Django pollapp-4 (creating views)

In Django, web pages and other content are delivered by views. Each view is represented by a Python function (or method, in the case of class-based views). Django will choose a view by examining the URL that’s requested (to be precise, the part of the URL after the domain name). To get from a URL to a view, Django uses what are known as ‘URLconfs’. A URLconf maps URL patterns to views.

Now let’s add a few more views to polls/views.py:

def detail(request, question_id):

            return HttpResponse("You're looking at question %s." % question_id)

           

def results(request, question_id):

            response = "You're looking at the results of question %s."

            return HttpResponse(response % question_id)

           

def vote(request, question_id):

            return HttpResponse("You're voting on question %s." % question_id)

Next we’ll wire these new views into the polls.urls module by adding the following path() calls:

    path('<int:question_id>/', views.detail, name='detail'),

    path('<int:question_id>/results/', views.results, name='results'),

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

Each view is responsible for doing one of two things: returning an HttpResponse object containing the content for the requested page, or raising an exception such as Http404. If we want we can modify our index() view, so that it displays the latest 5 poll questions in the system, separated by commas, according to publication date:

def index(request):

            latest_question_list = Question.objects.order_by('-pub_date')[:5]

            output = ', '.join([q.question_text for q in latest_question_list])

            return HttpResponse(output)

There’s a problem here, though: the page’s design is hard-coded in the view. If you want to change the way the page looks, you’ll have to edit this Python code. So let’s use Django’s template system to separate the design from Python by creating a template that the view can use.

First, create a directory called templates in your polls directory. Django will look for templates in there.

Your project’s TEMPLATES setting describes how Django will load and render templates. The default settings file configures a DjangoTemplates backend whose APP_DIRS option is set to True. By convention DjangoTemplates looks for a “templates” subdirectory in each of the INSTALLED_APPS.

Within the templates directory you have just created, create another directory called polls, and within that create a file called index.html. In other words, your template should be at polls/templates/polls/index.html. Because of how the app_directories template loader works as described above, you can refer to this template within Django as polls/index.html. Put the following code in that template:

{% if latest_question_list %}

    <ul>

    {% for question in latest_question_list %}

        <li><a href="/polls/{{ question.id }}/">{{ question.question_text }}</a></li>

    {% endfor %}

    </ul>

{% else %}

    <p>No polls are available.</p>

{% endif %}

Now let’s update our index view in polls/views.py to use the template:

def index(request):

            latest_question_list = Question.objects.order_by('-pub_date')[:5]

            template = loader.get_template('polls/index.html')

            context = {

        'latest_question_list': latest_question_list,

    }

            return HttpResponse(template.render(context, request))

That code loads the template called polls/index.html and passes it a context. The context is a dictionary mapping template variable names to Python objects. Load the page by pointing your browser at “/polls/”, and you should see a bulleted-list containing the created question in the previous post.

It’s a very common idiom to load a template, fill a context and return an HttpResponse object with the result of the rendered template. Django provides a shortcut. Here’s the full index() view, rewritten:

def index(request):

            latest_question_list = Question.objects.order_by('-pub_date')[:5]

    context = {'latest_question_list': latest_question_list}

    return render(request, 'polls/index.html', context)

Note that once we’ve done this in all these views, we no longer need to import loader and HttpResponse (you’ll want to keep HttpResponse if you still have the stub methods for detail, results, and vote).

The render() function takes the request object as its first argument, a template name as its second argument and a dictionary as its optional third argument. It returns an HttpResponse object of the given template rendered with the given context.

Now, let’s tackle the question detail view – the page that displays the question text for a given poll. Here’s the view:

from django.http import Http404

from django.shortcuts import render

from .models import Question

# ...

def detail(request, question_id):

    try:

        question = Question.objects.get(pk=question_id)

    except Question.DoesNotExist:

        raise Http404("Question does not exist")

return render(request, 'polls/detail.html', {'question': question})

The view raises the Http404 exception if a question with the requested ID doesn’t exist. It’s a very common idiom to use get() and raise Http404 if the object doesn’t exist. Django provides a shortcut. Here’s the detail() view, rewritten:

def detail(request, question_id):

            question = get_object_or_404(Question, pk=question_id)

            return render(request, 'polls/detail.html', {'question': question})

The get_object_or_404() function takes a Django model as its first argument and an arbitrary number of keyword arguments, which it passes to the get() function of the model’s manager. It raises Http404 if the object doesn’t exist.

There’s also a get_list_or_404() function, which works just as get_object_or_404() – except using filter() instead of get(). It raises Http404 if the list is empty. Back to the detail() view for our poll application. Given the context variable question, here’s what the polls/detail.html template might look like:

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

<ul>

{% for choice in question.choice_set.all %}

    <li>{{ choice.choice_text }}</li>

{% endfor %}

</ul>

The template system uses dot-lookup syntax to access variable attributes. In the example of {{ question.question_text }}, first Django does a dictionary lookup on the object question. Failing that, it tries an attribute lookup – which works, in this case. If attribute lookup had failed, it would’ve tried a list-index lookup.

Method-calling happens in the {% for %} loop: question.choice_set.all is interpreted as the Python code question.choice_set.all(), which returns an iterable of Choice objects and is suitable for use in the {% for %} tag.

Remember, when we wrote the link to a question in the polls/index.html template, the link was partially hardcoded like this:

<li><a href="/polls/{{ question.id }}/">{{ question.question_text }}</a></li>

The problem with this hardcoded, tightly-coupled approach is that it becomes challenging to change URLs on projects with a lot of templates. However, since you defined the name argument in the path() functions in the polls.urls module, you can remove a reliance on specific URL paths defined in your url configurations by using the {% url %} template tag:

<li><a href="{% url 'detail' question.id %}">{{ question.question_text }}</a></li>

The way this works is by looking up the URL definition as specified in the polls.urls module. You can see exactly where the URL name of ‘detail’ is defined below:

path('<int:question_id>/', views.detail, name='detail'),

Our project has just one app, polls but in real Django projects, there might be more. How does Django differentiate the URL names between them? For example, the polls app has a detail view, and so might an app on the same project that is for a blog. How does one make it so that Django knows which app view to create for a url when using the {% url %} template tag?

The answer is to add namespaces to your URLconf. In the polls/urls.py file, go ahead and add an app_name to set the application namespace:

app_name = 'polls'

Now change your polls/index.html template to point at the namespaced detail view:

<li><a href="{% url 'polls:detail' question.id %}">{{ question.question_text }}</a></li>

In the next post we’ll learn the basics about form processing and generic views.