=====================================
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."

.. _Tutorial 2: http://www.djangoproject.com/documentation/tutorial2/

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 weblog
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 "archive" 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, each view is represented by a simple Python function.

Design your URLs
================

The first step of writing views is to design your URL structure. You do this by
creating a Python module, called a URLconf. URLconfs are how Django associates
a given URL with given Python code.

When a user requests a Django-powered page, the system looks at the
``ROOT_URLCONF`` setting, which contains a string in Python dotted syntax.
Django loads that module and looks for a module-level variable called
``urlpatterns``, which is a sequence of tuples in the following format::

    (regular expression, Python callback function [, optional dictionary])

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.

When it finds a match, Django calls the Python callback function, with an
``HTTPRequest`` object as the first argument, any "captured" values from the
regular expression as keyword arguments, and, optionally, arbitrary keyword
arguments from the dictionary (an optional third item in the tuple).

For more on ``HTTPRequest`` objects, see the `request and response documentation`_.
For more details on URLconfs, see the `URLconf documentation`_.

When you ran ``python manage.py startproject mysite`` at the beginning of
Tutorial 1, it created a default URLconf in ``mysite/urls.py``. It also
automatically set your ``ROOT_URLCONF`` setting to point at that file::

    ROOT_URLCONF = 'mysite.urls'

Time for an example. Edit ``mysite/urls.py`` so it looks like this::

    from django.conf.urls.defaults import *

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

This is worth a review. When somebody requests a page from your Web site --
say, "/polls/23/", Django will load this 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. When it finds a regular
expression that matches -- ``r'^polls/(?P<poll_id>\d+)/$'`` -- it loads the
associated Python package/module: ``mysite.polls.views.detail``. That
corresponds to the function ``detail()`` in ``mysite/polls/views.py``.
Finally, it calls that ``detail()`` function like so::

    detail(request=<HttpRequest object>, poll_id='23')

The ``poll_id='23'`` part comes from ``(?P<poll_id>\d+)``. Using parenthesis around a
pattern "captures" the text matched by that pattern and sends it as an argument
to the view function; the ``?P<poll_id>`` defines the name that will be used to
identify the matched pattern; and ``\d+`` is a regular experession 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
``.php`` -- unless you have a sick sense of humor, in which case you can do
something like this::

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

But, don't do that. It's silly.

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 look for ``/myapp/``.

If you need help with regular expressions, see `Wikipedia's entry`_ and the
`Python documentation`_. Also, the O'Reilly book "Mastering Regular
Expressions" by Jeffrey Friedl is fantastic.

Finally, a performance note: these regular expressions are compiled the first
time the URLconf module is loaded. They're super fast.

.. _Wikipedia's entry: http://en.wikipedia.org/wiki/Regular_expression
.. _Python documentation: http://www.python.org/doc/current/lib/module-re.html
.. _request and response documentation: http://www.djangoproject.com/documentation/request_response/
.. _URLconf documentation: http://www.djangoproject.com/documentation/url_dispatch/

Write your first view
=====================

Well, we haven't created any views yet -- we just have the URLconf. But let's
make sure Django is following the URLconf properly.

Fire up the Django development Web server::

    python manage.py runserver

Now go to "http://localhost:8000/polls/" on your domain in your Web browser.
You should get a pleasantly-colored error page with the following message::

    ViewDoesNotExist at /polls/

    Tried index in module mysite.polls.views. Error was: 'module'
    object has no attribute 'index'

This error happened because you haven't written a function ``index()`` in the
module ``mysite/polls/views.py``.

Try "/polls/23/", "/polls/23/results/" and "/polls/23/vote/". The error
messages tell you which view Django tried (and failed to find, because you
haven't written any views yet).

Time to write the first view. Open the file ``mysite/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. Go to "/polls/" in your browser, and you
should see your text.

Now add the following view. It's slightly different, because it takes an
argument (which, remember, is passed in from whatever was captured by the
regular expression in the URLconf)::

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

Take a look in your browser, at "/polls/34/". It'll display whatever ID you
provide in the URL.

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 mysite.polls.models import Poll
    from django.http import HttpResponse

    def index(request):
        latest_poll_list = Poll.objects.all().order_by('-pub_date')
        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::

    from django.template import Context, loader
    from mysite.polls.models import Poll
    from django.http import HttpResponse

    def index(request):
        latest_poll_list = Poll.objects.all().order_by('-pub_date')
        t = loader.get_template('polls/index.html')
        c = Context({
            'latest_poll_list': latest_poll_list,
        })
        return HttpResponse(t.render(c))

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.

Reload the page. Now you'll see an error::

    TemplateDoesNotExist at /polls/
    polls/index.html

Ah. There's no template yet. First, create a directory, somewhere on your
filesystem, whose contents Django can access. (Django runs as whatever user
your server runs.) Don't put them under your document root, though. You
probably shouldn't make them public, just for security's sake.

Then edit ``TEMPLATE_DIRS`` in your ``settings.py`` to tell Django where it can
find templates -- just as you did in the "Customize the admin look and feel"
section of Tutorial 2.

