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diff --git a/parts/django/docs/topics/http/urls.txt b/parts/django/docs/topics/http/urls.txt new file mode 100644 index 0000000..2361297 --- /dev/null +++ b/parts/django/docs/topics/http/urls.txt @@ -0,0 +1,890 @@ +============== +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. + +.. _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.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 an + :class:`~django.http.HttpRequest` as its first argument and any values + captured in the regex as remaining arguments. + +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 +:func:`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: + +.. module:: django.conf.urls.defaults + +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 +--- + +.. versionadded:: 1.0 + +.. 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/$', index_view, name="main-view"), + ... + ) + +This function takes five arguments, most of which are optional:: + + url(regex, view, kwargs=None, name=None, prefix='') + +See `Naming URL patterns`_ for why the ``name`` parameter is useful. + +The ``prefix`` parameter has the same meaning as the first argument to +``patterns()`` and is only relevant when you're passing a string as the +``view`` parameter. + +handler404 +---------- + +.. data:: handler404 + +A callable, or a string representing the full Python import path to the view +that should be called if none of the URL patterns match. + +By default, this is ``'django.views.defaults.page_not_found'``. That default +value should suffice. + +.. versionchanged:: 1.2 + Previous versions of Django only accepted strings representing import paths. + +handler500 +---------- + +.. data:: handler500 + +A callable, or a string representing the full Python import path to the view +that should be called in case of server errors. Server errors happen when you +have runtime errors in view code. + +By default, this is ``'django.views.defaults.server_error'``. That default +value should suffice. + +.. versionchanged:: 1.2 + Previous versions of Django only accepted strings representing import paths. + +include +------- + +.. function:: include(<module or pattern_list>) + +A function that takes a full Python import path to another URLconf module that +should be "included" in this place. + +.. versionadded:: 1.1 + +:func:`include` also accepts as an argument an iterable that returns URL +patterns. + +See `Including other URLconfs`_ below. + +Notes on capturing text in URLs +=============================== + +Each captured argument is sent to the view as a plain Python string, regardless +of what sort of match the regular expression makes. For example, in this +URLconf line:: + + (r'^articles/(?P<year>\d{4})/$', '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 :doc:`Django overview </intro/overview>`:: + + from django.conf.urls.defaults import * + + 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+)/$', 'news.views.article_detail'), + ) + +In this example, each view has a common prefix -- ``'news.views'``. +Instead of typing that out for each entry in ``urlpatterns``, you can use the +first argument to the ``patterns()`` function to specify a prefix to apply to +each view function. + +With this in mind, the above example can be written more concisely as:: + + from django.conf.urls.defaults import * + + urlpatterns = patterns('news.views', + (r'^articles/(\d{4})/$', '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.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 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 ``$`` +(end-of-string match character) but do include a trailing slash. Whenever +Django encounters ``include()``, it chops off whatever part of the URL matched +up to that point and sends the remaining string to the included URLconf for +further processing. + +.. versionadded:: 1.1 + +Another possibility is to include additional URL patterns not by specifying the +URLconf Python module defining them as the `include`_ argument but by using +directly the pattern list as returned by `patterns`_ instead. For example:: + + from django.conf.urls.defaults import * + + extra_patterns = patterns('', + url(r'reports/(?P<id>\d+)/$', '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 +: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`: 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. + +.. _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. + +.. _naming-url-patterns: + +Naming URL patterns +=================== + +.. versionadded:: 1.0 + +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* +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: + +.. 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 +-------------- + +.. versionadded:: 1.1 + +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``. + + +Utility methods +=============== + +reverse() +--------- + +If you need to use something similar to the :ttag:`url` template tag in +your code, Django provides the following method (in the +``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. + +.. versionadded:: 1.1 + +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>`. + +.. 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. + +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 +the triple (view function, arguments, keyword arguments). + +If the URL does not resolve, the function raises an +:class:`~django.http.Http404` exception. + +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 + +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() + +.. versionadded:: 1.0 + +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 webserver, this is always ``"/"``, but it can be +changed, for instance by using ``django.root`` (see :ref:`How to use +Django with Apache and mod_python <howto-deployment-modpython>`).
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