Django documentation

Writing your first Django app, part 3

This tutorial begins where Tutorial 2 left off. We’re continuing the Web-poll application and will focus on creating the public interface – “views.”

Philosophy

A view is a “type” of Web page in your Django application that generally serves a specific function and has a specific template. For example, in a blog application, you might have the following views:

  • Blog homepage – displays the latest few entries.
  • Entry “detail” page – permalink page for a single entry.
  • Year-based archive page – displays all months with entries in the given year.
  • Month-based archive page – displays all days with entries in the given month.
  • Day-based archive page – displays all entries in the given day.
  • Comment action – handles posting comments to a given entry.

In our poll application, we’ll have the following four views:

  • Poll “index” page – displays the latest few polls.
  • Poll “detail” page – displays a poll question, with no results but with a form to vote.
  • Poll “results” page – displays results for a particular poll.
  • Vote action – handles voting for a particular choice in a particular poll.

In Django, web pages and other content are delivered by views. Each view is represented by a simple 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).

Now in your time on the web you may have come across such beauties as “ME2/Sites/dirmod.asp?sid=&type=gen&mod=Core+Pages&gid=A6CD4967199A42D9B65B1B”. You will be pleased to know that Django allows us much more elegant URL patterns than that.

A URL pattern is simply the general form of a URL - for example: /newsarchive/<year>/<month>/.

To get from a URL to a view, Django uses what are known as ‘URLconfs’. A URLconf maps URL patterns (described as regular expressions) to views.

This tutorial provides basic instruction in the use of URLconfs, and you can refer to django.core.urlresolvers for more information.

Write your first view

Let’s write the first view. Open the file polls/views.py and put the following Python code in it:

from django.http import HttpResponse

def index(request):
    return HttpResponse("Hello, world. You're at the poll index.")

This is the simplest view possible in Django. To call the view, we need to map it to a URL - and for this we need a URLconf.

To create a URLconf in the polls directory, create a file called urls.py. Your app directory should now look like:

polls/
    __init__.py
    admin.py
    models.py
    tests.py
    urls.py
    views.py

In the polls/urls.py file include the following code:

from django.conf.urls import patterns, url

from polls import views

urlpatterns = patterns('',
    url(r'^$', views.index, name='index')
)

The next step is to point the root URLconf at the polls.urls module. In mysite/urls.py insert an include(), leaving you with:

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

from django.contrib import admin
admin.autodiscover()

urlpatterns = patterns('',
    url(r'^polls/', include('polls.urls')),
    url(r'^admin/', include(admin.site.urls)),
)

You have now wired an index view into the URLconf. Go to http://localhost:8000/polls/ in your browser, and you should see the text “Hello, world. You’re at the poll index.”, which you defined in the index view.

The url() function is passed four arguments, two required: regex and view, and two optional: kwargs, and name. At this point, it’s worth reviewing what these arguments are for.

url() argument: regex

The term “regex” is a commonly used short form meaning “regular expression”, which is a syntax for matching patterns in strings, or in this case, url patterns. Django starts at the first regular expression and makes its way down the list, comparing the requested URL against each regular expression until it finds one that matches.

Note that these regular expressions do not search GET and POST parameters, or the domain name. For example, in a request to http://www.example.com/myapp/, the URLconf will look for myapp/. In a request to http://www.example.com/myapp/?page=3, the URLconf will also look for myapp/.

If you need help with regular expressions, see Wikipedia’s entry and the documentation of the re module. Also, the O’Reilly book “Mastering Regular Expressions” by Jeffrey Friedl is fantastic. In practice, however, you don’t need to be an expert on regular expressions, as you really only need to know how to capture simple patterns. In fact, complex regexes can have poor lookup performance, so you probably shouldn’t rely on the full power of regexes.

Finally, a performance note: these regular expressions are compiled the first time the URLconf module is loaded. They’re super fast (as long as the lookups aren’t too complex as noted above).

url() argument: view

When Django finds a regular expression match, Django calls the specified view function, with an HttpRequest object as the first argument and any “captured” values from the regular expression as other arguments. If the regex uses simple captures, values are passed as positional arguments; if it uses named captures, values are passed as keyword arguments. We’ll give an example of this in a bit.

url() argument: kwargs

Arbitrary keyword arguments can be passed in a dictionary to the target view. We aren’t going to use this feature of Django in the tutorial.

url() argument: name

Naming your URL lets you refer to it unambiguously from elsewhere in Django especially templates. This powerful feature allows you to make global changes to the url patterns of your project while only touching a single file.

