===================================== Writing your first Django app, part 3 ===================================== This tutorial begins where :doc:`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///``. 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 :mod:`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 :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\d+)/$', views.detail, name='detail'), # ex: /polls/5/results/ url(r'^(?P\d+)/results/$', views.results, name='results'), # ex: /polls/5/vote/ url(r'^(?P\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\d+)/$'`` resulting in a call to the ``detail()`` view like so:: detail(request=, poll_id='34') The ``poll_id='34'`` part comes from ``(?P\d+)``. Using parentheses around a pattern "captures" the text matched by that pattern and sends it as an argument to the view function; ``?P`` defines the name that will be used to identify the matched pattern; and ``\d+`` is a regular expression to match a sequence of digits (i.e., a number). Because the URL patterns are regular expressions, there really is no limit on what you can do with them. And there's no need to add URL cruft such as ``.html`` -- unless you want to, in which case you can do something like this:: (r'^polls/latest\.html$', '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 `. 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 :setting:`TEMPLATE_LOADERS` setting contains a list of callables that know how to import templates from various sources. One of the defaults is :class:`django.template.loaders.app_directories.Loader` which looks for a "templates" subdirectory in each of the :setting:`INSTALLED_APPS` - this is how Django knows to find the polls templates even though we didn't modify :setting:`TEMPLATE_DIRS`, as we did in :ref:`Tutorial 2 `. .. admonition:: 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 :doc:`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``. .. admonition:: 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: .. code-block:: html+django {% if latest_poll_list %} {% else %}

No polls are available.

{% 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 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: :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 from django.shortcuts import render from polls.models import Poll # ... 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, a file containing 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 from polls.models import Poll # ... 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

{{ poll.question }}

    {% for choice in poll.choice_set.all %}
  • {{ choice.choice_text }}
    • {% endfor %}
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 %}` 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 %}` tag. See the :doc:`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: .. code-block:: html+django
  • {{ poll.question }}
    • 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
  • {{ poll.question }}
    • .. 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\d+)/$', views.detail, name='detail'), ... If you want to change the URL of the polls detail view to something else, perhaps to something like ``polls/specifics/12/`` instead of doing it in the template (or templates) you would change it in ``polls/urls.py``:: ... # added the word 'specifics' url(r'^specifics/(?P\d+)/$', 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: .. code-block:: html+django
  • {{ poll.question }}
    • to point at the namespaced detail view: .. code-block:: html+django
  • {{ poll.question }}
    • When you're comfortable with writing views, read :doc:`part 4 of this tutorial ` to learn about simple form processing and generic views.