Writing your first Django app, part 4 | Django documentation (2024)

Where to get help:

If you’re having trouble going through this tutorial, please head over tothe Getting Help section of the FAQ.

Write a minimal form¶

Let’s update our poll detail template (“polls/detail.html”) from the lasttutorial, so that the template contains an HTML <form> element:

<form action="{% url 'polls:vote' question.id %}" method="post">{% csrf_token %}<fieldset> <legend><h1>{{ question.question_text }}</h1></legend> {% if error_message %}<p><strong>{{ error_message }}</strong></p>{% endif %} {% 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 %}</fieldset><input type="submit" value="Vote"></form>

A quick rundown:

• The above template displays a radio button for each question choice. Thevalue of each radio button is the associated question choice’s ID. Thename of each radio button is "choice". That means, when somebodyselects one of the radio buttons and submits the form, it’ll send thePOST data choice=# where # is the ID of the selected choice. This is thebasic concept of HTML forms.
• We set the form’s action to {% url 'polls:vote' question.id %}, and weset method="post". Using method="post" (as opposed tomethod="get") is very important, because the act of submitting thisform will alter data server-side. Whenever you create a form that altersdata server-side, use method="post". This tip isn’t specific toDjango; it’s good web development practice in general.
• forloop.counter indicates how many times the for tag has gonethrough its loop
• Since we’re creating a POST form (which can have the effect of modifyingdata), we need to worry about Cross Site Request Forgeries.Thankfully, you don’t have to worry too hard, because Django comes with ahelpful system for protecting against it. In short, all POST forms that aretargeted at internal URLs should use the {% csrf_token %}template tag.

Now, let’s create a Django view that handles the submitted data and doessomething with it. Remember, in Tutorial 3, wecreated a URLconf for the polls application that includes this line:

path("<int:question_id>/vote/", views.vote, name="vote"),

We also created a dummy implementation of the vote() function. Let’screate a real version. Add the following to polls/views.py:

from django.db.models import Ffrom django.http import HttpResponse, HttpResponseRedirectfrom django.shortcuts import get_object_or_404, renderfrom django.urls import reversefrom .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 = F("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-likeobject that lets you access submitted data by key name. In this case,request.POST['choice'] returns the ID of the selected choice, as astring. request.POST values arealways 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 onlyaltered via a POST call.

• request.POST['choice'] will raise KeyError ifchoice wasn’t provided in POST data. The above code checks forKeyError and redisplays the question form with an errormessage if choice isn’t given.

• F("votes") + 1 instructs the database to increase the vote count by 1.

• After incrementing the choice count, the code returns anHttpResponseRedirect rather than a normalHttpResponse.HttpResponseRedirect takes a single argument: theURL to which the user will be redirected (see the following point for howwe construct the URL in this case).

  As the Python comment above points out, you should always return anHttpResponseRedirect after successfully dealing withPOST data. This tip isn’t specific to Django; it’s good web developmentpractice in general.

• We are using the reverse() function in theHttpResponseRedirect 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 thevariable portion of the URL pattern that points to that view. In thiscase, 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 willthen call the 'results' view to display the final page.

As mentioned in Tutorial 3, request is anHttpRequest object. For more onHttpRequest objects, see the request andresponse documentation.

After somebody votes in a question, the vote() view redirects to the resultspage for the question. Let’s write that view:

from django.shortcuts import get_object_or_404, renderdef 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 thisredundancy 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 aresults page that gets updated each time you vote. If you submit the formwithout 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 very short – 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 fromthe database according to a parameter passed in the URL, loading a template andreturning the rendered template. Because this is so common, Django provides ashortcut, called the “generic views” system.

Generic views abstract common patterns to the point where you don’t even need towrite Python code to write an app. For example, theListView andDetailView generic viewsabstract the concepts of “display a list of objects” and“display a detail page for a particular type of object” respectively.

Let’s convert our poll app to use the generic views system, so we can delete abunch of our own code. We’ll 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. Introduce new views based on Django’s generic views.

Read on for details.

from django.urls import pathfrom . import viewsapp_name = "polls"urlpatterns = [ path("", views.IndexView.as_view(), name="index"), path("<int:pk>/", views.DetailView.as_view(), name="detail"), path("<int:pk>/results/", views.ResultsView.as_view(), name="results"), path("<int:question_id>/vote/", views.vote, name="vote"),]

from django.db.models import Ffrom django.http import HttpResponseRedirectfrom django.shortcuts import get_object_or_404, renderfrom django.urls import reversefrom django.views import genericfrom .models import Choice, Questionclass 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. ...