A clean, elegant URL scheme is an important detail in a high-quality Web application. Django lets you design URLs however you want, with no framework limitations.
There’s no .php or .cgi required, and certainly none of that 0,2097,1-1-1928,00 nonsense.
See Cool URIs don’t change, by World Wide Web creator Tim Berners-Lee, for excellent arguments on why URLs should be clean and usable.
To design URLs for an app, you create a Python module informally called a URLconf (URL configuration). This module is pure Python code and is a simple mapping between URL patterns (simple regular expressions) to Python functions (your views).
This mapping can be as short or as long as needed. It can reference other mappings. And, because it’s pure Python code, it can be constructed dynamically.
When a user requests a page from your Django-powered site, this is the algorithm the system follows to determine which Python code to execute:
Here’s a sample URLconf:
from django.conf.urls import patterns, url, include
urlpatterns = patterns('',
(r'^articles/2003/$', 'news.views.special_case_2003'),
(r'^articles/(\d{4})/$', 'news.views.year_archive'),
(r'^articles/(\d{4})/(\d{2})/$', 'news.views.month_archive'),
(r'^articles/(\d{4})/(\d{2})/(\d+)/$', 'news.views.article_detail'),
)
Notes:
Example requests:
The above example used simple, non-named regular-expression groups (via parenthesis) to capture bits of the URL and pass them as positional arguments to a view. In more advanced usage, it’s possible to use named regular-expression groups to capture URL bits and pass them as keyword arguments to a view.
In Python regular expressions, the syntax for named regular-expression groups is (?P<name>pattern), where name is the name of the group and pattern is some pattern to match.
Here’s the above example URLconf, rewritten to use named groups:
urlpatterns = patterns('',
(r'^articles/2003/$', 'news.views.special_case_2003'),
(r'^articles/(?P<year>\d{4})/$', 'news.views.year_archive'),
(r'^articles/(?P<year>\d{4})/(?P<month>\d{2})/$', 'news.views.month_archive'),
(r'^articles/(?P<year>\d{4})/(?P<month>\d{2})/(?P<day>\d{2})/$', 'news.views.article_detail'),
)
This accomplishes exactly the same thing as the previous example, with one subtle difference: The captured values are passed to view functions as keyword arguments rather than positional arguments. For example:
In practice, this means your URLconfs are slightly more explicit and less prone to argument-order bugs – and you can reorder the arguments in your views’ function definitions. Of course, these benefits come at the cost of brevity; some developers find the named-group syntax ugly and too verbose.
Here’s the algorithm the URLconf parser follows, with respect to named groups vs. non-named groups in a regular expression:
In both cases, any extra keyword arguments that have been given as per Passing extra options to view functions (below) will also be passed to the view.
The URLconf searches against the requested URL, as a normal Python string. This does not include GET or 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/.
The URLconf doesn’t look at the request method. In other words, all request methods – POST, GET, HEAD, etc. – will be routed to the same function for the same URL.
Each captured argument is sent to the view as a plain Python string, regardless of what sort of match the regular expression makes. For example, in this URLconf line:
(r'^articles/(?P<year>\d{4})/$', 'news.views.year_archive'),
...the year argument to news.views.year_archive() will be a string, not an integer, even though the \d{4} will only match integer strings.
A convenient trick is to specify default parameters for your views’ arguments. Here’s an example URLconf and view:
# URLconf
urlpatterns = patterns('',
(r'^blog/$', 'blog.views.page'),
(r'^blog/page(?P<num>\d+)/$', 'blog.views.page'),
)
# View (in blog/views.py)
def page(request, num="1"):
# Output the appropriate page of blog entries, according to num.
In the above example, both URL patterns point to the same view – blog.views.page – but the first pattern doesn’t capture anything from the URL. If the first pattern matches, the page() function will use its default argument for num, "1". If the second pattern matches, page() will use whatever num value was captured by the regex.
Each regular expression in a urlpatterns is compiled the first time it’s accessed. This makes the system blazingly fast.
urlpatterns should be a Python list, in the format returned by the function django.conf.urls.patterns(). Always use patterns() to create the urlpatterns variable.
When Django can’t find a regex matching the requested URL, or when an exception is raised, Django will invoke an error-handling view.
The views to use for these cases are specified by three variables. Their default values should suffice for most projects, but further customization is possible by assigning values to them.
See the documentation on customizing error views for the full details.
Such values can be set in your root URLconf. Setting these variables in any other URLconf will have no effect.
Values must be callables, or strings representing the full Python import path to the view that should be called to handle the error condition at hand.
The variables are:
You can specify a common prefix in your patterns() call, to cut down on code duplication.
