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])
When you ran django-admin.py startproject mysite at the beginning of
it created a default URLconf in mysite/urls.py. It alsoautomatically set your 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/$', 'mysite.polls.views.index'),
(r'^polls/(?P<poll_id>\d+)/$', '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'),
(r'^admin/', include(admin.site.urls)),
)
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.
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 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)
Write views that actually do something
Eachview is responsible for doing one of two things: Returning an HttpResponseobject containing thecontent 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. 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.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
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" .
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><a href="/polls/{{ poll.id }}/">{{ poll.question }}</a></li>
{% endfor %}
</ul>
{% else %}
<p>No polls are available.</p>
{% endif %}
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.
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})
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:
<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.
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 at this point, thanks to the strict directory structure that python manage.py startapp created, but one part of it is transferable to another Django installation with minimal fuss. Our poll app is pretty decoupled 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():
# ...
urlpatterns = patterns('',
(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/4/" 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 '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, and removing the lines registering the admin site:
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 "/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. Credit : https://docs.djangoproject.com
