.. _traversal_chapter: Traversal ========= When :term:`traversal` is used within a :mod:`repoze.bfg` application, the :mod:`repoze.bfg` *Router* parses the URL associated with the request. It splits the URL into individual path segments. Based on these path segments, :mod:`repoze.bfg` traverses a *model graph* in order to find a :term:`context`. It then attempts to find a :term:`view` based on the *type* of the context (specified by its Python class type or any :term:`interface` attached to it). If :mod:`repoze.bfg` finds a :term:`view` for the context, it calls it and returns a response to the user. The Model Graph --------------- When your application uses :term:`traversal` to resolve URLs to code, your application must supply a *model graph* to :mod:`repoze.bfg`. Users interact with your :mod:`repoze.bfg` -based application via a *router*, which is just a fancy :term:`WSGI` application. At system startup time, the router is configured with a callback known as a :term:`root factory`, supplied by the application developer. The root factory is passed a :term:`request` object and it is expected to return an object which represents the root of the model graph. All :term:`traversal` will begin at this root object. The root object is usually a *mapping* object (such as a Python dictionary). .. note:: If a :term:`root factory` is passed to the :mod:`repoze.bfg` "make_app" function as the value ``None``, a default root factory is used. This is most useful when you're using :term:`URL dispatch` and you don't care very much about traversing any particular graph to resolve URLs to code. It is also possible to use traversal and URL dispatch together. When both a root factory (and therefore traversal) *and* "routes" declarations (and therefore url dispatch) are used, the url dispatch routes are checked first, and if none match, :mod:`repoze.bfg` will fall back to using traversal to attempt to map the request to a view. If the name ``*traverse`` is in a route's ``path`` pattern, when it is matched, it is also possible to do traversal *after* a route has been matched. See :ref:`hybrid_chapter` for more information. .. warning:: In BFG 1.0 and prior versions, the root factory was passed a term WSGI *environment* object (a dictionary) while in BFG 1.1+ it is passed a request object. For backwards compatibility purposes, the request object passed to the root factory has a dictionary-like interface that emulates the WSGI environment, so code expecting the argument to be a dictionary will continue to work. Items contained within the object graph are analogous to the concept of :term:`model` objects used by many other frameworks (and :mod:`repoze.bfg` refers to them as models, as well). They are typically instances of Python classes. The model graph consists of *container* nodes and *leaf* nodes. There is only one difference between a *container* node and a *leaf* node: *container* nodes possess a ``__getitem__`` method while *leaf* nodes do not. The ``__getitem__`` method was chosen as the signifying difference between the two types of nodes because the presence of this method is how Python itself typically determines whether an object is "containerish" or not. A container node is presumed to be willing to return a child node or raise a ``KeyError`` based on a name passed to its ``__getitem__``. No leaf-level instance is required to have a ``__getitem__``. If leaf-level instances happen to have a ``__getitem__`` (through some historical inequity), you should subclass these node types and cause their ``__getitem__`` methods to simply raise a ``KeyError``. Or just disuse them and think up another strategy. Usually, the traversal root is a *container* node, and as such it contains other nodes. However, it doesn't *need* to be a container. Your model graph can be as shallow or as deep as you require. Traversal "stops" when :mod:`repoze.bfg` either reaches a leaf level model instance in your object graph or when the path segments implied by the URL "run out". The object that traversal "stops on" becomes the :term:`context`. .. _how_bfg_traverses: How :mod:`repoze.bfg` Processes a Request Using Traversal --------------------------------------------------------- .. image:: modelgraphtraverser.png When a user requests a page from your :mod:`repoze.bfg` -powered application, the system uses this algorithm to determine which Python code to execute: #. The request for the page is presented to the :mod:`repoze.bfg` :term:`router` in terms of a standard :term:`WSGI` request, which is represented by a WSGI environment and a ``start_response`` callable. #. The router creates a :term:`WebOb` request object based on the WSGI environment. #. The :term:`root factory` is called with the :term:`request`. It returns a :term:`root` object. #. The router uses the request's ``PATH_INFO`` information to determine the path segments to traverse. The leading slash is stripped off ``PATH_INFO``, and the remaining path segments are split on the slash character