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[gae.git] / python / lib / django_1_4 / docs / topics / http / urls.txt

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

.. module:: django.core.urlresolvers

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.

.. versionadded:: 1.4
    Django also allows to translate URLs according to the active language.
    This process is described in the
    :ref:`internationalization docs <url-internationalization>`.

.. _how-django-processes-a-request:

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 :setting:`ROOT_URLCONF` setting, but if the incoming
   ``HttpRequest`` object has an attribute called ``urlconf`` (set by
   middleware :ref:`request processing <request-middleware>`), its value
   will be used in place of the :setting:`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 :func:`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 :doc:`class based view
   </topics/class-based-views>`). The view gets passed an
   :class:`~django.http.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, 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:

* 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')``.

.. _Dive Into Python's explanation: http://diveintopython.net/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})/$', 'news.views.month_archive'),
        (r'^articles/(?P<year>\d{4})/(?P<month>\d{2})/(?P<day>\d{2})/$', '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.

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
:func:`django.conf.urls.patterns`. Always use ``patterns()`` to create
the ``urlpatterns`` variable.

``django.conf.urls`` utility functions
======================================

.. module:: django.conf.urls

.. deprecated:: 1.4
    Starting with Django 1.4 functions ``patterns``, ``url``, ``include`` plus
    the ``handler*`` symbols described below live in the ``django.conf.urls``
    module.

    Until Django 1.3 they were located in ``django.conf.urls.defaults``. You
    still can import them from there but it will be removed in Django 1.6.

patterns
--------

.. function:: patterns(prefix, pattern_description, ...)

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).

url
---

.. function:: url(regex, view, kwargs=None, name=None, prefix='')

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.

include
-------

.. function:: include(<module or pattern_list>)

A function that takes a full Python import path to another URLconf module that
should be "included" in this place.

:func:`include` also accepts as an argument an iterable that returns URL
patterns.

See `Including other URLconfs`_ below.

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 which can
be set in your root URLconf. Setting these variables in any other
URLconf will have no effect.

See the documentation on :ref:`customizing error views
<customizing-error-views>` for more details.

handler403
----------

.. data:: handler403

A callable, or a string representing the full Python import path to the view
that should be called if the user doesn't have the permissions required to
access a resource.

By default, this is ``'django.views.defaults.permission_denied'``. That default
value should suffice.

See the documentation about :ref:`the 403 (HTTP Forbidden) view
<http_forbidden_view>` for more information.

.. versionadded:: 1.4
    ``handler403`` is new in Django 1.4.

handler404
----------

.. data:: handler404

A callable, or 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.

.. versionchanged:: 1.2
    Previous versions of Django only accepted strings representing import paths.

handler500
----------

.. data:: handler500

A callable, or 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.

.. versionchanged:: 1.2
    Previous versions of Django only accepted strings representing import paths.

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 :doc:`Django overview </intro/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.

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 import patterns, url, include

    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 import patterns, url, include

    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 an excerpt of the URLconf for the `Django Web site`_
itself. It includes a number of other URLconfs::

    from django.conf.urls import patterns, url, 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()``, 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 import patterns, url, include

    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
:class:`~django.contrib.admin.AdminSite`; each
:class:`~django.contrib.admin.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.

.. _`Django Web site`: https://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.

.. _topics-http-defining-url-namespaces:

Defining URL namespaces
-----------------------

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 :ref:`named URL patterns <naming-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:

* An **application namespace**. This describes the name of the application
  that is being deployed. Every instance of a single application will have
  the same application namespace. For example, Django's admin application
  has the somewhat predictable application namespace of ``admin``.

* An **instance namespace**. This identifies a specific instance of an
  application. Instance namespaces should be unique across your entire
  project. However, an instance namespace can be the same as the
  application namespace. This is used to specify a default instance of an
  application. For example, the default Django Admin instance has an
  instance namespace of ``admin``.

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
:class:`~django.contrib.admin.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 :ref:`reversing namespaced
URLs <topics-http-reversing-url-namespaces>`.

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 :doc:`generic views </ref/generic-views>` and in the
:doc:`syndication framework </ref/contrib/syndication>` to pass metadata and
options to views.

.. 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 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.

Note that :doc:`class based views</topics/class-based-views>` must be
imported::

    from mysite.views import ClassBasedView

    urlpatterns = patterns('',
        (r'^myview/$', ClassBasedView.as_view()),
    )

.. _naming-url-patterns:

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 ``permalink()`` decorator or the :ttag:`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:

.. code-block:: html+django

    {% 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``.

.. _topics-http-reversing-url-namespaces:

URL namespaces
--------------

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:

1. First, Django looks for a matching application namespace (in this
   example, ``myapp``). This will yield a list of instances of that
   application.

2. 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 :func:`reverse()` function.

3. 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``).

4. If there is no default application instance, Django will pick the last
   deployed instance of the application, whatever its instance name may be.

5. 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 one of the instances is current - say, if we were rendering a utility page
  in the instance ``bar`` - ``myapp:index`` will resolve to the index page of
  the instance ``bar``.

* If there is no current instance - say, if we were rendering a page
  somewhere else on the site - ``myapp:index`` will resolve to the last
  registered instance of ``myapp``. Since there is no default instance,
  the last instance of ``myapp`` that is registered will be used. This could
  be ``foo`` or ``bar``, depending on the order they are introduced into the
  urlpatterns of the project.

* ``foo:index`` will always resolve to the index page of the instance ``foo``.

