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.
Let’s update our poll detail template (“polls/detail.html”) from the last
tutorial, so that the template contains an HTML <form> element:
<h1>{{ question.question_text }}</h1>
{% if error_message %}<p><strong>{{ error_message }}</strong></p>{% endif %}
<form action="{% url 'polls:vote' question.id %}" method="post">
{% csrf_token %}
{% for choice in question.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:
value of each radio button is the associated question 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=# where # is the ID of the selected choice. This is the
basic concept of HTML forms.action to {% url 'polls:vote' question.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 for tag has gone
through its loop{% csrf_token %} template tag.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:
url(r'^(?P<question_id>[0-9]+)/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.urls import reverse
from .models import Choice, Question
# ...
def vote(request, question_id):
question = get_object_or_404(Question, pk=question_id)
try:
selected_choice = question.choice_set.get(pk=request.POST['choice'])
except (KeyError, Choice.DoesNotExist):
# Redisplay the question voting form.
return render(request, 'polls/detail.html', {
'question': question,
'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=(question.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 question 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
'/polls/3/results/'
where the 3 is the value of question.id. This redirected URL will
then call the 'results' view to display the final page.
As mentioned in Tutorial 3, request is an
HttpRequest object. For more on
HttpRequest objects, see the request and
response documentation.
After somebody votes in a question, the vote() view redirects to the results
page for the question. Let’s write that view:
from django.shortcuts import get_object_or_404, render
def results(request, question_id):
question = get_object_or_404(Question, pk=question_id)
return render(request, 'polls/results.html', {'question': question})
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 polls/results.html template:
<h1>{{ question.question_text }}</h1>
<ul>
{% for choice in question.choice_set.all %}
<li>{{ choice.choice_text }} -- {{ choice.votes }} vote{{ choice.votes|pluralize }}</li>
{% endfor %}
</ul>
<a href="{% url 'polls:detail' question.id %}">Vote again?</a>
Now, go to /polls/1/ in your browser and vote in the question. 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.
Note
The code for our vote() view does have a small problem. It first gets
the selected_choice object from the database, then computes the new
value of votes, and then saves it back to the database. If two users of
your website try to vote at exactly the same time, this might go wrong:
The same value, let’s say 42, will be retrieved for votes. Then, for
both users the new value of 43 is computed and saved, but 44 would be the
expected value.
This is called a race condition. If you are interested, you can read Avoiding race conditions using F() to learn how you can solve this issue.
The detail() (from Tutorial 3) and results()
views are very simple – and, as mentioned above, redundant. The index()
view, 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:
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 and change it like so:
from django.conf.urls import url
from . import views
app_name = 'polls'
urlpatterns = [
url(r'^$', views.IndexView.as_view(), name='index'),
url(r'^(?P<pk>[0-9]+)/$', views.DetailView.as_view(), name='detail'),
url(r'^(?P<pk>[0-9]+)/results/$', views.ResultsView.as_view(), name='results'),
url(r'^(?P<question_id>[0-9]+)/vote/$', views.vote, name='vote'),
]
Note that the name of the matched pattern in the regexes of the second and third
patterns has changed from <question_id> to <pk>.
Next, we’re going to remove our old index, detail, and results
views and use Django’s generic views instead. To do so, open the
polls/views.py file and change it like so:
from django.shortcuts import get_object_or_404, render
from django.http import HttpResponseRedirect
from django.urls import reverse
from django.views import generic
from .models import Choice, Question
class IndexView(generic.ListView):
template_name = 'polls/index.html'
context_object_name = 'latest_question_list'
def get_queryset(self):
"""Return the last five published questions."""
return Question.objects.order_by('-pub_date')[:5]
class DetailView(generic.DetailView):
model = Question
template_name = 'polls/detail.html'
class ResultsView(generic.DetailView):
model = Question
template_name = 'polls/results.html'
def vote(request, question_id):
... # same as above, no changes needed.
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.”
model attribute.DetailView generic view
expects the primary key value captured from the URL to be called
"pk", so we’ve changed question_id to pk for the generic
views.By default, the DetailView generic
view uses a template called <app name>/<model name>_detail.html.
In our case, it would use the template "polls/question_detail.html". The
template_name attribute 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 question and latest_question_list
context variables. For DetailView the question variable is provided
automatically – since we’re using a Django model (Question), Django
is able to determine an appropriate name for the context variable.
However, for ListView, the automatically generated context variable is
question_list. To override this we provide the context_object_name
attribute, specifying that we want to use latest_question_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.
Run the server, and use your new polling app based on generic views.
For full details on generic views, see the generic views documentation.
When you’re comfortable with forms and generic views, read part 5 of this tutorial to learn about testing our polls app.
Jun 14, 2020