Here’s the example URLconf from the Django overview:
from django.conf.urls import patterns, url, include
urlpatterns = patterns('',
(r'^articles/(\d{4})/$', 'news.views.year_archive'),
(r'^articles/(\d{4})/(\d{2})/$', 'news.views.month_archive'),
(r'^articles/(\d{4})/(\d{2})/(\d+)/$', 'news.views.article_detail'),
)
In this example, each view has a common prefix – 'news.views'. Instead of typing that out for each entry in urlpatterns, you can use the first argument to the patterns() function to specify a prefix to apply to each view function.
With this in mind, the above example can be written more concisely as:
from django.conf.urls import patterns, url, include
urlpatterns = patterns('news.views',
(r'^articles/(\d{4})/$', 'year_archive'),
(r'^articles/(\d{4})/(\d{2})/$', 'month_archive'),
(r'^articles/(\d{4})/(\d{2})/(\d+)/$', 'article_detail'),
)
Note that you don’t put a trailing dot (".") in the prefix. Django puts that in automatically.
In practice, you’ll probably end up mixing and matching views to the point where the views in your urlpatterns won’t have a common prefix. However, you can still take advantage of the view prefix shortcut to remove duplication. Just add multiple patterns() objects together, like this:
Old:
from django.conf.urls import patterns, url, include
urlpatterns = patterns('',
(r'^$', 'myapp.views.app_index'),
(r'^(?P<year>\d{4})/(?P<month>[a-z]{3})/$', 'myapp.views.month_display'),
(r'^tag/(?P<tag>\w+)/$', 'weblog.views.tag'),
)
New:
from django.conf.urls import patterns, url, include
urlpatterns = patterns('myapp.views',
(r'^$', 'app_index'),
(r'^(?P<year>\d{4})/(?P<month>[a-z]{3})/$','month_display'),
)
urlpatterns += patterns('weblog.views',
(r'^tag/(?P<tag>\w+)/$', 'tag'),
)
At any point, your urlpatterns can “include” other URLconf modules. This essentially “roots” a set of URLs below other ones.
For example, here’s an excerpt of the URLconf for the Django Web site itself. It includes a number of other URLconfs:
from django.conf.urls import patterns, include
urlpatterns = patterns('',
# ... snip ...
(r'^comments/', include('django.contrib.comments.urls')),
(r'^community/', include('django_website.aggregator.urls')),
(r'^contact/', include('django_website.contact.urls')),
(r'^r/', include('django.conf.urls.shortcut')),
# ... snip ...
)
Note that the regular expressions in this example don’t have a $ (end-of-string match character) but do include a trailing slash. Whenever Django encounters include() (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.
Another possibility is to include additional URL patterns not by specifying the URLconf Python module defining them as the include() argument but by using directly the pattern list as returned by patterns() instead. For example, consider this URLconf:
from django.conf.urls import patterns, url, include
extra_patterns = patterns('',
url(r'^reports/(?P<id>\d+)/$', 'credit.views.report'),
url(r'^charge/$', 'credit.views.charge'),
)
urlpatterns = patterns('',
url(r'^$', 'apps.main.views.homepage'),
(r'^help/', include('apps.help.urls')),
(r'^credit/', include(extra_patterns)),
)
In this example, the /credit/reports/ URL will be handled by the credit.views.report() Django view.
An included URLconf receives any captured parameters from parent URLconfs, so the following example is valid:
# In settings/urls/main.py
urlpatterns = patterns('',
(r'^(?P<username>\w+)/blog/', include('foo.urls.blog')),
)
# In foo/urls/blog.py
urlpatterns = patterns('foo.views',
(r'^$', 'blog.index'),
(r'^archive/$', 'blog.archive'),
)
In the above example, the captured "username" variable is passed to the included URLconf, as expected.
URLconfs have a hook that lets you pass extra arguments to your view functions, as a Python dictionary.
Any URLconf tuple can have an optional third element, which should be a dictionary of extra keyword arguments to pass to the view function.
For example:
urlpatterns = patterns('blog.views',
(r'^blog/(?P<year>\d{4})/$', 'year_archive', {'foo': 'bar'}),
)
In this example, for a request to /blog/2005/, Django will call blog.views.year_archive(year='2005', foo='bar').
This technique is used in the syndication framework to pass metadata and options to views.
Dealing with conflicts
It’s possible to have a URL pattern which captures named keyword arguments, and also passes arguments with the same names in its dictionary of extra arguments. When this happens, the arguments in the dictionary will be used instead of the arguments captured in the URL.
Similarly, you can pass extra options to include(). When you pass extra options to include(), each line in the included URLconf will be passed the extra options.
