URL dispatcher¶

Overview¶

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.

How Django processes a request¶

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:

1. Django determines the root URLconf module to use. Ordinarily, this is the value of the ROOT_URLCONF setting, but if the incoming HttpRequest object has an attribute called urlconf (set by middleware request processing), its value will be used in place of the ROOT_URLCONF setting.
2. Django loads that Python module and looks for the variable urlpatterns. This should be a Python list, in the format returned by the function django.conf.urls.patterns().
3. Django runs through each URL pattern, in order, and stops at the first one that matches the requested URL.
4. Once one of the regexes matches, Django imports and calls the given view, which is a simple Python function (or a class based view). The view gets passed an HttpRequest as its first argument and any values captured in the regex as remaining arguments.
5. If no regex matches, or if an exception is raised during any point in this process, Django invokes an appropriate error-handling view. See Error handling below.

Example¶

Here’s a sample URLconf:

from django.conf.urls import patterns

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:

• To capture a value from the URL, just put parenthesis around it.
• There’s no need to add a leading slash, because every URL has that. For example, it’s ^articles, not ^/articles.
• The 'r' in front of each regular expression string is optional but recommended. It tells Python that a string is “raw” – that nothing in the string should be escaped. See Dive Into Python’s explanation.

Example requests:

• A request to /articles/2005/03/ would match the third entry in the list. Django would call the function news.views.month_archive(request, '2005', '03').
• /articles/2005/3/ would not match any URL patterns, because the third entry in the list requires two digits for the month.
• /articles/2003/ would match the first pattern in the list, not the second one, because the patterns are tested in order, and the first one is the first test to pass. Feel free to exploit the ordering to insert special cases like this.
• /articles/2003 would not match any of these patterns, because each pattern requires that the URL end with a slash.
• /articles/2003/03/03/ would match the final pattern. Django would call the function news.views.article_detail(request, '2003', '03', '03').

Named groups¶

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:

• A request to /articles/2005/03/ would call the function news.views.month_archive(request, year='2005', month='03'), instead of news.views.month_archive(request, '2005', '03').
• A request to /articles/2003/03/03/ would call the function news.views.article_detail(request, year='2003', month='03', day='03').

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.

What the URLconf searches against¶

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.

Notes on capturing text in URLs¶

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.

Performance¶

Each regular expression in a urlpatterns is compiled the first time it’s accessed. This makes the system blazingly fast.

Syntax of the urlpatterns variable¶

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.

Error handling¶

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:

• handler404 – See django.conf.urls.handler404.
• handler500 – See django.conf.urls.handler500.
• handler403 – See django.conf.urls.handler403.

The view prefix¶

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

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

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.

from django.conf.urls import patterns

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'),
)

from django.conf.urls import patterns

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'),
)

Including other URLconfs¶

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.

# 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'),
)

Passing extra options to view functions¶

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.

# main.py
urlpatterns = patterns('',
    (r'^blog/', include('inner'), {'blogid': 3}),
)

# inner.py
urlpatterns = patterns('',
    (r'^archive/$', 'mysite.views.archive'),
    (r'^about/$', 'mysite.views.about'),
)

# 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}),
)

Passing callable objects instead of strings¶

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()),
)

Reverse resolution of URLs¶

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:

• Starting with a URL requested by the user/browser, it calls the right Django view providing any arguments it might need with their values as extracted from the URL.
• Starting with the identification of the corresponding Django view plus the values of arguments that would be passed to it, obtain the associated URL.

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:

• In templates: Using the url template tag.
• In Python code: Using the django.core.urlresolvers.reverse() function.
• In higher level code related to handling of URLs of Django model instances: The get_absolute_url() method.

from django.conf.urls import patterns, url

urlpatterns = patterns('',
    #...
    url(r'^articles/(\d{4})/$', 'news.views.year_archive'),
    #...
)

<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>

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,)))

Naming URL patterns¶

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.

URL namespaces¶

(r'^help/', include('apps.help.urls', namespace='foo', app_name='bar')),

(<patterns object>, <application namespace>, <instance namespace>)

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')),