to form a traversal sequence, so a request with a ``PATH_INFO`` variable of ``/a/b/c`` maps to the traversal sequence ``[u'a', u'b', u'c']``. Note that each of the path segments in the sequence is converted to Unicode using the UTF-8 decoding (if the decoding fails, a ``TypeError`` is raised). #. :term:`Traversal` begins at the root object returned by the root factory. For the traversal sequence ``[u'a', u'b', u'c']``, the root object's ``__getitem__`` is called with the name ``a``. Traversal continues through the sequence. In our example, if the root object's ``__getitem__`` called with the name ``a`` returns an object (aka "object ``a``"), that object's ``__getitem__`` is called with the name ``b``. If object A returns an object when asked for ``b``, "object ``b``"'s ``__getitem__`` is then asked for the name ``c``, and may return "object ``c``". #. Traversal ends when a) the entire path is exhausted or b) when any graph element raises a ``KeyError`` from its ``__getitem__`` or c) when any non-final path element traversal does not have a ``__getitem__`` method (resulting in a ``NameError``) or d) when any path element is prefixed with the set of characters ``@@`` (indicating that the characters following the ``@@`` token should be treated as a "view name"). #. When traversal ends for any of the reasons in the previous step, the the last object found during traversal is deemed to be the :term:`context`. If the path has been exhausted when traversal ends, the "view name" is deemed to be the empty string (``''``). However, if the path was *not* exhausted before traversal terminated, the first remaining path element is treated as the view name. Any subsequent path elements after the view name are deemed the :term:`subpath`. The subpath is always a sequence of path segments that come from ``PATH_INFO`` that are "left over" after traversal has completed. For instance, if ``PATH_INFO`` was ``/a/b`` and the root returned an "object ``a``", and "object ``a``" subsequently returned an "object ``b``", the router deems that the context is "object ``b``", the view name is the empty string, and the subpath is the empty sequence. On the other hand, if ``PATH_INFO`` was ``/a/b/c`` and "object ``a``" was found but raised a ``KeyError`` for the name ``b``, the router deems that the context is "object ``a``", the view name is ``b`` and the subpath is ``('c',)``. #. If a :term:`authentication policy` is configured, the router performs a permission lookup. If a permission declaration is found for the view name and context implied by the current request, an :term:`authorization policy` is consulted to see if the "current user" (all determined by the the authentication policy) can perform the action. If he can, processing continues. If he cannot, the ``forbidden`` view is called (see :ref:`changing_the_forbidden_view`). #. Armed with the context, the view name, and the subpath, the router performs a view lookup. It attempts to look up a view from the :mod:`repoze.bfg` :term:`application registry` using the view name and the context. If a view function is found, it is called with the context and the request. It returns a response, which is fed back upstream. If a view is not found, the ``notfound`` view is called (see :ref:`changing_the_notfound_view`). In either case, the result is returned upstream via the :term:`WSGI` protocol. .. _debug_notfound_section: ``NotFound`` Errors ------------------- It's useful to be able to debug ``NotFound`` errors when they occur unexpectedly due to an application registry misconfiguration. To debug these errors, use the ``BFG_DEBUG_NOTFOUND`` environment variable or the ``debug_notfound`` configuration file setting. Details of why a view was not found will be printed to ``stderr``, and the browser representation of the error will include the same information. See :ref:`environment_chapter` for more information about how and where to set these values. A Traversal Example ------------------- Let's pretend the user asks for ``http://example.com/foo/bar/baz/biz/buz.txt``. Let's pretend that the request's ``PATH_INFO`` in that case is ``/foo/bar/baz/biz/buz.txt``. Let's further pretend that when this request comes in that we're traversing the following graph:: /-- | |-- foo | ----bar Here's what happens: - :mod:`repoze.bfg` traverses the root, and attempts to find foo, which it finds. - :mod:`repoze.bfg` traverses foo, and attempts to find bar, which it finds. - :mod:`repoze.bfg` traverses bar, and attempts to find baz, which it does not find ('bar' raises a ``KeyError`` when asked for baz). The fact that it does not find "baz" at this point does not signify an error condition. It signifies that: - the :term:`context` is bar (the context is the last item found during traversal). - the :term:`view name` is ``baz`` - the :term:`subpath` is ``('biz', 'buz.txt')`` Because it's the "context", :mod:`repoze.bfg` examines "bar" to find out what "type" it is. Let's say it finds that the context is an ``IBar`` type (because "bar" happens to have an attribute attached to it that indicates it's an ``IBar``). Using the "view name" ("baz") and the type, it asks the :term:`application registry` (configured separately, via ``configure.zcml``) this question: - Please find me a :term:`view` with the name "baz" that can be used for the type ``IBar``. Let's say it finds no matching view type. It then returns the result of the ``notfound`` view. The request ends. Everyone is sad. But! For this graph:: /-- | |-- foo | ----bar | ----baz | biz The user asks for ``http://example.com/foo/bar/baz/biz/buz.txt`` - :mod:`repoze.bfg` traverses foo, and attempts to find bar, which it finds. - :mod:`repoze.bfg` traverses bar, and attempts to find baz, which it finds. - :mod:`repoze.bfg` traverses baz, and attempts to find biz, which it finds. - :mod:`repoze.bfg` traverses biz, and attempts to find "buz.txt" which it does not find. The fact that it does not find "buz.txt" at this point does not signify an error condition. It signifies that: - the :term:`context` is biz (the context is the last item found during traversal). - the :term:`view name` is "buz.txt" - the :term:`subpath` is an empty sequence ( ``()`` ). Because it's the "context", :mod:`repoze.bfg` examines "biz" to find out what "type" it is. Let's say it finds that the context an ``IBiz`` type (because "biz" happens to have an attribute attached to it that happens indicates it's an ``IBiz``). Using the "view name" ("buz.txt") and the type, it asks the :term:`application registry` this question: - Please find me a :term:`view` with the name "buz.txt" that can be used for type ``IBiz``. Let's say that question is answered "here you go, here's a bit of code that is willing to deal with that case", and returns a :term:`view`. It is passed the "biz" object as the "context" and the current :term:`WebOb` :term:`request` as the "request". It returns a :term:`response`. There are two special cases: - During traversal you will often end up with a :term:`view name` that is the empty string. This indicates that :mod:`repoze.bfg` should look up the *default view*. The default view is a view that is registered with no name or a view which is registered with a name that equals the empty string. - If any path segment element begins with the special characters ``@@`` (think of them as goggles), that segment is considered the "view name" immediately and traversal stops there. This allows you to address views that may have the same names as model instance names in the graph unambiguously. Traversal-Related Side Effects ------------------------------ The :term:`subpath` will always be available to a view as a the ``subpath`` attribute of the :term:`request` object. It will be a sequence containing zero or more elements (which will be Unicode objects). The :term:`view name` will always be available to a view as the ``view_name`` attribute of the :term:`request` object. It will be a single string (possibly the empty string if we're rendering a default view). The :term:`root` will always be available to a view as the ``root`` attribute of the :term:`request` object. It will be the model object at which traversal started (the root). The :term:`context` will always be available to a view as the ``context`` attribute of the :term:`request` object. It will be the context object implied by the current request. The "traversal path" will always be available to a view as the ``traversed`` attribute of the :term:`request` object. It will be a sequence representing the ordered set of names that were used to traverse to the :term:`context`, not including the view name or subpath. If there is a virtual root associated with request, the virtual root path is included within the traversal path. The :term:`virtual root` will always be available to a view as the ``virtual_root`` attribute of the :term:`request` object. It will be the virtual root object implied by the current request. See :ref:`vhosting_chapter` for more information about virtual roots. The :term:`virtual root` *path* will always be available to a view as the ``virtual_root_path`` attribute of the :term:`request` object. It will be a sequence representing the ordered set of names that were used to traverse to the virtual root object. See :ref:`vhosting_chapter` for more information about virtual roots. Unicode and Traversal --------------------- The traversal machinery by default attempts to first URL-unquote and then Unicode-decode each path element in ``PATH_INFO`` from its natural byte string (``str`` type) representation. URL unquoting is performed using the Python standard library ``urllib.unquote`` function. Conversion from a URL-decoded string into Unicode is attempted using the UTF-8 encoding. If any URL-unquoted path segment in ``PATH_INFO`` is not decodeable using the UTF-8 decoding, a TypeError is raised. A segment will be fully URL-unquoted and UTF8-decoded before it is passed it to the ``__getitem__`` of any model object during traversal.