Typically, they are used to put each application’s URLs into their own namespace. This prevents the reverse() Django function and the {% url %} template function from returning the wrong URL because the URL-pattern name happened to match in another app.
What I have in my project-level urls.py file is the following:
from django.conf.urls.defaults import *
from django.conf import settings
from django.contrib import admin
admin.autodiscover()
urlpatterns = patterns('',
url(r'^$', 'main.views.main', name="main"),
url(r'^login$', 'django.contrib.auth.views.login', name="login"),
url(r'^logout$', 'django.contrib.auth.views.logout',
{"next_page": "https://stackoverflow.com/"}, name="logout"),
# Admin
url(r'^admin/doc/', include('django.contrib.admindocs.urls')),
url(r'^admin/', include(admin.site.urls)),
)
# Auto-add the applications.
for app in settings.LOCAL_APPS:
urlpatterns += patterns('',
url(r'^{0}/'.format(app), include(app + '.urls', namespace=app)),
)
Note the last section: this goes through the applications I have installed (settings.LOCAL_APPS is a setting I added that contains only my apps; it gets added to INSTALLED_APPS which has other things like South), looks for a urls.py in each of them, and imports those URLs into a namespace named after the app, and also puts those URLs into a URL subdirectory named after the app.
So, for example, if I have an app named hosts, and hosts/urls.py looks like:
from django.conf.urls.defaults import *
urlpatterns = patterns('hosts.views',
url(r'^$', 'show_hosts', name="list"),
)
Now my views.py can call reverse("hosts:list") to get the URL to the page that calls hosts.views.show_hosts, and it will look something like "/hosts/". Same goes for {% url "hosts:list" %} in a template. This way I don’t have to worry about colliding with a URL named “list” in another app, and I don’t have to prefix every name with hosts_.
Note that the login page is at {% url "login" %} since it wasn’t given a namespace.