.. index:: single: url dispatch .. _urldispatch_chapter: URL Dispatch ============ It is common for :mod:`repoze.bfg` developers to rely on :term:`traversal` to map URLs to code. However, :mod:`repoze.bfg` can also map URLs to code via :term:`URL dispatch`. The presence of ```` statements in a :term:`ZCML` file used by your application or the presence of calls to the :meth:`repoze.bfg.configuration.Configurator.add_route` method in imperative configuration within your application is a sign that you're using URL dispatch. Using the ``add_route`` configurator method or ```` statements in ZCML allows you to declaratively map URLs to code. The syntax of the pattern matching language used by :mod:`repoze.bfg` is close to that of :term:`Routes`. It often makes a lot of sense to use :term:`URL dispatch` instead of :term:`traversal` in an application that has no natural hierarchy. For instance, if all the data in your application lives in a relational database, and that relational database has no self-referencing tables that form a natural hierarchy, URL dispatch is easier to use than traversal, and is often a more natural fit for creating an application that manipulates "flat" data. Concept and Usage ----------------- The URL dispatch features of :mod:`repoze.bfg` allow you to either augment or replace :term:`traversal`, allowing URL dispatch to have the "first crack" (and potentially the *only* crack) at resolving a given URL to :term:`context` and :term:`view name`. To allow for URL dispatch to be used, the :mod:`repoze.bfg` framework allows you to inject ``route`` ZCML directives into your application's ``configure.zcml`` file. The mod:`repoze.bfg` :term:`Router` checks an incoming request against a *routes map* to find a :term:`context` and a :term:`view callable` before :term:`traversal` has a chance to find these things first. If a route matches, a :term:`context` is generated and :mod:`repoze.bfg` will call the :term:`view callable` found due to the context and the request. If no route matches, :mod:`repoze.bfg` will fail over to calling the :term:`root factory` callable passed to the :term:`Configurator` for the application (usually a traversal function). A root factory is not required for purely URL-dispatch-based apps: if the root factory callable is passed as ``None`` to the :term:`Configurator`, :mod:`repoze.bfg` will return a :exc:`NotFound` error to the user's browser when no routes match. .. note:: See :ref:`modelspy_project_section` for an example of a simple root factory callable that will use traversal. .. index:: single: add_route The ``add_route`` Configurator Method ------------------------------------- The :meth:`repoze.bfg.configuration.Configurator.add_route` method adds a single :term:`route configuration` to the :term:`application registry`. Here's an example: .. ignore-next-block .. code-block:: python config.add_route('myroute', '/prefix/:one/:two') .. index:: single: ZCML directive; route Configuring a Route via ZCML ---------------------------- Instead of using the imperative method of adding a route, you can use :term:`ZCML` for the same purpose. For example: .. code-block:: xml :linenos: See :ref:`route_directive` for full ``route`` ZCML directive documentation. .. note:: The documentation that follows in this chapter assumes that :term:`ZCML` will be used to perform route configuration. .. index:: pair: URL dispatch; matchdict The Matchdict ------------- The main purpose of a route is to match (nor not match) the ``PATH_INFO`` present in the WSGI environment provided during a request against a URL path pattern. When this URL path pattern is matched, a dictionary is placed on the request named ``matchdict`` with the values that match patterns in the ``path`` element. If the URL pattern does not match, no matchdict is generated. .. index:: pair: URL dispatch; path pattern syntax .. _route_path_pattern_syntax: Path Pattern Syntax -------------------- The path pattern syntax is simple. The path may start with a slash character. If the path does not start with a slash character, an implicit slash will be prepended to it at matching time. For example, the following paths are equivalent: .. code-block:: text :foo/bar/baz and: .. code-block:: text /:foo/bar/baz A path segment (an individual item between ``/`` characters in the path) may either be a literal string (e.g. ``foo``) *or* it may segment replacement marker (e.g. ``:foo``). A segment replacement marker is in the format ``:name``, where this means "accept any characters up to the next slash and use this as the ``name`` matchdict