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+++ b/parts/django/docs/intro/tutorial03.txt Sat Jan 08 11:20:57 2011 +0530
@@ -0,0 +1,546 @@
+=====================================
+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 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
+:setting:`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
+:class:`~django.http.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 :class:`~django.http.HttpRequest` objects, see the
+:doc:`/ref/request-response`. For more details on URLconfs, see the
+:doc:`/topics/http/urls`.
+
+When you ran ``django-admin.py startproject mysite`` at the beginning of
+Tutorial 1, it created a default URLconf in ``mysite/urls.py``. It also
+automatically set your :setting:`ROOT_URLCONF` setting (in ``settings.py``) 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 *
+
+ from django.contrib import admin
+ admin.autodiscover()
+
+ urlpatterns = patterns('',
+ (r'^polls/$', 'polls.views.index'),
+ (r'^polls/(?P<poll_id>\d+)/$', 'polls.views.detail'),
+ (r'^polls/(?P<poll_id>\d+)/results/$', 'polls.views.results'),
+ (r'^polls/(?P<poll_id>\d+)/vote/$', 'polls.views.vote'),
+ (r'^admin/', include(admin.site.urls)),
+ )
+
+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 :setting:`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+)/$'`` -- it loads the
+function ``detail()`` from ``polls/views.py``. Finally, it calls that
+``detail()`` function like so::
+
+ detail(request=<HttpRequest object>, poll_id='23')
+
+The ``poll_id='23'`` part comes from ``(?P<poll_id>\d+)``. Using parentheses
+around a pattern "captures" the text matched by that pattern and sends it as an
+argument to the view function; the ``?P<poll_id>`` defines the name that will be
+used to identify the matched pattern; and ``\d+`` is a regular expression to
+match a sequence of digits (i.e., a number).
+
+Because the URL patterns are regular expressions, there really is no limit on
+what you can do with them. And there's no need to add URL cruft such as ``.php``
+-- unless you have a sick sense of humor, in which case you can do something
+like this::
+
+ (r'^polls/latest\.php$', '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://docs.python.org/library/re.html
+
+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:
+
+.. code-block:: bash
+
+ 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 polls.views. Error was: 'module'
+ object has no attribute 'index'
+
+This error happened because you haven't written a function ``index()`` in the
+module ``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 ``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 lets add a few more views. These views are slightly different, because
+they take 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)
+
+ 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)
+
+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.
+
+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 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 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 :setting:`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:
+
+.. 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 %}
+
+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.
+
+A shortcut: render_to_response()
+--------------------------------
+
+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_to_response
+ from 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
+:mod:`~django.template.loader`, :class:`~django.template.Context` and
+:class:`~django.http.HttpResponse`.
+
+The :func:`~django.shortcuts.render_to_response` function takes a template name
+as its first argument and a dictionary as its optional second argument. It
+returns an :class:`~django.http.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 :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: get_object_or_404()
+-------------------------------
+
+It's a very common idiom to use :meth:`~django.db.models.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_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 :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 module's :meth:`~django.db.models.QuerySet.get` function. 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.
+
+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.QuerySet.filter` instead of
+:meth:`~django.db.models.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``, 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
+:func:`django.views.defaults.page_not_found` by default.
+
+Four 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.
+
+ * 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 :setting:`DEBUG` is set to ``False`` (in your settings module) and if
+ you didn't create a ``404.html`` file, an ``Http500`` is raised instead.
+ So remember to create a ``404.html``.
+
+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 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 }}</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 :doc:`template guide </topics/templates>` for more about 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/$', 'polls.views.index'),
+ (r'^polls/(?P<poll_id>\d+)/$', 'polls.views.detail'),
+ (r'^polls/(?P<poll_id>\d+)/results/$', 'polls.views.results'),
+ (r'^polls/(?P<poll_id>\d+)/vote/$', 'polls.views.vote'),
+ )
+
+Namely, ``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 :func:`~django.conf.urls.defaults.patterns`, like so::
+
+ urlpatterns = patterns('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.
+
+Since you generally don't want the prefix for one app to be applied to every
+callback in your URLconf, you can concatenate multiple
+:func:`~django.conf.urls.defaults.patterns`. Your full ``mysite/urls.py`` might
+now look like this::
+
+ from django.conf.urls.defaults import *
+
+ from django.contrib import admin
+ admin.autodiscover()
+
+ urlpatterns = patterns('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'),
+ )
+
+ urlpatterns += patterns('',
+ (r'^admin/', include(admin.site.urls)),
+ )
+
+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 transferable 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 ``polls/urls.py``. Then, change
+``mysite/urls.py`` to remove the poll-specific URLs and insert an
+:func:`~django.conf.urls.defaults.include`, leaving you with::
+
+ # This also imports the include function
+ from django.conf.urls.defaults import *
+
+ from django.contrib import admin
+ admin.autodiscover()
+
+ urlpatterns = patterns('',
+ (r'^polls/', include('polls.urls')),
+ (r'^admin/', include(admin.site.urls)),
+ )
+
+:func:`~django.conf.urls.defaults.include` simply references another URLconf.
+Note that the regular expression doesn't have a ``$`` (end-of-string match
+character) but has the trailing slash. Whenever Django encounters
+:func:`~django.conf.urls.defaults.include`, it chops off whatever part of the
+URL matched up to that point and sends the remaining string to the included
+URLconf for further processing.
+
+Here's what happens if a user goes to "/polls/34/" in this system:
+
+ * Django will find the match at ``'^polls/'``
+
+ * Then, Django will strip off the matching text (``"polls/"``) and send the
+ remaining text -- ``"34/"`` -- to the 'polls.urls' URLconf for
+ further processing.
+
+Now that we've decoupled that, we need to decouple the ``polls.urls``
+URLconf by removing the leading "polls/" from each line, and removing the
+lines registering the admin site. Your ``polls.urls`` file should now look like
+this::
+
+ from django.conf.urls.defaults import *
+
+ urlpatterns = patterns('polls.views',
+ (r'^$', 'index'),
+ (r'^(?P<poll_id>\d+)/$', 'detail'),
+ (r'^(?P<poll_id>\d+)/results/$', 'results'),
+ (r'^(?P<poll_id>\d+)/vote/$', 'vote'),
+ )
+
+The idea behind :func:`~django.conf.urls.defaults.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 path root, and the app will still work.
+
+All the poll app cares about is its relative path, not its absolute path.
+
+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.