Writing more views

Now let’s add a few more views to polls/views.py. These views are slightly different, because they take an argument:

def detail(request, poll_id):
    return HttpResponse("You're looking at poll %s." % poll_id)

def results(request, poll_id):
    return HttpResponse("You're looking at the results of poll %s." % poll_id)

def vote(request, poll_id):
    return HttpResponse("You're voting on poll %s." % poll_id)

Wire these new views into the polls.urls module by adding the following url() calls:

from django.conf.urls import patterns, url

from polls import views

urlpatterns = patterns('',
    # ex: /polls/
    url(r'^$', views.index, name='index'),
    # ex: /polls/5/
    url(r'^(?P<poll_id>\d+)/$', views.detail, name='detail'),
    # ex: /polls/5/results/
    url(r'^(?P<poll_id>\d+)/results/$', views.results, name='results'),
    # ex: /polls/5/vote/
    url(r'^(?P<poll_id>\d+)/vote/$', views.vote, name='vote'),
)

Take a look in your browser, at “/polls/34/”. It’ll run the detail() method and display whatever ID you provide in the URL. Try “/polls/34/results/” and “/polls/34/vote/” too – these will display the placeholder results and voting pages.

When somebody requests a page from your Web site – say, “/polls/34/”, Django will load the mysite.urls Python module because it’s pointed to by the ROOT_URLCONF setting. It finds the variable named urlpatterns and traverses the regular expressions in order. The include() functions we are using simply reference other URLconfs. Note that the regular expressions for the include() functions don’t have a $ (end-of-string match character) but rather a trailing slash. Whenever Django encounters include(), it chops off whatever part of the URL matched up to that point and sends the remaining string to the included URLconf for further processing.

The idea behind include() is to make it easy to plug-and-play URLs. Since polls are in their own URLconf (polls/urls.py), they can be placed under “/polls/”, or under “/fun_polls/”, or under “/content/polls/”, or any other path root, and the app will still work.

Here’s what happens if a user goes to “/polls/34/” in this system:

  • Django will find the match at '^polls/'

  • Then, Django will strip off the matching text ("polls/") and send the remaining text – "34/" – to the ‘polls.urls’ URLconf for further processing which matches r'^(?P<poll_id>\d+)/$' resulting in a call to the detail() view like so:

    detail(request=<HttpRequest object>, poll_id='34')

The poll_id='34' part comes from (?P<poll_id>\d+). Using parentheses around a pattern “captures” the text matched by that pattern and sends it as an argument to the view function; ?P<poll_id> defines the name that will be used to identify the matched pattern; and \d+ is a regular expression to match a sequence of digits (i.e., a number).

Because the URL patterns are regular expressions, there really is no limit on what you can do with them. And there’s no need to add URL cruft such as .html – unless you want to, in which case you can do something like this:

(r'^polls/latest\.html$', 'polls.views.index'),

But, don’t do that. It’s silly.

Write views that actually do something

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. The rest is up to you.

Your view can read records from a database, or not. It can use a template system such as Django’s – or a third-party Python template system – or not. It can generate a PDF file, output XML, create a ZIP file on the fly, anything you want, using whatever Python libraries you want.

All Django wants is that HttpResponse. Or an exception.

Because it’s convenient, let’s use Django’s own database API, which we covered in Tutorial 1. Here’s one stab at the index() view, which displays the latest 5 poll questions in the system, separated by commas, according to publication date:

from django.http import HttpResponse

from polls.models import Poll

def index(request):
    latest_poll_list = Poll.objects.order_by('-pub_date')[:5]
    output = ', '.join([p.question for p in latest_poll_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.

Django’s TEMPLATE_LOADERS setting contains a list of callables that know how to import templates from various sources. One of the defaults is django.template.loaders.app_directories.Loader which looks for a “templates” subdirectory in each of the INSTALLED_APPS - this is how Django knows to find the polls templates even though we didn’t modify TEMPLATE_DIRS, as we did in Tutorial 2.

Organizing templates

We could have all our templates together, in one big templates directory, and it would work perfectly well. However, this template belongs to the polls application, so unlike the admin template we created in the previous tutorial, we’ll put this one in the application’s template directory (polls/templates) rather than the project’s (templates). We’ll discuss in more detail in the reusable apps tutorial why we do this.

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 simply as polls/index.html.

Template namespacing

Now we might be able to get away with putting our templates directly in polls/templates (rather than creating another polls subdirectory), but it would actually be a bad idea. Django will choose the first template it finds whose name matches, and if you had a template with the same name in a different application, Django would be unable to distinguish between them. We need to be able to point Django at the right one, and the easiest way to ensure this is by namespacing them. That is, by putting those templates inside another directory named for the application itself.

Put the following code in that template:

{% if latest_poll_list %}
    <ul>
    {% for poll in latest_poll_list %}
        <li><a href="/polls/{{ poll.id }}/">{{ poll.question }}</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:

from django.http import HttpResponse
from django.template import RequestContext, loader

from polls.models import Poll

def index(request):
    latest_poll_list = Poll.objects.order_by('-pub_date')[:5]
    template = loader.get_template('polls/index.html')
    context = RequestContext(request, {
        'latest_poll_list': latest_poll_list,
    })
    return HttpResponse(template.render(context))

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 “What’s up” poll from Tutorial 1. The link points to the poll’s detail page.

