::-- limodou [2005-07-20 01:57:56]

Contents

  1. Writing your first Django app, part 3
    1. Philosophy
    2. Design your URLs
    3. Write your first view
    4. Write views that actually do something
    5. Raising 404
    6. Write a 404 (page not found) view
    7. Write a 500 (server error) view
    8. Use the template system
    9. Simplifying the URLconfs
    10. Decoupling the URLconfs
    11. Coming soon

1. Writing your first Django app, part 3

By Adrian Holovaty <[email protected]>

This tutorial begins where Tutorial 2 left off. We're continuing the Web-poll application and will focus on creating the public interface -- "views."

1.1. 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.

1.2. 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 request 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).

When you ran django-admin.py startproject myproject at the beginning of Tutorial 1, it created a default URLconf in myproject/settings/urls/main.py. It also automatically set your ROOT_URLCONF setting to point at that file: 

ROOT_URLCONF = 'myproject.settings.urls.main'

Time for an example. Edit myproject/settings/urls/main.py so it looks like this: 

from django.conf.urls.defaults import *

urlpatterns = patterns('',
    (r'^polls/$', 'myproject.apps.polls.views.polls.index'),
    (r'^polls/(?P<poll_id>\d+)/$', 'myproject.apps.polls.views.polls.detail'),
    (r'^polls/(?P<poll_id>\d+)/results/$', 'myproject.apps.polls.views.polls.results'),
    (r'^polls/(?P<poll_id>\d+)/vote/$', 'myproject.apps.polls.views.polls.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: myproject.polls.views.polls.detail. That corresponds to the function detail() in myproject/polls/views/polls.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 (?<name>pattern) "captures" the text matched by pattern and sends it as a keyword argument to the view function.

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$', 'myproject.apps.polls.views.polls.index'),

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

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.

1.3. 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.

Set your DJANGO_SETTINGS_MODULE environment variable to your main settings (myproject.settings.main), as we did with the admin settings in Tutorial 2. Then, fire up the Django development Web server, as we also did in Tutorial 2: 

django-admin.py runserver

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

ViewDoesNotExist: Tried myproject.apps.polls.views.polls.index.
No module named polls

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

Time to write the first view. Create the file myproject/apps/polls/views/polls.py and put the following Python code in it: 

from django.utils.httpwrappers import HttpResponse

def index(request):
    return HttpResponse("Hello, world. You're at the poll index.")

This is the simplest view possible. Restart your development server and go to "/polls/". 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.

1.4. 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 django.models.polls import polls
from django.utils.httpwrappers import HttpResponse

def index(request):
    latest_poll_list = polls.get_list(order_by=[('pub_date', 'DESC')],
        limit=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.core import template_loader
from django.core.extensions import DjangoContext as Context
from django.models.polls import polls
from django.utils.httpwrappers import HttpResponse

def index(request):
    latest_poll_list = polls.get_list(order_by=[('pub_date', 'DESC')],
        limit=5)
    t = template_loader.get_template('polls/index')
    c = Context(request, {
        'latest_poll_list': latest_poll_list,
    })
    return HttpResponse(t.render(c))

That code loads the template called "polls/index" 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: Your TEMPLATE_DIRS settings is empty.
Change it to point to at least one template directory.

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 main.py settings file 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. Django requires that templates have ".html" extension. Note that our template_loader.get_template('polls/index') 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 %}

Templates are read from disk at each page request, so you don't have to restart the server to see changes. Load the page in your Web browser, and you should see a bulleted-list containing the "What's up" poll from Tutorial 1.

1.5. 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.core.exceptions import Http404
def detail(request, poll_id):
    try:
        p = polls.get_object(id__exact=poll_id)
    except polls.PollDoesNotExist:
        raise Http404
    t = template_loader.get_template('polls/detail')
    c = Context(request, {
        'poll': p,
    })
    return HttpResponse(t.render(c))

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

1.6. 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.

Two 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.

1.7. 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.

1.8. 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.get_choice_list %}
    <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.get_choice_list is interpreted as the Python code poll.get_choice_list(), which returns a list of Choice objects and is suitable for iteration via the {% for %} tag.

See the template guide for full details on how templates work.

1.9. 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/$', 'myproject.apps.polls.views.polls.index'),
    (r'^polls/(?P<poll_id>\d+)/$', 'myproject.apps.polls.views.polls.detail'),
    (r'^polls/(?P<poll_id>\d+)/results/$', 'myproject.apps.polls.views.polls.results'),
    (r'^polls/(?P<poll_id>\d+)/vote/$', 'myproject.apps.polls.views.polls.vote'),
)

Namely, myproject.apps.polls.views.polls 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('myproject.apps.polls.views.polls',
    (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.

1.10. 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 django-admin.py startapp created, but one part of it is coupled to the Django settings: The URLconf.

We've been editing the URLs in myproject/settings/urls/main.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.

Just copy the file myproject/settings/urls/main.py to myproject/apps/polls/urls/polls.py, which had already been created, as a stub, by django-admin.py startapp.

Then, change myproject/settings/urls/main.py to remove the poll-specific URLs and insert an include(): 

(r'^polls/', include('myproject.apps.polls.urls.polls')),

Notes:

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 'myproject.apps.polls.urls.polls' urlconf for further processing.

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

urlpatterns = patterns('myproject.apps.polls.views.polls',
    (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.

1.11. Coming soon

The tutorial ends here for the time being. But check back within 48 hours for the next installments:

  • Advanced view features: Form processing
  • Using the RSS framework
  • Using the cache framework
  • Using the comments framework
  • Advanced admin features: Permissions

  • Advanced admin features: Custom JavaScript