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+=====================================
+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: ../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+)/$'`` -- it loads the
+associated Python package/module: ``mysite.polls.views.detail``. That
+corresponds to the function ``detail()`` in ``mysite/polls/views.py``.
+Finally, it calls that ``detail()`` function like so::
+
+ detail(request=<HttpRequest object>, poll_id='23')
+
+The ``poll_id='23'`` part comes from ``(?P<poll_id>\d+)``. Using parenthesis around a
+pattern "captures" the text matched by that pattern and sends it as an argument
+to the view function; the ``?P<poll_id>`` defines the name that will be used to
+identify the matched pattern; and ``\d+`` is a regular expression to match a sequence of
+digits (i.e., a number).
+
+Because the URL patterns are regular expressions, there really is no limit on
+what you can do with them. And there's no need to add URL cruft such as
+``.php`` -- unless you have a sick sense of humor, in which case you can do
+something like this::
+
+ (r'^polls/latest\.php$', '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: ../request_response/
+.. _URLconf 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')[: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::
+
+ 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')[:5]
+ 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')[:5]
+ return render_to_response('polls/index.html', {'latest_poll_list': latest_poll_list})
+
+Note that once we've done this in all these views, we no longer need to import ``loader``, ``Context`` and ``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()`` 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()`` 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 ``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``, 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: ../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: ../tutorial4/