A shortcut: render()

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:

from django.shortcuts import render

from polls.models import Poll

def index(request):
    latest_poll_list = Poll.objects.all().order_by('-pub_date')[:5]
    context = {'latest_poll_list': latest_poll_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, RequestContext 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.

Raising a 404 error

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

from django.http import Http404
# ...
def detail(request, poll_id):
    try:
        poll = Poll.objects.get(pk=poll_id)
    except Poll.DoesNotExist:
        raise Http404
    return render(request, 'polls/detail.html', {'poll': poll})

The new concept here: The view raises the Http404 exception if a poll with the requested ID doesn’t exist.

We’ll discuss what you could put in that polls/detail.html template a bit later, but if you’d like to quickly get the above example working, a file containing just:

{{ poll }}

will get you started for now.

A shortcut: get_object_or_404()

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:

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

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.

Philosophy

Why do we use a helper function get_object_or_404() instead of automatically catching the ObjectDoesNotExist exceptions at a higher level, or having the model API raise Http404 instead of ObjectDoesNotExist?

Because that would couple the model layer to the view layer. One of the foremost design goals of Django is to maintain loose coupling. Some controlled coupling is introduced in the django.shortcuts module.

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.

Write a 404 (page not found) view

When you raise Http404 from within a view, Django will load a special view devoted to handling 404 errors. It finds it by looking for the variable handler404 in your root URLconf (and only in your root URLconf; setting handler404 anywhere else will have no effect), which is a string in Python dotted syntax – the same format the normal URLconf callbacks use. A 404 view itself has nothing special: It’s just a normal view.

You normally won’t have to bother with writing 404 views. If you don’t set handler404, the built-in view django.views.defaults.page_not_found() is used by default. Optionally, you can create a 404.html template in the root of your template directory. The default 404 view will then use that template for all 404 errors when DEBUG is set to False (in your settings module). If you do create the template, add at least some dummy content like “Page not found”.

Warning

If DEBUG is set to False, all responses will be “Bad Request (400)” unless you specify the proper ALLOWED_HOSTS as well (something like ['localhost', '127.0.0.1'] for local development).

A couple more things to note about 404 views:

  • If DEBUG is set to True (in your settings module) then your 404 view will never be used (and thus the 404.html template will never be rendered) because the traceback will be displayed instead.
  • The 404 view is also called if Django doesn’t find a match after checking every regular expression in the URLconf.

Write a 500 (server error) view

Similarly, your root URLconf may define a handler500, which points to a view to call in case of server errors. Server errors happen when you have runtime errors in view code.

Likewise, you should create a 500.html template at the root of your template directory and add some content like “Something went wrong”.

Use the template system

Back to the detail() view for our poll application. Given the context variable poll, here’s what the polls/detail.html template might look like:

<h1>{{ poll.question }}</h1>
<ul>
{% for choice in poll.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 {{ poll.question }}, first Django does a dictionary lookup on the object poll. 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: poll.choice_set.all is interpreted as the Python code poll.choice_set.all(), which returns an iterable of Choice objects and is suitable for use in the {% for %} tag.

See the template guide for more about templates.

Removing hardcoded URLs in templates

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

<li><a href="/polls/{{ poll.id }}/">{{ poll.question }}</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 url() 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' poll.id %}">{{ poll.question }}</a></li>

Note

If {% url 'detail' poll.id %} (with quotes) doesn’t work, but {% url detail poll.id %} (without quotes) does, that means you’re using a version of Django < 1.5. In this case, add the following declaration at the top of your template:

{% load url from future %}

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:

...
# the 'name' value as called by the {% url %} template tag
url(r'^(?P<poll_id>\d+)/$', views.detail, name='detail'),
...

If you want to change the URL of the polls detail view to something else, perhaps to something like polls/specifics/12/ instead of doing it in the template (or templates) you would change it in polls/urls.py:

...
# added the word 'specifics'
url(r'^specifics/(?P<poll_id>\d+)/$', views.detail, name='detail'),
...

Namespacing URL names

The tutorial project has just one app, polls. In real Django projects, there might be five, ten, twenty apps or 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 root URLconf. In the mysite/urls.py file (the project’s urls.py, not the application’s), go ahead and change it to include namespacing:

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

from django.contrib import admin
admin.autodiscover()

urlpatterns = patterns('',
    url(r'^polls/', include('polls.urls', namespace="polls")),
    url(r'^admin/', include(admin.site.urls)),
)

Now change your polls/index.html template from:

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

to point at the namespaced detail view:

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

When you’re comfortable with writing views, read part 4 of this tutorial to learn about simple form processing and generic views.

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