Writing your first Django app, part 4

This tutorial begins where Tutorial 3 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:

<h1>{{ poll.question }}</h1>

{% if error_message %}<p><strong>{{ error_message }}</strong></p>{% endif %}

<form action="/polls/{{ poll.id }}/vote/" 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 }}</label><br />
{% endfor %}
<input type="submit" value="Vote" />

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 /polls/{{ poll.id }}/vote/, 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 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 {% csrf_token %} template tag.

The {% csrf_token %} tag requires information from the request object, which is not normally accessible from within the template context. To fix this, a small adjustment needs to be made to the detail view, so that it looks like the following:

from django.template import RequestContext
# ...
def detail(request, poll_id):
    p = get_object_or_404(Poll, pk=poll_id)
    return render_to_response('polls/detail.html', {'poll': p},

The details of how this works are explained in the documentation for RequestContext.

Now, let's create a Django view that handles the submitted data and does something with it. Remember, in Tutorial 3, we created a URLconf for the polls application that includes this line:

(r'^(?P<poll_id>\d+)/vote/$', '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_to_response
from django.http import HttpResponseRedirect, HttpResponse
from django.core.urlresolvers import reverse
from django.template import RequestContext
from polls.models import Choice, Poll
# ...
def vote(request, poll_id):
    p = get_object_or_404(Poll, pk=poll_id)
        selected_choice = p.choice_set.get(pk=request.POST['choice'])
    except (KeyError, Choice.DoesNotExist):
        # Redisplay the poll voting form.
        return render_to_response('polls/detail.html', {
            'poll': p,
            'error_message': "You didn't select a choice.",
        }, context_instance=RequestContext(request))
        selected_choice.votes += 1
        # 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.views.results', args=(p.id,)))

This code includes a few things we haven't covered yet in this tutorial:

  • request.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. request.POST values are always strings.

    Note that Django also provides request.GET for accessing GET data in the same way -- but we're explicitly using request.POST in our code, to ensure that data is only altered via a POST call.

  • request.POST['choice'] will raise KeyError if choice wasn't provided in POST data. The above code checks for KeyError and redisplays the poll form with an error message if choice isn't given.

  • After incrementing the choice count, the code returns an HttpResponseRedirect rather than a normal HttpResponse. 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 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 reverse() function in the 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 reverse() call will return a string like


    ... where the 3 is the value of p.id. This redirected URL will then call the 'results' view to display the final page. Note that you need to use the full name of the view here (including the prefix).

As mentioned in Tutorial 3, request is a HttpRequest object. For more on HttpRequest objects, see the request and response documentation.

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):
    p = get_object_or_404(Poll, pk=poll_id)
    return render_to_response('polls/results.html', {'poll': p})

This is almost exactly the same as the detail() view from Tutorial 3. The only difference is the template name. We'll fix this redundancy later.

Now, create a results.html template:

<h1>{{ poll.question }}</h1>

{% for choice in poll.choice_set.all %}
    <li>{{ choice.choice }} -- {{ choice.votes }} vote{{ choice.votes|pluralize }}</li>
{% endfor %}

<a href="/polls/{{ 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 Tutorial 3) 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.

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. It looks like this, according to the tutorial so far:

from django.conf.urls import patterns, include, url

urlpatterns = patterns('polls.views',
    url(r'^$', 'index'),
    url(r'^(?P<poll_id>\d+)/$', 'detail'),
    url(r'^(?P<poll_id>\d+)/results/$', 'results'),
    url(r'^(?P<poll_id>\d+)/vote/$', 'vote'),

Change it like so:

from django.conf.urls import patterns, include, url
from django.views.generic import DetailView, ListView
from polls.models import Poll

urlpatterns = patterns('',
    url(r'^(?P<poll_id>\d+)/vote/$', 'polls.views.vote'),

We're using two generic views here: ListView and 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 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.
  • We've added a name, poll_results, to the results view so that we have a way to refer to its URL later on (see the documentation about naming URL patterns for information). We're also using the url() function from django.conf.urls here. It's a good habit to use url() when you are providing a pattern name like this.

By default, the 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 DetailView behind the scenes.

Similarly, the ListView generic view uses a default template called <app name>/<model name>_list.html; we use template_name to tell 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.

The last thing to do is fix the URL handling to account for the use of generic views. In the vote view above, we used the reverse() function to avoid hard-coding our URLs. Now that we've switched to a generic view, we'll need to change the reverse() call to point back to our new generic view. We can't simply use the view function anymore -- generic views can be (and are) used multiple times -- but we can use the name we've given:

return HttpResponseRedirect(reverse('poll_results', args=(p.id,)))

Run the server, and use your new polling app based on generic views.

For full details on generic views, see the generic views documentation.

Coming soon

The tutorial ends here for the time being. Future installments of the tutorial will cover:

  • Advanced form processing
  • Using the RSS framework
  • Using the cache framework
  • Using the comments framework
  • Advanced admin features: Permissions
  • Advanced admin features: Custom JavaScript

In the meantime, you might want to check out some pointers on where to go from here