For example, these two URLconf sets are functionally identical:
Set one:
# main.py
urlpatterns = patterns('',
(r'^blog/', include('inner'), {'blogid': 3}),
)
# inner.py
urlpatterns = patterns('',
(r'^archive/$', 'mysite.views.archive'),
(r'^about/$', 'mysite.views.about'),
)
Set two:
# main.py
urlpatterns = patterns('',
(r'^blog/', include('inner')),
)
# inner.py
urlpatterns = patterns('',
(r'^archive/$', 'mysite.views.archive', {'blogid': 3}),
(r'^about/$', 'mysite.views.about', {'blogid': 3}),
)
Note that extra options will always be passed to every line in the included URLconf, regardless of whether the line’s view actually accepts those options as valid. For this reason, this technique is only useful if you’re certain that every view in the included URLconf accepts the extra options you’re passing.
Some developers find it more natural to pass the actual Python function object rather than a string containing the path to its module. This alternative is supported – you can pass any callable object as the view.
For example, given this URLconf in “string” notation:
urlpatterns = patterns('',
(r'^archive/$', 'mysite.views.archive'),
(r'^about/$', 'mysite.views.about'),
(r'^contact/$', 'mysite.views.contact'),
)
You can accomplish the same thing by passing objects rather than strings. Just be sure to import the objects:
from mysite.views import archive, about, contact
urlpatterns = patterns('',
(r'^archive/$', archive),
(r'^about/$', about),
(r'^contact/$', contact),
)
The following example is functionally identical. It’s just a bit more compact because it imports the module that contains the views, rather than importing each view individually:
from mysite import views
urlpatterns = patterns('',
(r'^archive/$', views.archive),
(r'^about/$', views.about),
(r'^contact/$', views.contact),
)
The style you use is up to you.
Note that if you use this technique – passing objects rather than strings – the view prefix (as explained in “The view prefix” above) will have no effect.
Note that class based views must be imported:
from mysite.views import ClassBasedView
urlpatterns = patterns('',
(r'^myview/$', ClassBasedView.as_view()),
)
A common need when working on a Django project is the possibility to obtain URLs in their final forms either for embedding in generated content (views and assets URLs, URLs shown to the user, etc.) or for handling of the navigation flow on the server side (redirections, etc.)
It is strongly desirable not having to hard-code these URLs (a laborious, non-scalable and error-prone strategy) or having to devise ad-hoc mechanisms for generating URLs that are parallel to the design described by the URLconf and as such in danger of producing stale URLs at some point.
In other words, what’s needed is a DRY mechanism. Among other advantages it would allow evolution of the URL design without having to go all over the project source code to search and replace outdated URLs.
The piece of information we have available as a starting point to get a URL is an identification (e.g. the name) of the view in charge of handling it, other pieces of information that necessarily must participate in the lookup of the right URL are the types (positional, keyword) and values of the view arguments.
Django provides a solution such that the URL mapper is the only repository of the URL design. You feed it with your URLconf and then it can be used in both directions:
The first one is the usage we’ve been discussing in the previous sections. The second one is what is known as reverse resolution of URLs, reverse URL matching, reverse URL lookup, or simply URL reversing.
Django provides tools for performing URL reversing that match the different layers where URLs are needed:
Consider again this URLconf entry:
from django.conf.urls import patterns, url
urlpatterns = patterns('',
#...
url(r'^articles/(\d{4})/$', 'news.views.year_archive'),
#...
)
According to this design, the URL for the archive corresponding to year nnnn is /articles/nnnn/.
You can obtain these in template code by using:
<a href="{% url 'news.views.year_archive' 2012 %}">2012 Archive</a>
{# Or with the year in a template context variable: #}
<ul>
{% for yearvar in year_list %}
<li><a href="{% url 'news.views.year_archive' yearvar %}">{{ yearvar }} Archive</a></li>
{% endfor %}
</ul>
Or in Python code:
from django.core.urlresolvers import reverse
from django.http import HttpResponseRedirect
def redirect_to_year(request):
# ...
year = 2006
# ...
return HttpResponseRedirect(reverse('news.views.year_archive', args=(year,)))
If, for some reason, it was decided that the URLs where content for yearly article archives are published at should be changed then you would only need to change the entry in the URLconf.
In some scenarios where views are of a generic nature, a many-to-one relationship might exist between URLs and views. For these cases the view name isn’t a good enough identificator for it when it comes the time of reversing URLs. Read the next section to know about the solution Django provides for this.