value." For example, the following pattern defines one literal segment ("foo") and two dynamic segments ("baz", and "bar"): .. code-block:: text foo/:baz/:bar The above pattern will match these URLs, generating the following matchdicts: .. code-block:: text foo/1/2 -> {'baz':u'1', 'bar':u'2'} foo/abc/def -> {'baz':u'abc', 'bar':u'def'} It will not match the following patterns however: .. code-block:: text foo/1/2/ -> No match (trailing slash) bar/abc/def -> First segment literal mismatch Note that values representing path segments matched with a ``:segment`` match will be url-unquoted and decoded from UTF-8 into Unicode within the matchdict. So for instance, the following pattern: .. code-block:: text foo/:bar When matching the following URL: .. code-block:: text foo/La%20Pe%C3%B1a The matchdict will look like so (the value is URL-decoded / UTF-8 decoded): .. code-block:: text {'bar':u'La Pe\xf1a'} If the pattern has a ``*`` in it, the name which follows it is considered a "remainder match". A remainder match *must* come at the end of the path pattern. Unlike segment replacement markers, it does not need to be preceded by a slash. For example: .. code-block:: text foo/:baz/:bar*traverse The above pattern will match these URLs, generating the following matchdicts: .. code-block:: text foo/1/2/ -> {'baz':1, 'bar':2, 'traverse':()} foo/abc/def/a/b/c -> {'baz':abc, 'bar':def, 'traverse':('a', 'b', 'c')} Note that when a ``*stararg`` remainder match is matched, the value put into the matchdict is turned into a tuple of path segments representing the remainder of the path. These path segments are url-unquoted and decoded from UTF-8 into Unicode. For example, for the following pattern: .. code-block:: text foo/*traverse When matching the following path: .. code-block:: text /foo/La%20Pe%C3%B1a/a/b/c Will generate the following matchdict: .. code-block:: text {'traverse':(u'La Pe\xf1a', u'a', u'b', u'c')} .. index:: triple: ZCML directive; route; examples ```` Statement Examples ------------------------------ Let's check out some examples of how ```` statements might be commonly declared. Example 1 ~~~~~~~~~ The simplest route declaration: .. code-block:: xml :linenos: When the URL matches ``/hello.html``, the view callable at the Python dotted path name ``mypackage.views.hello_view`` will be called with a default context object and the request. See :ref:`views_chapter` for more information about views. The ``mypackage.views`` module referred to above might look like so: .. code-block:: python :linenos: from webob import Response def hello_view(request): return Response('Hello!') .. note: the ``context`` attribute of the ``request`` object passed to the above view will be an instance of the :class:`repoze.bfg.urldispatch.DefaultRoutesContext` class. This is the type of object created for a context when there is no "factory" specified in the ``route`` declaration. It is a mapping object, a lot like a dictionary. When using :term:`url dispatch` exclusively in an application (as opposed to using both url dispatch and :term:`traversal`), the :term:`context` of the view isn't always terribly interesting, particularly if you never use a ``factory`` attribute on your route definitions. However, if you do use a ``factory`` attribute on your route definitions, you may be very interested in the :term:`context` of the view. :mod:`repoze.bfg` supports view callables defined with two arguments: ``context`` and ``request``. For example, the below view statement is completely equivalent to the above view statement: .. code-block:: python :linenos: from webob import Response def hello_view(context, request): return Response('Hello!') The ``context`` passed to this view will be an instance returned by the default root factory or an instance returned by the ``factory`` argument to your route definition. Even if you use the request-only argument format in view callables, you can still get to the ``context`` of the view (if necessary) by accessing ``request.context``. See :ref:`request_and_context_view_definitions` for more information. Example 2 ~~~~~~~~~ Below is an example of some more complicated route statements you might add to your ``configure.zcml``: .. code-block:: xml :linenos: The above configuration will allow :mod:`repoze.bfg` to service URLs in these forms: .. code-block:: text /ideas/ /users/ /tags/ When a URL matches the pattern ``/ideas/``, the view registered with the name ``idea`` will be called. This will be the view available at the dotted Python pathname ``mypackage.views.idea_view``. Example 3 ~~~~~~~~~ The context object