=====================================
Writing your first Django app, part 3
=====================================

This tutorial begins where :doc:`Tutorial 2 </intro/tutorial02>` 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, each view is represented by a simple Python function.

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. Now we have a problem, how does
this view get called? For that we need to map it to a URL, in Django this is
done in a configuration file called a URLconf.

.. admonition:: What is a URLconf?

    In Django, web pages and other content are delivered by views and
    determining which view is called is done by Python modules informally
    titled 'URLconfs'. These modules are pure Python code and are a simple
    mapping between URL patterns (as simple regular expressions) to Python
    callback functions (your views). This tutorial provides basic instruction
    in their use, and you can refer to :mod:`django.core.urlresolvers` for
    more information.

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 :func:`~django.conf.urls.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 :func:`~django.conf.urls.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.

:func:`~django.conf.urls.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 :mod:`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).

.. _Wikipedia's entry: http://en.wikipedia.org/wiki/Regular_expression

:func:`~django.conf.urls.url` argument: view
--------------------------------------------

When Django finds a regular expression match, Django calls the specified view
function, with an :class:`~django.http.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.

:func:`~django.conf.urls.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.

:func:`~django.conf.urls.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 news views into the ``polls.urls`` module by adding the following
:func:`~django.conf.urls.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
:setting:`ROOT_URLCONF` setting. It finds the variable named ``urlpatterns``
and traverses the regular expressions in order. The
:func:`~django.conf.urls.include` functions we are using simply reference
other URLconfs. Note that the regular expressions for the
:func:`~django.conf.urls.include` functions don't have a ``$`` (end-of-string
match character) but rather a trailing slash. Whenever Django encounters
:func:`~django.conf.urls.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 :func:`~django.conf.urls.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
:class:`~django.http.HttpResponse` object containing the content for the
requested page, or raising an exception such as :exc:`~django.http.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 :class:`~django.http.HttpResponse`. Or an exception.

Because it's convenient, let's use Django's own database API, which we covered
in :doc:`Tutorial 1 </intro/tutorial01>`. 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.

First, create a directory ``polls`` in your template directory you specified
in setting:`TEMPLATE_DIRS`. Within that, create a file called ``index.html``.
Put the following code in that template:

.. code-block:: html+django

    {% 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 use that html template in our index view::

    from django.http import HttpResponse
    from django.template import Context, 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 = Context({
            '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 in your Web browser, 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.

.. admonition:: Organizing Templates

    Rather than one big templates directory, you can also store templates
    within each app. We'll discuss this in more detail in the :doc:`reusable
    apps tutorial</intro/reusable-apps>`.

A shortcut: :func:`~django.shortcuts.render`
--------------------------------------------

It's a very common idiom to load a template, fill a context and return an
:class:`~django.http.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
:mod:`~django.template.loader`, :class:`~django.template.Context` and
:class:`~django.http.HttpResponse` (you'll want to keep ``HttpResponse`` if you
still have the stub methods for ``detail``, ``results``, and ``vote``).

The :func:`~django.shortcuts.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 :class:`~django.http.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 :exc:`~django.http.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, just::

    {{ poll }}

will get you started for now.

A shortcut: :func:`~django.shortcuts.get_object_or_404`
-------------------------------------------------------

It's a very common idiom to use :meth:`~django.db.models.query.QuerySet.get`
and raise :exc:`~django.http.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 :func:`~django.shortcuts.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 :meth:`~django.db.models.query.QuerySet.get` function of the
model's manager. It raises :exc:`~django.http.Http404` if the object doesn't
exist.

.. admonition:: Philosophy

    Why do we use a helper function :func:`~django.shortcuts.get_object_or_404`
    instead of automatically catching the
    :exc:`~django.core.exceptions.ObjectDoesNotExist` exceptions at a higher
    level, or having the model API raise :exc:`~django.http.Http404` instead of
    :exc:`~django.core.exceptions.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 :mod:`django.shortcuts` module.

There's also a :func:`~django.shortcuts.get_list_or_404` function, which works
just as :func:`~django.shortcuts.get_object_or_404` -- except using
:meth:`~django.db.models.query.QuerySet.filter` instead of
:meth:`~django.db.models.query.QuerySet.get`. It raises
:exc:`~django.http.Http404` if the list is empty.

Write a 404 (page not found) view
=================================

When you raise :exc:`~django.http.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 :func:`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 :setting:`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".

A couple more things to note about 404 views:

* If :setting:`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:

.. code-block:: html+django

    <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 :ttag:`{% for %}<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 :ttag:`{% for %}<for>` tag.

See the :doc:`template guide </topics/templates>` 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:

.. code-block:: html+django

    <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 :func:`~django.conf.urls.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:

.. code-block:: html+django

    <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:

    .. code-block:: html+django

        {% 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, 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:

.. code-block:: html+django

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

to point at the namespaced detail view:

.. code-block:: html+django

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

When you're comfortable with writing views, read :doc:`part 4 of this tutorial
</intro/tutorial04>` to learn about simple form processing and generic views.