When you've done that, create a directory ``polls`` in your template directory.
Within that, create a file called ``index.html``. Note that our
``loader.get_template('polls/index.html')`` code from above maps to
"[template_directory]/polls/index.html" on the filesystem.

Put the following code in that template::

    {% if latest_poll_list %}
        <ul>
        {% for poll in latest_poll_list %}
            <li>{{ poll.question }}</li>
        {% endfor %}
        </ul>
    {% else %}
        <p>No polls are available.</p>
    {% endif %}

Load the page in your Web browser, and you should see a bulleted-list
containing the "What's up" poll from Tutorial 1.

A shortcut: render_to_response()
--------------------------------

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_to_response
    from mysite.polls.models import Poll

    def index(request):
        latest_poll_list = Poll.objects.all().order_by('-pub_date')
        return render_to_response('polls/index.html', {'latest_poll_list': latest_poll_list})

Note that we no longer need to import ``loader``, ``Context`` or
``HttpResponse``.

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

Raising 404
===========

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:
            p = Poll.objects.get(pk=poll_id)
        except Poll.DoesNotExist:
            raise Http404
        return render_to_response('polls/detail.html', {'poll': p})

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

A shortcut: get_object_or_404()
-------------------------------

It's a very common idiom to use ``get_object()`` and raise ``Http404`` if the
object doesn't exist. Django provides a shortcut. Here's the ``detail()`` view,
rewritten::

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

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

.. admonition:: Philosophy

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

    Because that would couple the model layer to the view layer. One of the
    foremost design goals of Django is to maintain loose coupling.

There's also a ``get_list_or_404()`` function, which works just as
``get_object_or_404()`` -- except using ``get_list()`` instead of
``get_object()``. 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``, 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. By default, URLconfs
have the following line up top::

    from django.conf.urls.defaults import *

That takes care of setting ``handler404`` in the current module. As you can see
in ``django/conf/urls/defaults.py``, ``handler404`` is set to
``'django.views.defaults.page_not_found'`` by default.

Three more things to note about 404 views:

    * The 404 view is also called if Django doesn't find a match after checking
      every regular expression in the URLconf.
    * If you don't define your own 404 view -- and simply use the default,
      which is recommended -- you still have one obligation: To create a
      ``404.html`` template in the root of your template directory. The default
      404 view will use that template for all 404 errors.
    * If ``DEBUG`` is set to ``True`` (in your settings module) then your 404
      view will never be used, and the traceback will be displayed instead.

Write a 500 (server error) view
===============================

Similarly, URLconfs 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.

Use the template system
=======================

Back to our ``polls.detail`` view. Given the context variable ``poll``, here's
what the template might look like::

    <h1>{{ poll.question }}</h1>
    <ul>
    {% for choice in poll.choice_set.all %}
        <li>{{ choice.choice }}</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 attribute lookup -- which works,
in this case. If attribute lookup had failed, it would've tried calling the
method ``question()`` on the poll object.

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 full details on how templates work.

.. _template guide: http://www.djangoproject.com/documentation/templates/

Simplifying the URLconfs
========================

Take some time to play around with the views and template system. As you edit
the URLconf, you may notice there's a fair bit of redundancy in it::

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

Namely, ``mysite.polls.views`` is in every callback.

Because this is a common case, the URLconf framework provides a shortcut for
common prefixes. You can factor out the common prefixes and add them as the
first argument to ``patterns()``, like so::

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

This is functionally identical to the previous formatting. It's just a bit
tidier.

Decoupling the URLconfs
=======================

While we're at it, we should take the time to decouple our poll-app URLs from
our Django project configuration. Django apps are meant to be pluggable -- that
is, each particular app should be transferrable to another Django installation
with minimal fuss.

Our poll app is pretty decoupled at this point, thanks to the strict directory
structure that ``python manage.py startapp`` created, but one part of it is
coupled to the Django settings: The URLconf.

We've been editing the URLs in ``mysite/urls.py``, but the URL design of an
app is specific to the app, not to the Django installation -- so let's move the
URLs within the app directory.

Copy the file ``mysite/urls.py`` to ``mysite/polls/urls.py``. Then,
change ``mysite/urls.py`` to remove the poll-specific URLs and insert an
``include()``::

    (r'^polls/', include('mysite.polls.urls')),

``include()``, simply, references another URLconf. Note that the regular
expression doesn't have a ``$`` (end-of-string match character) but has the
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.

Here's what happens if a user goes to "/polls/34/" in this system:

* Django will find the match at ``'^polls/'``
* It will strip off the matching text (``"polls/"``) and send the remaining
  text -- ``"34/"`` -- to the 'mysite.polls.urls' urlconf for
  further processing.

Now that we've decoupled that, we need to decouple the
'mysite.polls.urls' urlconf by removing the leading "polls/" from each
line::

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

The idea behind ``include()`` and URLconf decoupling is to make it easy to
plug-and-play URLs. Now that polls are in their own URLconf, they can be placed
under "/polls/", or under "/fun_polls/", or under "/content/polls/", or any
other URL root, and the app will still work.

All the poll app cares about is its relative URLs, not its absolute URLs.

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

.. _part 4 of this tutorial: http://www.djangoproject.com/documentation/tutorial4/