passed to a view found as the result of URL dispatch will by default be an instance of the object returned by the default :term:`root factory`. You can override this behavior by passing in a ``factory`` argument to the ZCML directive for a particular route. The ``factory`` should be a callable that accepts a :term:`request` and returns an instance of a class that will be the context used by the view. An example of using a route with a factory: .. code-block:: xml :linenos: The above route will manufacture an ``Idea`` model as a :term:`context`, assuming that ``mypackage.models.Idea`` resolves to a class that accepts a request in its ``__init__``. .. note:: Values prefixed with a period (``.``) for the ``factory`` and ``view`` attributes of a ``route`` (such as ``.models.Idea`` and ``.views.idea_view``) above) mean "relative to the Python package directory in which this :term:`ZCML` file is stored". So if the above ``route`` declaration was made inside a ``configure.zcml`` file that lived in the ``hello`` package, you could replace the relative ``.models.Idea`` with the absolute ``hello.models.Idea`` Either the relative or absolute form is functionally equivalent. It's often useful to use the relative form, in case your package's name changes. It's also shorter to type. If no route matches in the above configuration, :mod:`repoze.bfg` will call the "fallback" :term:`root factory` callable provided to the :term:`Configurator` constructor. If the "fallback" root factory is None, a :exc:`NotFound` error will be raised when no route matches. .. note:: See :ref:`using_model_interfaces` for more information about how views are found when interfaces are attached to a context. You can also map classes to views; interfaces are not used then. Example 4 ~~~~~~~~~ An example of configuring a ``view`` declaration in ``configure.zcml`` that maps a context found via URL dispatch to a view function is as follows: .. code-block:: xml :linenos: The ``.models`` module referred to above might look like so: .. code-block:: python :linenos: class Article(object): def __init__(self, request): self.__dict__.update(request.matchdict) def is_root(self): return self.article == 'root' The ``.views`` module referred to above might look like so: .. code-block:: python :linenos: from webob import Response def article_view(context, request): if context.is_root(): return Response('Root article') else: return Response('Article with name %s' % context.article) The effect of this configuration: when this :mod:`repoze.bfg` application runs, if any URL matches the pattern ``archives/:article``, the ``.views.articles_view`` view will be called with its :term:`context` as a instance of the ``Article`` class. The ``Article`` instance will have keys and values matching the keys and values in the routing dictionary associated with the request. In this case in particular, when a user visits ``/archives/something``, the context will be an instance of the Article class and it will have an ``article`` attribute with the value of ``something``. .. index:: pair: URL dispatch; catching root URL Catching the Root URL --------------------- It's not entirely obvious how to use a route to catch the root URL ("/"). To do so, give the empty string as a path in a ZCML ``route`` declaration: .. code-block:: xml :linenos: Or provide the literal string ``/`` as the path: .. code-block:: xml :linenos: .. index:: pair: URL dispatch; generating route URLs Generating Route URLs --------------------- Use the :func:`repoze.bfg.url.route_url` function to generate URLs based on route paths. For example, if you've configured a route in ZCML with the ``name`` "foo" and the ``path`` ":a/:b/:c", you might do this. .. ignore-next-block .. code-block:: python :linenos: from repoze.bfg.url import route_url url = route_url('foo', request, a='1', b='2', c='3') This would return something like the string ``http://example.com/1/2/3`` (at least if the current protocol and hostname implied ``http:/example.com``). See the :func:`repoze.bfg.url.route_url` API documentation for more information. .. index:: pair: URL dispatch; slash-redirecting Redirecting to Slash-Appended Routes ------------------------------------ For behavior like Django's ``APPEND_SLASH=True``, use the :func:`repoze.bfg.view.append_slash_notfound_view` view as the :term:`Not Found view` in your application. When this view is the Not Found view (indicating that no view was found), and any routes have been defined in the configuration of your application, if the value of ``PATH_INFO`` does not already end in a slash, and