mirror of
https://github.com/django/django.git
synced 2024-11-18 07:26:04 +00:00
281 lines
12 KiB
Plaintext
281 lines
12 KiB
Plaintext
=====================================
|
|
Writing your first Django app, part 4
|
|
=====================================
|
|
|
|
This tutorial begins where :doc:`Tutorial 3 </intro/tutorial03>` left off. We're
|
|
continuing the Web-poll application and will focus on simple form processing and
|
|
cutting down our code.
|
|
|
|
Write a simple form
|
|
===================
|
|
|
|
Let's update our poll detail template ("polls/detail.html") from the last
|
|
tutorial, so that the template contains an HTML ``<form>`` element:
|
|
|
|
.. code-block:: html+django
|
|
|
|
<h1>{{ poll.question }}</h1>
|
|
|
|
{% if error_message %}<p><strong>{{ error_message }}</strong></p>{% endif %}
|
|
|
|
<form action="{% url 'polls:vote' poll.id %}" method="post">
|
|
{% csrf_token %}
|
|
{% for choice in poll.choice_set.all %}
|
|
<input type="radio" name="choice" id="choice{{ forloop.counter }}" value="{{ choice.id }}" />
|
|
<label for="choice{{ forloop.counter }}">{{ choice.choice_text }}</label><br />
|
|
{% endfor %}
|
|
<input type="submit" value="Vote" />
|
|
</form>
|
|
|
|
A quick rundown:
|
|
|
|
* The above template displays a radio button for each poll choice. The
|
|
``value`` of each radio button is the associated poll choice's ID. The
|
|
``name`` of each radio button is ``"choice"``. That means, when somebody
|
|
selects one of the radio buttons and submits the form, it'll send the
|
|
POST data ``choice=3``. This is HTML Forms 101.
|
|
|
|
* We set the form's ``action`` to ``{% url 'polls:vote' poll.id %}``, and we
|
|
set ``method="post"``. Using ``method="post"`` (as opposed to
|
|
``method="get"``) is very important, because the act of submitting this
|
|
form will alter data server-side. Whenever you create a form that alters
|
|
data server-side, use ``method="post"``. This tip isn't specific to
|
|
Django; it's just good Web development practice.
|
|
|
|
* ``forloop.counter`` indicates how many times the :ttag:`for` tag has gone
|
|
through its loop
|
|
|
|
* Since we're creating a POST form (which can have the effect of modifying
|
|
data), we need to worry about Cross Site Request Forgeries.
|
|
Thankfully, you don't have to worry too hard, because Django comes with
|
|
a very easy-to-use system for protecting against it. In short, all POST
|
|
forms that are targeted at internal URLs should use the
|
|
:ttag:`{% csrf_token %}<csrf_token>` template tag.
|
|
|
|
Now, let's create a Django view that handles the submitted data and does
|
|
something with it. Remember, in :doc:`Tutorial 3 </intro/tutorial03>`, we
|
|
created a URLconf for the polls application that includes this line::
|
|
|
|
url(r'^(?P<poll_id>\d+)/vote/$', views.vote, name='vote'),
|
|
|
|
We also created a dummy implementation of the ``vote()`` function. Let's
|
|
create a real version. Add the following to ``polls/views.py``::
|
|
|
|
from django.shortcuts import get_object_or_404, render
|
|
from django.http import HttpResponseRedirect, HttpResponse
|
|
from django.core.urlresolvers import reverse
|
|
from polls.models import Choice, Poll
|
|
# ...
|
|
def vote(request, poll_id):
|
|
p = get_object_or_404(Poll, pk=poll_id)
|
|
try:
|
|
selected_choice = p.choice_set.get(pk=request.POST['choice'])
|
|
except (KeyError, Choice.DoesNotExist):
|
|
# Redisplay the poll voting form.
|
|
return render(request, 'polls/detail.html', {
|
|
'poll': p,
|
|
'error_message': "You didn't select a choice.",
|
|
})
|
|
else:
|
|
selected_choice.votes += 1
|
|
selected_choice.save()
|
|
# Always return an HttpResponseRedirect after successfully dealing
|
|
# with POST data. This prevents data from being posted twice if a
|
|
# user hits the Back button.
|
|
return HttpResponseRedirect(reverse('polls:results', args=(p.id,)))
|
|
|
|
This code includes a few things we haven't covered yet in this tutorial:
|
|
|
|
* :attr:`request.POST <django.http.HttpRequest.POST>` is a dictionary-like
|
|
object that lets you access submitted data by key name. In this case,
|
|
``request.POST['choice']`` returns the ID of the selected choice, as a
|
|
string. :attr:`request.POST <django.http.HttpRequest.POST>` values are
|
|
always strings.
