==============
URL dispatcher
==============

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.

.. _Cool URIs don't change: http://www.w3.org/Provider/Style/URI

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 (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.

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 looks at the ``ROOT_URLCONF`` setting in your `settings file`_.
       This should be a string representing the full Python import path to your
       URLconf. For example: ``"mydjangoapps.urls"``.
    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.defaults.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. The view gets passed a
       `request object`_ as its first argument and any values captured in the
       regex as remaining arguments.

.. _settings file: http://www.djangoproject.com/documentation/settings/
.. _request object: http://www.djangoproject.com/documentation/request_response/#httprequest-objects

Example
=======

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:

    * ``from django.conf.urls.defaults import *`` makes the ``patterns()``
      function available.

    * 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/2003/03/3/`` would match the final pattern. Django would call
      the function ``news.views.article_detail(request, '2003', '03', '3')``.

.. _Dive Into Python's explanation: http://diveintopython.org/regular_expressions/street_addresses.html#re.matching.2.3

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})/$', 'news.views.month_archive'),
        (r'^articles/(?P<year>\d{4})/(?P<month>\d{2})/(?P<day>\d+)/$', '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/2003/03/3/`` would call the function
      ``news.views.article_detail(request, year='2003', month='03', day='3')``.

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.

The matching/grouping algorithm
-------------------------------

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.

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.

Syntax of the urlpatterns variable
==================================

``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:

patterns
--------

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])

...where ``optional dictionary`` is optional. (See
_`Passing extra options to view functions` below.)

handler404
----------

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.

handler500
----------

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.

include
-------

A function that takes a full Python import path to another URLconf that should
be "included" in this place. See _`Including other URLconfs` below.

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.

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.defaults import *

    urlpatterns = patterns('',
        (r'^articles/(\d{4})/$', 'myproject.news.views.year_archive'),
        (r'^articles/(\d{4})/(\d{2})/$', 'myproject.news.views.month_archive'),
        (r'^articles/(\d{4})/(\d{2})/(\d+)/$', 'myproject.news.views.article_detail'),
    )

In this example, each view has a common prefix -- ``'myproject.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('myproject.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.

.. _Django overview: http://www.djangoproject.com/documentation/overview/

Multiple view prefixes
----------------------

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'),
    )

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 the URLconf for the `Django website`_ 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.comments')),
    )

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.

.. _`Django website`: http://www.djangoproject.com/

Captured parameters
-------------------

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.

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 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.

.. _generic views: http://www.djangoproject.com/documentation/generic_views/
.. _syndication framework: http://www.djangoproject.com/documentation/syndication/