if the value of ``PATH_INFO`` *plus* a slash matches any route's path, do an HTTP redirect to the slash-appended ``PATH_INFO``. Let's use an example, because this behavior is a bit magical. If this your route configuration is looks like so, and the ``append_slash_notfound_view`` is configured in your application: .. code-block:: xml :linenos: If a request enters the application with the ``PATH_INFO`` value of ``/no_slash``, the first route will match. If a request enters the application with the ``PATH_INFO`` value of ``/no_slash/``, *no* route will match, and the slash-appending "not found" view will *not* find a matching route with an appended slash. However, if a request enters the application with the ``PATH_INFO`` value of ``/has_slash/``, the second route will match. If a request enters the application with the ``PATH_INFO`` value of ``/has_slash``, a route *will* be found by the slash appending notfound view. An HTTP redirect to ``/has_slash/`` will be returned to the user's browser. Note that this will *lose* ``POST`` data information (turning it into a GET), so you shouldn't rely on this to redirect POST requests. To configure the slash-appending not found view in your application, change the application's ``configure.zcml``, adding the following stanza: .. code-block:: xml :linenos: See :ref:`view_module` and :ref:`changing_the_notfound_view` for more information about the slash-appending not found view and for a more general description of how to configure a not found view. .. note:: This feature is new as of :mod:`repoze.bfg` 1.1. Cleaning Up After a Request --------------------------- Often it's required that some cleanup be performed at the end of a request when a database connection is involved. When :term:`traversal` is used, this cleanup is often done as a side effect of the traversal :term:`root factory`. Often the root factory will insert an object into the WSGI environment that performs some cleanup when its ``__del__`` method is called. When URL dispatch is used, however, no special root factory is required, so sometimes that option is not open to you. Instead of putting this cleanup logic in the root factory, however, you can cause a subscriber to be fired when a new request is detected; the subscriber can do this work. For example, let's say you have a ``mypackage`` :mod:`repoze.bfg` application package that uses SQLAlchemy, and you'd like the current SQLAlchemy database session to be removed after each request. Put the following in the ``mypackage.run`` module: .. ignore-next-block .. code-block:: python :linenos: from mypackage.sql import DBSession class Cleanup: def __init__(self, cleaner): self.cleaner = cleaner def __del__(self): self.cleaner() def handle_teardown(event): environ = event.request.environ environ['mypackage.sqlcleaner'] = Cleanup(DBSession.remove) Then in the ``configure.zcml`` of your package, inject the following: .. code-block:: xml This will cause the DBSession to be removed whenever the WSGI environment is destroyed (usually at the end of every request). .. index:: pair: URL dispatch; security .. _using_security_with_urldispatch: Using :mod:`repoze.bfg` Security With URL Dispatch -------------------------------------------------- :mod:`repoze.bfg` provides its own security framework which consults a :term:`authorization policy` before allowing any application code to be called. This framework operates in terms of ACLs (Access Control Lists, see :ref:`security_chapter` for more information about the :mod:`repoze.bfg` authorization subsystem). A common thing to want to do is to attach an ``__acl__`` to the context object dynamically for declarative security purposes. You can use the ``factory`` argument that points at a factory which attaches a custom ``__acl__`` to an object at its creation time. Such a ``factory`` might look like so: .. code-block:: python :linenos: class Article(object): def __init__(self, request): matchdict = request.matchdict article = matchdict.get('article', None) if article == '1': self.__acl__ = [ (Allow, 'editor', 'view') ] If the route ``archives/:article`` is matched, and the article number is ``1``, :mod:`repoze.bfg` will generate an ``Article`` :term:`context` with an ACL on it that allows the ``editor`` principal the ``view`` permission. Obviously you can do more generic things that inspect the routes match dict to see if the ``article`` argument matches a particular string; our sample ``Article`` factory class is not very ambitious. .. note:: See :ref:`security_chapter` for more information about :mod:`repoze.bfg` security and ACLs.