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246 lines
10 KiB
Plaintext
246 lines
10 KiB
Plaintext
=====================================
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Writing your first Django app, part 4
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=====================================
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By Adrian Holovaty <holovaty@gmail.com>
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This tutorial begins where `Tutorial 3`_ left off. We're continuing the Web-poll
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application and will focus on simple form processing and 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 from the last tutorial, so that the
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template contains an HTML ``<form>`` element::
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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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{% for choice in poll.get_choice_list %}
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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 }}</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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Now, let's create a Django view that handles the submitted data and does
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something with it. Remember, in `Tutorial 3`_, we create a URLconf that
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included this line::
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(r'^polls/(?P<poll_id>\d+)/vote/$', 'myproject.apps.polls.views.polls.vote'),
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So let's create a ``vote()`` function in ``myproject/apps/polls/views/polls.py``::
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from django.core import template_loader
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from django.core.extensions import DjangoContext as Context
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from django.models.polls import choices, polls
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from django.utils.httpwrappers import HttpResponse, HttpResponseRedirect
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from django.core.exceptions import Http404
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def vote(request, poll_id):
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try:
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p = polls.get_object(pk=poll_id)
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except polls.PollDoesNotExist:
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raise Http404
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try:
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selected_choice = p.get_choice(pk=request.POST['choice'])
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except (KeyError, choices.ChoiceDoesNotExist):
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# Redisplay the poll voting form.
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t = template_loader.get_template('polls/detail')
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c = Context(request, {
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'poll': p,
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'error_message': "You didn't select a choice.",
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})
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return HttpResponse(t.render(c))
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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('/polls/%s/results/' % p.id)
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This code includes a few things we haven't covered yet in this tutorial:
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* ``request.POST`` is a dictionary-like object that lets you access
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submitted data by key name. In this case, ``request.POST['choice']``
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returns the ID of the selected choice, as a string. ``request.POST``
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values are always strings.
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Note that Django also provides ``request.GET`` for accessing GET data
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in the same way -- but we're explicitly using ``request.POST`` in our
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code, to ensure that data is only altered via a POST call.
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* ``request.POST['choice']`` will raise ``KeyError`` if ``choice`` wasn't
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provided in POST data. The above code checks for ``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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``HttpResponseRedirect`` rather than a normal ``HttpResponse``.
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``HttpResponseRedirect`` takes a single argument: the URL to which the
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user will be redirected. You should leave off the "http://" and domain
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name if you can. That helps your app become portable across domains.
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As the Python comment above points out, you should always return an
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``HttpResponseRedirect`` after successfully dealing with POST data. This
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tip isn't specific to Django; it's just good Web development practice.
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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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try:
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p = polls.get_object(pk=poll_id)
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except polls.PollDoesNotExist:
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raise Http404
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t = template_loader.get_template('polls/results')
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c = Context(request, {
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'poll': p,
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})
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return HttpResponse(t.render(c))
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This is almost exactly the same as the ``detail()`` view from `Tutorial 3`_.
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The only difference is the template name. We'll fix this redundancy later.
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Now, create a ``results.html`` template::
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<h1>{{ poll.question }}</h1>
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<ul>
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{% for choice in poll.get_choice_list %}
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<li>{{ choice.choice }} -- {{ choice.votes }} vote{{ choice.votes|pluralize }}</li>
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{% endfor %}
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</ul>
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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 `Tutorial 3`_) and ``results()`` views are stupidly
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simple -- and, as mentioned above, redundant. The ``index()`` view (also from
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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.
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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.py URLconf. It looks like this, according to the tutorial
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so far::
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from django.conf.urls.defaults import *
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urlpatterns = patterns('myproject.apps.polls.views.polls',
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(r'^$', 'index'),
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(r'^(?P<poll_id>\d+)/$', 'detail'),
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(r'^(?P<poll_id>\d+)/results/$', 'results'),
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(r'^(?P<poll_id>\d+)/vote/$', 'vote'),
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)
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Change it like so::
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from django.conf.urls.defaults import *
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info_dict = {
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'app_label': 'polls',
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'module_name': 'polls',
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}
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urlpatterns = patterns('',
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(r'^$', 'django.views.generic.list_detail.object_list', info_dict),
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(r'^(?P<object_id>\d+)/$', 'django.views.generic.list_detail.object_detail', info_dict),
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(r'^(?P<object_id>\d+)/results/$', 'django.views.generic.list_detail.object_detail', dict(info_dict, template_name='polls/results')),
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(r'^(?P<poll_id>\d+)/vote/$', 'myproject.apps.polls.views.polls.vote'),
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)
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We're using two generic views here: ``object_list`` and ``object_detail``.
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Respectively, those two views abstract the concepts of "display a list of
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objects" and "display a detail page for a particular type of object."
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* Each generic view needs to know which ``app_label`` and ``module_name``
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it's acting on. Thus, we've defined ``info_dict``, a dictionary that's
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passed to each of the generic views via the third parameter to the URL
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tuples.
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* The ``object_detail`` generic view expects that the ID value captured
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from the URL is called ``"object_id"``, so we've changed ``poll_id`` to
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``object_id`` for the generic views.
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By default, the ``object_detail`` generic view uses a template called
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``<app_label>/<module_name>_detail``. In our case, it'll use the template
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``"polls/polls_detail"``. Thus, rename your ``polls/detail.html`` template to
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``polls/polls_detail.html``, and change the ``template_loader.get_template()``
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line in ``vote()``.
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Similarly, the ``object_list`` generic view uses a template called
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``<app_label>/<module_name>_list``. Thus, rename ``polls/index.html`` to
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``polls/polls_list.html``.
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Because we have more than one entry in the URLconf that uses ``object_detail``
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for the polls app, we manually specify a template name for the results view:
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``template_name='polls/results'``. Otherwise, both views would use the same
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template. Note that we use ``dict()`` to return an altered dictionary in place.
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The generic views pass ``object`` and ``object_list`` to their templates, so
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change your templates so that ``latest_poll_list`` becomes ``object_list`` and
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``poll`` becomes ``object``.
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Finally, you can delete the ``index()``, ``detail()`` and ``results()`` views
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from ``polls/views/polls.py``. We don't need them anymore.
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For full details on generic views, see the `generic views documentation`_.
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.. _generic views documentation: http://www.djangoproject.com/documentation/generic_views/
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Coming soon
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===========
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The tutorial ends here for the time being. But check back soon for the next
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installments:
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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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.. _Tutorial 3: http://www.djangoproject.com/documentation/tutorial3/
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