
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from django.shortcuts import get_object_or_404, render_to_response
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from django.http import HttpResponseRedirect, HttpResponse
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from django.core.urlresolvers import reverse
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from django.template import RequestContext
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from polls.models import Choice, Poll
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# ...
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def vote(request, poll_id):
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p = get_object_or_404(Poll, pk=poll_id)
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try:
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selected_choice = p.choice_set.get(pk=request.POST['choice'])
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except (KeyError, Choice.DoesNotExist):
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# Redisplay the poll voting form.
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return render_to_response('polls/detail.html', {
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'poll': p,
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'error_message': "You didn't select a choice.",
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}, context_instance=RequestContext(request))
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else:
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selected_choice.votes += 1
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selected_choice.save()
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# Always return an HttpResponseRedirect after successfully dealing
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# with POST data. This prevents data from being posted twice if a
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# user hits the Back button.
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return HttpResponseRedirect(reverse('polls.views.results', args=(p.id,)))
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This code includes a few things we haven't covered yet in this tutorial:
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* :attr:`request.POST <django.http.HttpRequest.POST>` is a dictionary-like
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object that lets you access submitted data by key name. In this case,
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``request.POST['choice']`` returns the ID of the selected choice, as a
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string. :attr:`request.POST <django.http.HttpRequest.POST>` values are
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always strings.
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Note that Django also provides :attr:`request.GET
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<django.http.HttpRequest.GET>` for accessing GET data in the same way --
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but we're explicitly using :attr:`request.POST
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<django.http.HttpRequest.POST>` in our code, to ensure that data is only
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altered via a POST call.
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* ``request.POST['choice']`` will raise :exc:`KeyError` if ``choice`` wasn't
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provided in POST data. The above code checks for :exc:`KeyError` and
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redisplays the poll form with an error message if ``choice`` isn't given.
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* After incrementing the choice count, the code returns an
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:class:`~django.http.HttpResponseRedirect` rather than a normal
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:class:`~django.http.HttpResponse`.
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:class:`~django.http.HttpResponseRedirect` takes a single argument: the
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URL to which the user will be redirected (see the following point for how
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we construct the URL in this case).
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As the Python comment above points out, you should always return an
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:class:`~django.http.HttpResponseRedirect` after successfully dealing with
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POST data. This tip isn't specific to Django; it's just good Web
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development practice.
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* We are using the :func:`~django.core.urlresolvers.reverse` function in the
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:class:`~django.http.HttpResponseRedirect` constructor in this example.
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This function helps avoid having to hardcode a URL in the view function.
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It is given the name of the view that we want to pass control to and the
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variable portion of the URL pattern that points to that view. In this
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case, using the URLconf we set up in Tutorial 3, this
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:func:`~django.core.urlresolvers.reverse` call will return a string like
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::
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'/polls/3/results/'
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... where the ``3`` is the value of ``p.id``. This redirected URL will
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then call the ``'results'`` view to display the final page. Note that you
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need to use the full name of the view here (including the prefix).
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As mentioned in Tutorial 3, ``request`` is a :class:`~django.http.HttpRequest`
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object. For more on :class:`~django.http.HttpRequest` objects, see the
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:doc:`request and response documentation </ref/request-response>`.
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After somebody votes in a poll, the ``vote()`` view redirects to the results
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page for the poll. Let's write that view::
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def results(request, poll_id):
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p = get_object_or_404(Poll, pk=poll_id)
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return render_to_response('polls/results.html', {'poll': p})
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This is almost exactly the same as the ``detail()`` view from :doc:`Tutorial 3
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</intro/tutorial03>`. The only difference is the template name. We'll fix this
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redundancy later.
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Now, create a ``results.html`` template:
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.. code-block:: html+django
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<h1>{{ poll.question }}</h1>
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<ul>
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{% for choice in poll.choice_set.all %}
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<li>{{ choice.choice_text }} -- {{ choice.votes }} vote{{ choice.votes|pluralize }}</li>
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{% endfor %}
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</ul>
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<a href="/polls/{{ poll.id }}/">Vote again?</a>
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Now, go to ``/polls/1/`` in your browser and vote in the poll. You should see a
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results page that gets updated each time you vote. If you submit the form
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without having chosen a choice, you should see the error message.
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Use generic views: Less code is better
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======================================
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The ``detail()`` (from :doc:`Tutorial 3 </intro/tutorial03>`) and ``results()``
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views are stupidly simple -- and, as mentioned above, redundant. The ``index()``
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view (also from Tutorial 3), which displays a list of polls, is similar.
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These views represent a common case of basic Web development: getting data from
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the database according to a parameter passed in the URL, loading a template and
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returning the rendered template. Because this is so common, Django provides a
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shortcut, called the "generic views" system.
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Generic views abstract common patterns to the point where you don't even need
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to write Python code to write an app.
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Let's convert our poll app to use the generic views system, so we can delete a
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bunch of our own code. We'll just have to take a few steps to make the
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conversion. We will:
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1. Convert the URLconf.
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2. Delete some of the old, unneeded views.
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3. Fix up URL handling for the new views.
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Read on for details.