If there was also a default instance - i.e., an instance named `myapp` - the
following would happen:

* If one of the instances is current - say, if we were rendering a utility page
  in the instance ``bar`` - ``myapp:index`` will resolve to the index page of
  the instance ``bar``.

* If there is no current instance - say, if we were rendering a page somewhere
  else on the site - ``myapp:index`` will resolve to the index page of the
  default instance.

* ``foo:index`` will again resolve to the index page of the instance ``foo``.


``django.core.urlresolvers`` utility functions
==============================================

.. currentmodule:: django.core.urlresolvers

reverse()
---------

If you need to use something similar to the :ttag:`url` template tag in
your code, Django provides the following function (in the
:mod:`django.core.urlresolvers` module):

.. function:: reverse(viewname, [urlconf=None, args=None, kwargs=None, current_app=None])

``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]))

.. _URL pattern name: `Naming URL patterns`_

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
:ref:`namespaced URL resolution strategy <topics-http-reversing-url-namespaces>`.

You can use ``kwargs`` instead of ``args``. For example::

    >>> reverse('admin:app_list', kwargs={'app_label': 'auth'})
    '/admin/auth/'

``args`` and ``kwargs`` cannot be passed to ``reverse()`` at the same time.

.. admonition:: 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.

.. note::

    The string returned by :meth:`~django.core.urlresolvers.reverse` is already
    :ref:`urlquoted <uri-and-iri-handling>`. For example::

        >>> reverse('cities', args=[u'Orléans'])
        '.../Orl%C3%A9ans/'

    Applying further encoding (such as :meth:`~django.utils.http.urlquote` or
    ``urllib.quote``) to the output of :meth:`~django.core.urlresolvers.reverse`
    may produce undesirable results.

reverse_lazy()
--------------

.. versionadded:: 1.4

A lazily evaluated version of `reverse()`_.

.. function:: reverse_lazy(viewname, [urlconf=None, args=None, kwargs=None, current_app=None])

It is useful for when you need to use a URL reversal before your project's
URLConf is loaded. Some common cases where this function is necessary are:

* providing a reversed URL as the ``url`` attribute of a generic class-based
  view.

* providing a reversed URL to a decorator (such as the ``login_url`` argument
  for the :func:`django.contrib.auth.decorators.permission_required`
  decorator).

* providing a reversed URL as a default value for a parameter in a function's
  signature.

resolve()
---------

The :func:`django.core.urlresolvers.resolve` function can be used for
resolving URL paths to the corresponding view functions. It has the
following signature:

.. function:: resolve(path, urlconf=None)

``path`` is the URL path you want to resolve. As with
:func:`~django.core.urlresolvers.reverse`, you don't need to
worry about the ``urlconf`` parameter. The function returns a
:class:`ResolverMatch` object that allows you
to access various meta-data about the resolved URL.

If the URL does not resolve, the function raises an
:class:`~django.http.Http404` exception.

.. class:: ResolverMatch

    .. attribute:: ResolverMatch.func

        The view function that would be used to serve the URL

    .. attribute:: ResolverMatch.args

        The arguments that would be passed to the view function, as
        parsed from the URL.

    .. attribute:: ResolverMatch.kwargs

        The keyword arguments that would be passed to the view
        function, as parsed from the URL.

    .. attribute:: ResolverMatch.url_name

        The name of the URL pattern that matches the URL.

    .. attribute:: ResolverMatch.app_name

        The application namespace for the URL pattern that matches the
        URL.

    .. attribute:: ResolverMatch.namespace

        The instance namespace for the URL pattern that matches the
        URL.

    .. attribute:: ResolverMatch.namespaces

        The list of individual namespace components in the full
        instance namespace for the URL pattern that matches the URL.
        i.e., if the namespace is ``foo:bar``, then namespaces will be
        ``['foo', 'bar']``.

A :class:`ResolverMatch` object can then be interrogated to provide
information about the URL pattern that matches a URL::

    # Resolve a URL
    match = resolve('/some/path/')
    # Print the URL pattern that matches the URL
    print match.url_name

A :class:`ResolverMatch` object can also be assigned to a triple::

    func, args, kwargs = resolve('/some/path/')

.. versionchanged:: 1.3
    Triple-assignment exists for backwards-compatibility. Prior to
    Django 1.3, :func:`~django.core.urlresolvers.resolve` returned a
    triple containing (view function, arguments, keyword arguments);
    the :class:`ResolverMatch` object (as well as the namespace and pattern
    information it provides) is not available in earlier Django releases.

One possible use of :func:`~django.core.urlresolvers.resolve` would be to test
whether 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


permalink()
-----------

The :func:`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 :func:`django.db.models.permalink` for more.

get_script_prefix()
-------------------

.. function:: get_script_prefix()

Normally, you should always use :func:`~django.core.urlresolvers.reverse` or
:func:`~django.db.models.permalink` to define URLs within your application.
However, if your application constructs part of the URL hierarchy itself, you
may occasionally need to generate URLs. In that case, you need to be able to
find the base URL of the Django project within its Web server
(normally, :func:`~django.core.urlresolvers.reverse` takes care of this for
you). In that case, you can call ``get_script_prefix()``, which will return the
script prefix portion of the URL for your Django project. If your Django
project is at the root of its Web server, this is always ``"/"``, but it can be
changed, for instance  by using ``django.root`` (see :doc:`How to use
Django with Apache and mod_python </howto/deployment/modpython>`).