|
|
|
|
Note that Django also provides :attr:`request.GET
|
|
<django.http.HttpRequest.GET>` for accessing GET data in the same way --
|
|
but we're explicitly using :attr:`request.POST
|
|
<django.http.HttpRequest.POST>` in our code, to ensure that data is only
|
|
altered via a POST call.
|
|
|
|
* ``request.POST['choice']`` will raise :exc:`~exceptions.KeyError` if
|
|
``choice`` wasn't provided in POST data. The above code checks for
|
|
:exc:`~exceptions.KeyError` and redisplays the poll form with an error
|
|
message if ``choice`` isn't given.
|
|
|
|
* After incrementing the choice count, the code returns an
|
|
:class:`~django.http.HttpResponseRedirect` rather than a normal
|
|
:class:`~django.http.HttpResponse`.
|
|
:class:`~django.http.HttpResponseRedirect` takes a single argument: the
|
|
URL to which the user will be redirected (see the following point for how
|
|
we construct the URL in this case).
|
|
|
|
As the Python comment above points out, you should always return an
|
|
:class:`~django.http.HttpResponseRedirect` after successfully dealing with
|
|
POST data. This tip isn't specific to Django; it's just good Web
|
|
development practice.
|
|
|
|
* We are using the :func:`~django.core.urlresolvers.reverse` function in the
|
|
:class:`~django.http.HttpResponseRedirect` constructor in this example.
|
|
This function helps avoid having to hardcode a URL in the view function.
|
|
It is given the name of the view that we want to pass control to and the
|
|
variable portion of the URL pattern that points to that view. In this
|
|
case, using the URLconf we set up in Tutorial 3, this
|
|
:func:`~django.core.urlresolvers.reverse` call will return a string like
|
|
::
|
|
|
|
'/polls/3/results/'
|
|
|
|
... where the ``3`` is the value of ``p.id``. This redirected URL will
|
|
then call the ``'results'`` view to display the final page.
|
|
|
|
As mentioned in Tutorial 3, ``request`` is a :class:`~django.http.HttpRequest`
|
|
object. For more on :class:`~django.http.HttpRequest` objects, see the
|
|
:doc:`request and response documentation </ref/request-response>`.
|
|
|
|
After somebody votes in a poll, the ``vote()`` view redirects to the results
|
|
page for the poll. Let's write that view::
|
|
|
|
def results(request, poll_id):
|
|
poll = get_object_or_404(Poll, pk=poll_id)
|
|
return render(request, 'polls/results.html', {'poll': poll})
|
|
|
|
This is almost exactly the same as the ``detail()`` view from :doc:`Tutorial 3
|
|
</intro/tutorial03>`. The only difference is the template name. We'll fix this
|
|
redundancy later.
|
|
|
|
Now, create a ``polls/results.html`` template:
|
|
|
|
.. code-block:: html+django
|
|
|
|
<h1>{{ poll.question }}</h1>
|
|
|
|
<ul>
|
|
{% for choice in poll.choice_set.all %}
|
|
<li>{{ choice.choice_text }} -- {{ choice.votes }} vote{{ choice.votes|pluralize }}</li>
|
|
{% endfor %}
|
|
</ul>
|
|
|
|
<a href="{% url 'polls:detail' poll.id %}">Vote again?</a>
|
|
|
|
Now, go to ``/polls/1/`` in your browser and vote in the poll. You should see a
|
|
results page that gets updated each time you vote. If you submit the form
|
|
without having chosen a choice, you should see the error message.
|
|
|
|
Use generic views: Less code is better
|
|
======================================
|
|
|
|
The ``detail()`` (from :doc:`Tutorial 3 </intro/tutorial03>`) and ``results()``
|
|
views are stupidly simple -- and, as mentioned above, redundant. The ``index()``
|
|
view (also from Tutorial 3), which displays a list of polls, is similar.
|
|
|
|
These views represent a common case of basic Web development: getting data from
|
|
the database according to a parameter passed in the URL, loading a template and
|
|
returning the rendered template. Because this is so common, Django provides a
|
|
shortcut, called the "generic views" system.
|
|
|
|
Generic views abstract common patterns to the point where you don't even need
|
|
to write Python code to write an app.