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.. admonition:: Why the code-shuffle?
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Generally, when writing a Django app, you'll evaluate whether generic views
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are a good fit for your problem, and you'll use them from the beginning,
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rather than refactoring your code halfway through. But this tutorial
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intentionally has focused on writing the views "the hard way" until now, to
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focus on core concepts.
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You should know basic math before you start using a calculator.
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First, open the ``polls/urls.py`` URLconf. It looks like this, according to the
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tutorial so far::
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from django.conf.urls import patterns, include, url
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urlpatterns = patterns('polls.views',
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url(r'^$', 'index'),
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url(r'^(?P<poll_id>\d+)/$', 'detail'),
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url(r'^(?P<poll_id>\d+)/results/$', 'results'),
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url(r'^(?P<poll_id>\d+)/vote/$', 'vote'),
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)
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Change it like so::
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from django.conf.urls import patterns, include, url
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from django.views.generic import DetailView, ListView
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from polls.models import Poll
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urlpatterns = patterns('',
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url(r'^$',
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ListView.as_view(
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queryset=Poll.objects.order_by('-pub_date')[:5],
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context_object_name='latest_poll_list',
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template_name='polls/index.html')),
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url(r'^(?P<pk>\d+)/$',
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DetailView.as_view(
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model=Poll,
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template_name='polls/detail.html')),
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url(r'^(?P<pk>\d+)/results/$',
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DetailView.as_view(
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model=Poll,
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template_name='polls/results.html'),
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name='poll_results'),
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url(r'^(?P<poll_id>\d+)/vote/$', 'polls.views.vote'),
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)
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We're using two generic views here:
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:class:`~django.views.generic.list.ListView` and
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:class:`~django.views.generic.detail.DetailView`. Respectively, those
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two views abstract the concepts of "display a list of objects" and
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"display a detail page for a particular type of object."
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* Each generic view needs to know what model it will be acting
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upon. This is provided using the ``model`` parameter.
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* The :class:`~django.views.generic.list.DetailView` generic view
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expects the primary key value captured from the URL to be called
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``"pk"``, so we've changed ``poll_id`` to ``pk`` for the generic
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views.
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* We've added a name, ``poll_results``, to the results view so
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that we have a way to refer to its URL later on (see the
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documentation about :ref:`naming URL patterns
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<naming-url-patterns>` for information). We're also using the
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:func:`~django.conf.urls.url` function from
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:mod:`django.conf.urls` here. It's a good habit to use
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:func:`~django.conf.urls.url` when you are providing a
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pattern name like this.
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By default, the :class:`~django.views.generic.list.DetailView` generic
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view uses a template called ``<app name>/<model name>_detail.html``.
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In our case, it'll use the template ``"polls/poll_detail.html"``. The
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``template_name`` argument is used to tell Django to use a specific
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template name instead of the autogenerated default template name. We
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also specify the ``template_name`` for the ``results`` list view --
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this ensures that the results view and the detail view have a
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different appearance when rendered, even though they're both a
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:class:`~django.views.generic.list.DetailView` behind the scenes.
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Similarly, the :class:`~django.views.generic.list.ListView` generic
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view uses a default template called ``<app name>/<model
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name>_list.html``; we use ``template_name`` to tell
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:class:`~django.views.generic.list.ListView` to use our existing
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``"polls/index.html"`` template.
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In previous parts of the tutorial, the templates have been provided
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with a context that contains the ``poll`` and ``latest_poll_list``
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context variables. For DetailView the ``poll`` variable is provided
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automatically -- since we're using a Django model (``Poll``), Django
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is able to determine an appropriate name for the context variable.
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However, for ListView, the automatically generated context variable is
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``poll_list``. To override this we provide the ``context_object_name``
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option, specifying that we want to use ``latest_poll_list`` instead.
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As an alternative approach, you could change your templates to match
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the new default context variables -- but it's a lot easier to just
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tell Django to use the variable you want.
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You can now delete the ``index()``, ``detail()`` and ``results()``
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views from ``polls/views.py``. We don't need them anymore -- they have
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been replaced by generic views.
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The last thing to do is fix the URL handling to account for the use of
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generic views. In the vote view above, we used the
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:func:`~django.core.urlresolvers.reverse` function to avoid
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hard-coding our URLs. Now that we've switched to a generic view, we'll
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need to change the :func:`~django.core.urlresolvers.reverse` call to
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point back to our new generic view. We can't simply use the view
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function anymore -- generic views can be (and are) used multiple times
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-- but we can use the name we've given::
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return HttpResponseRedirect(reverse('poll_results', args=(p.id,)))
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Run the server, and use your new polling app based on generic views.
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For full details on generic views, see the :doc:`generic views documentation
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</topics/class-based-views/index>`.
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Coming soon
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===========
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The tutorial ends here for the time being. Future installments of the tutorial
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will cover:
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* Advanced form processing
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* Using the RSS framework
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* Using the cache framework
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* Using the comments framework
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* Advanced admin features: Permissions
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* Advanced admin features: Custom JavaScript
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In the meantime, you might want to check out some pointers on :doc:`where to go
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from here </intro/whatsnext>`
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