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 (as simple regular expressions) to Python callback 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.defaults import *
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})/$', 'news.views.month_archive'),
(r'^articles/(?P<year>\d{4})/(?P<month>\d{2})/(?P<day>\d+)/$', '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:
If there are any named arguments, it will use those, ignoring non-named arguments. Otherwise, it will pass all non-named arguments as positional arguments.
In both cases, it will pass any extra keyword arguments as keyword arguments. See "Passing extra options to view functions" below.
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.
urlpatterns should be a Python list, in the format returned by the function django.conf.urls.defaults.patterns(). Always use patterns() to create the urlpatterns variable.
Convention is to use from django.conf.urls.defaults import * at the top of your URLconf. This gives your module access to these objects:
A function that takes a prefix, and an arbitrary number of URL patterns, and returns a list of URL patterns in the format Django needs.
The first argument to patterns() is a string prefix. See The view prefix below.
The remaining arguments should be tuples in this format:
(regular expression, Python callback function [, optional dictionary [, optional name]])
...where optional dictionary and optional name are optional. (See Passing extra options to view functions below.)
Note
Because patterns() is a function call, it accepts a maximum of 255 arguments (URL patterns, in this case). This is a limit for all Python function calls. This is rarely a problem in practice, because you'll typically structure your URL patterns modularly by using include() sections. However, on the off-chance you do hit the 255-argument limit, realize that patterns() returns a Python list, so you can split up the construction of the list.
urlpatterns = patterns('',
...
)
urlpatterns += patterns('',
...
)
Python lists have unlimited size, so there's no limit to how many URL patterns you can construct. The only limit is that you can only create 254 at a time (the 255th argument is the initial prefix argument).
You can use the url() function, instead of a tuple, as an argument to patterns(). This is convenient if you want to specify a name without the optional extra arguments dictionary. For example:
urlpatterns = patterns('',
url(r'^index/$', index_view, name="main-view"),
...
)
This function takes five arguments, most of which are optional:
url(regex, view, kwargs=None, name=None, prefix='')
See Naming URL patterns for why the name parameter is useful.
The prefix parameter has the same meaning as the first argument to patterns() and is only relevant when you're passing a string as the view parameter.
A string representing the full Python import path to the view that should be called if none of the URL patterns match.
By default, this is 'django.views.defaults.page_not_found'. That default value should suffice.
A string representing the full Python import path to the view that should be called in case of server errors. Server errors happen when you have runtime errors in view code.
By default, this is 'django.views.defaults.server_error'. That default value should suffice.
A function that takes a full Python import path to another URLconf module that should be "included" in this place.
include() also accepts as an argument an iterable that returns URL patterns.
See Including other URLconfs below.
Each captured argument is sent to the view as a plain Python string, regardless of what sort of match the regular expression makes. For example, in this URLconf line:
(r'^articles/(?P<year>\d{4})/$', '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.
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.defaults import *
urlpatterns = patterns('',
(r'^articles/(\d{4})/$', 'mysite.news.views.year_archive'),
(r'^articles/(\d{4})/(\d{2})/$', 'mysite.news.views.month_archive'),
(r'^articles/(\d{4})/(\d{2})/(\d+)/$', 'mysite.news.views.article_detail'),
)
In this example, each view has a common prefix -- 'mysite.news.views'. Instead of typing that out for each entry in urlpatterns, you can use the first argument to the patterns() function to specify a prefix to apply to each view function.
With this in mind, the above example can be written more concisely as:
from django.conf.urls.defaults import *
urlpatterns = patterns('mysite.news.views',
(r'^articles/(\d{4})/$', '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.defaults import *
urlpatterns = patterns('',
(r'^$', 'django.views.generic.date_based.archive_index'),
(r'^(?P<year>\d{4})/(?P<month>[a-z]{3})/$', 'django.views.generic.date_based.archive_month'),
(r'^tag/(?P<tag>\w+)/$', 'weblog.views.tag'),
)
New:
from django.conf.urls.defaults import *
urlpatterns = patterns('django.views.generic.date_based',
(r'^$', 'archive_index'),
(r'^(?P<year>\d{4})/(?P<month>[a-z]{3})/$','archive_month'),
)
urlpatterns += patterns('weblog.views',
(r'^tag/(?P<tag>\w+)/$', '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 the URLconf for the Django Web site itself. It includes a number of other URLconfs:
from django.conf.urls.defaults import *
urlpatterns = patterns('',
(r'^weblog/', include('django_website.apps.blog.urls.blog')),
(r'^documentation/', include('django_website.apps.docs.urls.docs')),
(r'^comments/', include('django.contrib.comments.urls')),
)
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(), 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:
from django.conf.urls.defaults import *
extra_patterns = patterns('',
url(r'reports/(?P<id>\d+)/$', 'credit.views.report', name='credit-reports'),
url(r'charge/$', 'credit.views.charge', name='credit-charge'),
)
urlpatterns = patterns('',
url(r'^$', 'apps.main.views.homepage', name='site-homepage'),
(r'^help/', include('apps.help.urls')),
(r'^credit/', include(extra_patterns)),
)
This approach can be seen in use when you deploy an instance of the Django Admin application. The Django Admin is deployed as instances of a AdminSite; each AdminSite instance has an attribute urls that returns the url patterns available to that instance. It is this attribute that you include() into your projects urlpatterns when you deploy the admin instance.