It’s fairly common to use the same view function in multiple URL patterns in your URLconf. For example, these two URL patterns both point to the archive view:
urlpatterns = patterns('',
(r'^archive/(\d{4})/$', archive),
(r'^archive-summary/(\d{4})/$', archive, {'summary': True}),
)
This is completely valid, but it leads to problems when you try to do reverse URL matching (through the reverse() function or the url template tag). Continuing this example, if you wanted to retrieve the URL for the archive view, Django’s reverse URL matcher would get confused, because two URL patterns point at that view.
To solve this problem, Django supports named URL patterns. That is, you can give a name to a URL pattern in order to distinguish it from other patterns using the same view and parameters. Then, you can use this name in reverse URL matching.
Here’s the above example, rewritten to use named URL patterns:
urlpatterns = patterns('',
url(r'^archive/(\d{4})/$', archive, name="full-archive"),
url(r'^archive-summary/(\d{4})/$', archive, {'summary': True}, "arch-summary"),
)
With these names in place (full-archive and arch-summary), you can target each pattern individually by using its name:
{% url 'arch-summary' 1945 %}
{% url 'full-archive' 2007 %}
Even though both URL patterns refer to the archive view here, using the name parameter to url() allows you to tell them apart in templates.
The string used for the URL name can contain any characters you like. You are not restricted to valid Python names.
Note
When you name your URL patterns, make sure you use names that are unlikely to clash with any other application’s choice of names. If you call your URL pattern comment, and another application does the same thing, there’s no guarantee which URL will be inserted into your template when you use this name.
Putting a prefix on your URL names, perhaps derived from the application name, will decrease the chances of collision. We recommend something like myapp-comment instead of comment.
When you need to deploy multiple instances of a single application, it can be helpful to be able to differentiate between instances. This is especially important when using named URL patterns, since multiple instances of a single application will share named URLs. Namespaces provide a way to tell these named URLs apart.
A URL namespace comes in two parts, both of which are strings:
Namespaced URLs are specified using the ':' operator. For example, the main index page of the admin application is referenced using 'admin:index'. This indicates a namespace of 'admin', and a named URL of 'index'.
Namespaces can also be nested. The named URL 'foo:bar:whiz' would look for a pattern named 'whiz' in the namespace 'bar' that is itself defined within the top-level namespace 'foo'.
When given a namespaced URL (e.g. 'myapp:index') to resolve, Django splits the fully qualified name into parts, and then tries the following lookup:
First, Django looks for a matching application namespace (in this example, 'myapp'). This will yield a list of instances of that application.
If there is a current application defined, Django finds and returns the URL resolver for that instance. The current application can be specified as an attribute on the template context - applications that expect to have multiple deployments should set the current_app attribute on any Context or RequestContext that is used to render a template.
The current application can also be specified manually as an argument to the django.core.urlresolvers.reverse() function.
If there is no current application. Django looks for a default application instance. The default application instance is the instance that has an instance namespace matching the application namespace (in this example, an instance of the myapp called 'myapp').
If there is no default application instance, Django will pick the last deployed instance of the application, whatever its instance name may be.
If the provided namespace doesn’t match an application namespace in step 1, Django will attempt a direct lookup of the namespace as an instance namespace.
If there are nested namespaces, these steps are repeated for each part of the namespace until only the view name is unresolved. The view name will then be resolved into a URL in the namespace that has been found.
To show this resolution strategy in action, consider an example of two instances of myapp: one called 'foo', and one called 'bar'. myapp has a main index page with a URL named 'index'. Using this setup, the following lookups are possible:
If there was also a default instance - i.e., an instance named 'myapp' - the following would happen:
URL namespaces of included URLconfs can be specified in two ways.
Firstly, you can provide the application and instance namespaces as arguments to django.conf.urls.include() when you construct your URL patterns. For example,:
(r'^help/', include('apps.help.urls', namespace='foo', app_name='bar')),
This will include the URLs defined in apps.help.urls into the application namespace 'bar', with the instance namespace 'foo'.
Secondly, you can include an object that contains embedded namespace data. If you include() an object as returned by patterns(), the URLs contained in that object will be added to the global namespace. However, you can also include() a 3-tuple containing:
(<patterns object>, <application namespace>, <instance namespace>)
For example:
help_patterns = patterns('',
url(r'^basic/$', 'apps.help.views.views.basic'),
url(r'^advanced/$', 'apps.help.views.views.advanced'),
)
(r'^help/', include(help_patterns, 'bar', 'foo')),
This will include the nominated URL patterns into the given application and instance namespace.
For example, the Django Admin is deployed as instances of AdminSite. AdminSite objects have a urls attribute: A 3-tuple that contains all the patterns in the corresponding admin site, plus the application namespace 'admin', and the name of the admin instance. It is this urls attribute that you include() into your projects urlpatterns when you deploy an Admin instance.
Dec 23, 2012