.. _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 ZCML are a sign that you're using URL dispatch. Using ```` 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 maniplates "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. When any ``route`` ZCML directive is present in an application's ``configure.zcml``, "under the hood" :mod:`repoze.bfg` wraps the :term:`root factory` in a special ``RoutesRootFactory`` instance. The wrapper instance then acts as the effective root factory. When it acts as a root factory, it is willing to check the requested URL against a *routes map* to find a :term:`context` and a :term:`view` before traversal has a chance to find it first. If a route matches, a :term:`context` is generated and :mod:`repoze.bfg` will call the :term:`view` specified with 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 application in it's ``make_app`` function (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 ``make_app`` function, :mod:`repoze.bfg` will return a 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. .. _route_zcml_directive: The ``route`` ZCML Directive ---------------------------- The ``route`` ZCML directive has these possible attributes. All attributes are optional unless the description names them as required. path The `route path `_, e.g. ``ideas/:idea``. This attribute is required. name The `route name `_, e.g. ``myroute``. This attribute is required. view The Python dotted-path name to a function that will be used as a view callable when this route matches. e.g. ``mypackage.views.my_view``. view_for The Python dotted-path name to a class or an interface that the :term:`context` of the view should match for the view named by the route to be used. This attribute is only useful if the ``view`` attribute is used. If this attribute is not specified, the default (``None``) will be used. permission The permission name required to invoke the view. e.g. ``edit``. (see :ref:`using_security_with_urldispatch` for more information about permissions). factory The Python dotted-path name to a function that will generate a :mod:`repoze.bfg` context object when this route matches. e.g. ``mypackage.models.MyFactoryClass``. If this argument is not specified, a default root factory will be used. request_type A string representing an HTTP method name, e.g. ``GET`` or ``POST`` or an interface representing a :term:`request type`. The Matchdict ------------- The main purpose of a route is to match (nor not match) the ``PATH_INFO`` 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. 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:: :foo/bar/baz and:: /: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"):: foo/:baz/:bar The above pattern will match these URLs, generating the followng matchdicts:: foo/1/2 -> {'baz':u'1', 'bar':u'2'} foo/abc/def -> {'baz':u'abc', 'bar':u'2'} It will not match the following patterns however:: 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:: foo/:bar When matching the following URL:: foo/La%20Pe%C3%B1a The matchdict will look like so (the value is URL-decoded / UTF-8 decoded):: {'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:: foo/:baz/:bar*traverse The above pattern will match these URLs, generating the followng matchdicts:: foo/1/2/ -> {'baz':1, 'bar':2, 'traverse':()} foo/abc/def/a/b/c -> {'baz':abc, 'bar':2, '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:: foo/*traverse When matching the following path:: /foo/La%20Pe%C3%B1a/a/b/c Will generate the following matchdict:: {'traverse':(u'La Pe\xf1a', u'a', u'b', u'c')} Example 1 --------- The simplest route delcaration: .. 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(context, request): return Response('Hello!') In this case the context object passed to the view will be an instance of the ``repoze.bfg.urldispatch.DefaultRoutesContext``. This is the type of obejct created for a context when there is no "factory" specified in the ``route`` declaration. It is a mapping object, a lot like a dictionary. 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:: bash :linenos: /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 WSGI environment 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 context, assuming that ``mypackage.models.Idea`` resolves to a class that accepts a WSGI environment 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 it during ``make_app`. If the "fallback" root factory is None, a ``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, environ): self.__dict__.update(environ['repoze.bfg.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``. 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: Generating Route URLs --------------------- Use the :mod:`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. .. code-block:: python 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 :mod:`repoze.bfg.url.route_url` API documentation for more information. 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`` BFG 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: .. code-block:: python 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). .. _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, environ): matchdict = environ['bfg.routes.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.