An included URLconf receives any captured parameters from parent URLconfs, so the following example is valid:
# In settings/urls/main.py
urlpatterns = patterns('',
(r'^(?P<username>\w+)/blog/', include('foo.urls.blog')),
)
# In foo/urls/blog.py
urlpatterns = patterns('foo.views',
(r'^$', 'blog.index'),
(r'^archive/$', 'blog.archive'),
)
In the above example, the captured "username" variable is passed to the included URLconf, as expected.
When you need to deploy multiple instances of a single application, it can be helpful to be able to differentiate between instances. This is especially important when using named URL patterns, since multiple instances of a single application will share named URLs. Namespaces provide a way to tell these named URLs apart.
A URL namespace comes in two parts, both of which are strings:
URL Namespaces can be specified in two ways.
Firstly, you can provide the application and instance namespace as arguments to include() when you construct your URL patterns. For example,:
(r'^help/', include('apps.help.urls', namespace='foo', app_name='bar')),
This will include the URLs defined in apps.help.urls into the application namespace bar, with the instance namespace foo.
Secondly, you can include an object that contains embedded namespace data. If you include() a patterns object, that object will be added to the global namespace. However, you can also include() an object that contains a 3-tuple containing:
(<patterns object>, <application namespace>, <instance namespace>)
This will include the nominated URL patterns into the given application and instance namespace. For example, the urls attribute of Django's AdminSite object returns a 3-tuple that contains all the patterns in an admin site, plus the name of the admin instance, and the application namespace admin.
Once you have defined namespaced URLs, you can reverse them. For details on reversing namespaced urls, see the documentation on reversing namespaced URLs.
URLconfs have a hook that lets you pass extra arguments to your view functions, as a Python dictionary.
Any URLconf tuple can have an optional third element, which should be a dictionary of extra keyword arguments to pass to the view function.
For example:
urlpatterns = patterns('blog.views',
(r'^blog/(?P<year>\d{4})/$', 'year_archive', {'foo': 'bar'}),
)
In this example, for a request to /blog/2005/, Django will call the blog.views.year_archive() view, passing it these keyword arguments:
year='2005', foo='bar'
This technique is used in generic views and 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/$', views.contact),
)
The style you use is up to you.
Note that if you use this technique -- passing objects rather than strings -- the view prefix (as explained in "The view prefix" above) will have no effect.
It's fairly common to use the same view function in multiple URL patterns in your URLconf. For example, these two URL patterns both point to the archive view:
urlpatterns = patterns('',
(r'^archive/(\d{4})/$', archive),
(r'^archive-summary/(\d{4})/$', archive, {'summary': True}),
)
This is completely valid, but it leads to problems when you try to do reverse URL matching (through the permalink() decorator or the url template tag). Continuing this example, if you wanted to retrieve the URL for the archive view, Django's reverse URL matcher would get confused, because two URLpatterns point at that view.
To solve this problem, Django supports named URL patterns. That is, you can give a name to a URL pattern in order to distinguish it from other patterns using the same view and parameters. Then, you can use this name in reverse URL matching.
Here's the above example, rewritten to use named URL patterns:
urlpatterns = patterns('',
url(r'^archive/(\d{4})/$', 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.
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 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 first 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:
If you need to use something similar to the url template tag in your code, Django provides the following method (in the django.core.urlresolvers module):
viewname is either the function name (either a function reference, or the string version of the name, if you used that form in urlpatterns) or the URL pattern name. Normally, you won't need to worry about the urlconf parameter and will only pass in the positional and keyword arguments to use in the URL matching. For example:
from django.core.urlresolvers import reverse
def myview(request):
return HttpResponseRedirect(reverse('arch-summary', args=[1945]))
The reverse() function can reverse a large variety of regular expression patterns for URLs, but not every possible one. The main restriction at the moment is that the pattern cannot contain alternative choices using the vertical bar ("|") character. You can quite happily use such patterns for matching against incoming URLs and sending them off to views, but you cannot reverse such patterns.
The current_app argument allows you to provide a hint to the resolver indicating the application to which the currently executing view belongs. This current_app argument is used as a hint to resolve application namespaces into URLs on specific application instances, according to the namespaced URL resolution strategy.
Make sure your views are all correct
As part of working out which URL names map to which patterns, the reverse() function has to import all of your URLconf files and examine the name of each view. This involves importing each view function. If there are any errors whilst importing any of your view functions, it will cause reverse() to raise an error, even if that view function is not the one you are trying to reverse.
Make sure that any views you reference in your URLconf files exist and can be imported correctly. Do not include lines that reference views you haven't written yet, because those views will not be importable.
The django.core.urlresolvers.resolve() function can be used for resolving URL paths to the corresponding view functions. It has the following signature:
path is the URL path you want to resolve. As with reverse() above, you don't need to worry about the urlconf parameter. The function returns the triple (view function, arguments, keyword arguments).
For example, it can be used for testing if a view would raise a Http404 error before redirecting to it:
from urlparse import urlparse
from django.core.urlresolvers import resolve
from django.http import HttpResponseRedirect, Http404
def myview(request):
next = request.META.get('HTTP_REFERER', None) or '/'
response = HttpResponseRedirect(next)
# modify the request and response as required, e.g. change locale
# and set corresponding locale cookie
view, args, kwargs = resolve(urlparse(next)[2])
kwargs['request'] = request
try:
view(*args, **kwargs)
except Http404:
return HttpResponseRedirect('/')
return response
The django.db.models.permalink() decorator is useful for writing short methods that return a full URL path. For example, a model's get_absolute_url() method. See django.db.models.permalink() for more.
Sep 20, 2009