|
|
|
|
Let's convert our poll app to use the generic views system, so we can delete a
|
|
bunch of our own code. We'll just have to take a few steps to make the
|
|
conversion. We will:
|
|
|
|
1. Convert the URLconf.
|
|
|
|
2. Delete some of the old, unneeded views.
|
|
|
|
3. Fix up URL handling for the new views.
|
|
|
|
Read on for details.
|
|
|
|
.. admonition:: Why the code-shuffle?
|
|
|
|
Generally, when writing a Django app, you'll evaluate whether generic views
|
|
are a good fit for your problem, and you'll use them from the beginning,
|
|
rather than refactoring your code halfway through. But this tutorial
|
|
intentionally has focused on writing the views "the hard way" until now, to
|
|
focus on core concepts.
|
|
|
|
You should know basic math before you start using a calculator.
|
|
|
|
First, open the ``polls/urls.py`` URLconf and change it like so::
|
|
|
|
from django.conf.urls import patterns, url
|
|
from django.views.generic import DetailView, ListView
|
|
from polls.models import Poll
|
|
|
|
urlpatterns = patterns('',
|
|
url(r'^$',
|
|
ListView.as_view(
|
|
queryset=Poll.objects.order_by('-pub_date')[:5],
|
|
context_object_name='latest_poll_list',
|
|
template_name='polls/index.html'),
|
|
name='index'),
|
|
url(r'^(?P<pk>\d+)/$',
|
|
DetailView.as_view(
|
|
model=Poll,
|
|
template_name='polls/detail.html'),
|
|
name='detail'),
|
|
url(r'^(?P<pk>\d+)/results/$',
|
|
DetailView.as_view(
|
|
model=Poll,
|
|
template_name='polls/results.html'),
|
|
name='results'),
|
|
url(r'^(?P<poll_id>\d+)/vote/$', 'polls.views.vote', name='vote'),
|
|
)
|
|
|
|
We're using two generic views here:
|
|
:class:`~django.views.generic.list.ListView` and
|
|
:class:`~django.views.generic.detail.DetailView`. Respectively, those
|
|
two views abstract the concepts of "display a list of objects" and
|
|
"display a detail page for a particular type of object."
|
|
|
|
* Each generic view needs to know what model it will be acting
|
|
upon. This is provided using the ``model`` parameter.
|
|
|
|
* The :class:`~django.views.generic.detail.DetailView` generic view
|
|
expects the primary key value captured from the URL to be called
|
|
``"pk"``, so we've changed ``poll_id`` to ``pk`` for the generic
|
|
views.
|
|
|
|
By default, the :class:`~django.views.generic.detail.DetailView` generic
|
|
view uses a template called ``<app name>/<model name>_detail.html``.
|
|
In our case, it'll use the template ``"polls/poll_detail.html"``. The
|
|
``template_name`` argument is used to tell Django to use a specific
|
|
template name instead of the autogenerated default template name. We
|
|
also specify the ``template_name`` for the ``results`` list view --
|
|
this ensures that the results view and the detail view have a
|
|
different appearance when rendered, even though they're both a
|
|
:class:`~django.views.generic.detail.DetailView` behind the scenes.
|
|
|
|
Similarly, the :class:`~django.views.generic.list.ListView` generic
|
|
view uses a default template called ``<app name>/<model
|
|
name>_list.html``; we use ``template_name`` to tell
|
|
:class:`~django.views.generic.list.ListView` to use our existing
|
|
``"polls/index.html"`` template.
|
|
|
|
In previous parts of the tutorial, the templates have been provided
|
|
with a context that contains the ``poll`` and ``latest_poll_list``
|
|
context variables. For ``DetailView`` the ``poll`` variable is provided
|
|
automatically -- since we're using a Django model (``Poll``), Django
|
|
is able to determine an appropriate name for the context variable.
|
|
However, for ListView, the automatically generated context variable is
|
|
``poll_list``. To override this we provide the ``context_object_name``
|
|
option, specifying that we want to use ``latest_poll_list`` instead.
|
|
As an alternative approach, you could change your templates to match
|
|
the new default context variables -- but it's a lot easier to just
|
|
tell Django to use the variable you want.
|
|
|
|
You can now delete the ``index()``, ``detail()`` and ``results()``
|
|
views from ``polls/views.py``. We don't need them anymore -- they have
|
|
been replaced by generic views.
|
|
|
|
Run the server, and use your new polling app based on generic views.
|
|
|
|
For full details on generic views, see the :doc:`generic views documentation
|
|
</topics/class-based-views/index>`.
|
|
|
|
When you're comfortable with forms and generic views, read :doc:`part 5 of this
|
|
tutorial</intro/tutorial05>` to learn about testing our polls app.
|