==============

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: ../settings/

.. _request object: ../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})/$', '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.

.. _Django overview: ../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: ../generic_views/

.. _syndication framework: ../syndication/

.. admonition:: 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.

Passing extra options to ``include()``

--------------------------------------

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 the included URLconf accepts the extra options you're passing.

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.