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 repoze.bfg
 ==========
 
-``repoze.bfg`` is a system for routing to applications based on graph
-traversal and adapter lookups (ala Zope).
+``repoze.bfg`` is a system for routing web requests to applications
+based on graph traversal.  It is inspired by Zope's publisher, and
+uses Zope libraries to do much of its work.  However, it is less
+ambitious and less featureful than any released version of Zope's
+publisher.
 
+``repoze.bfg`` uses the WSGI protocol to handle requests and
+responses, and integrates Zope, Paste, and WebOb libraries to form the
+basis for a simple web object publishing framework.
+
+Graph Traversal
+---------------
+
+In many popular web frameworks, a "URL dispatcher" is used to
+associate a particular URL with a bit of code (known somewhat
+ambiguously as a "controller" or "view" depending upon the particular
+vocabulary religion to which you subscribe).  These systems allow the
+developer to create "urlconfs" or "routes" to controller/view Python
+code using pattern matching against URL components.  Examples:
+`Django's URL dispatcher
+<http://www.djangoproject.com/documentation/url_dispatch/>`_ and the
+`Routes URL mapping system <http://routes.groovie.org/>`_ .
+
+It is however possible to map URLs to code slightly differently, using
+object graph traversal. The venerable Zope and CherryPy web frameworks
+offer traversal-based URL dispatch.  ``repoze.bfg`` also provides
+graph-traversal-based dispatch of URLs to code.  Graph-traversal based
+dispatching is useful if you like the URL to be representative of an
+arbitrary hierarchy of potentially heterogeneous items.
+
+Non-graph traversal based URL dispatch can easily handle URLs such as
+``http://example.com/members/Chris``, where it's assumed that each
+item "below" ``members`` in the URL represents a member in the system.
+You just match everything "below" ``members`` to a particular view.
+They are not very good, however, at inferring the difference between
+sets of URLs such as ``http://example.com/members/Chris/document`` vs.
+``http://example.com/members/Chris/stuff/page`` wherein you'd like the
+``document`` in the first URL to represent, e.g. a PDF document, and
+``/stuff/page`` in the second to represent, e.g. an OpenOffice
+document in a "stuff" folder.  It takes more pattern matching
+assertions to be able to make URLs like these work in URL-dispatch
+based systems.  Over time, the more assertions you make, the more
+fragile the URL dispatch lookup logic becomes.  URL-dispatch based
+systems don't deal very well with URLs that represent arbitrary-depth
+hierarchies.
+
+Graph traversal works well if you need to divine meaning out of these
+types of "ambiguous" URLs and URLs that represent arbitrary-depth
+hierarchies.  Each URL segment represents a single traversal through
+an edge of the graph.  So a URL like ``http://example.com/a/b/c`` can
+be thought of as a graph traversal on the example.com site through the
+edges "a", "b", and "c".
+
+Graph traversal is materially more complex than URL-based dispatch,
+however, if only because it requires the construction and maintenance
+of a graph, and it requires the developer to think about mapping URLs
+to code in terms of traversing the graph.  (How's *that* for
+self-referential! ;-) That said, for developers comfortable with Zope,
+in particular, and comfortable with hierarchical data stores like
+ZODB, mapping a URL to a graph traversal it's a natural way to think
+about creating a web application.  In essence, the choice to use graph
+traversal vs. URL dispatch is largely "religious" in some sense and
+often doesn't make sense for completely "square" data, but old habits
+die hard for folks used to graph-traversal-based lookup.
+``repoze.bfg`` is for those folks.
+
+How ``repoze.bfg`` is Configured
+--------------------------------
+
+Users interact with your ``repoze.bfg``-based application via a
+"router", which is itself a WSGI application.  At system startup time,
+the router must be configured with a root object from which all
+traversal will begin.  The root object is a mapping object, such as a
+Python dictionary.  In fact, all items contained in the graph are
+either leaf nodes (these have no __getitem__) or container nodes
+(these do have a __getitem__).
+
+Items contained within the graph are analogous to the concept of
+``model`` objects used by many other frameworks.  They are typically
+instances of classes.  Each containerish instance is willing to return
+a child or raise a KeyError based on a name passed to its __getitem__.
+No leaf-level instance is required to have a __getitem__.
+
+Similarities with Other Frameworks
+----------------------------------
+
+The Django docs state that Django is an "MTV" framework in their `FAQ
+<http://www.djangoproject.com/documentation/faq/>`_.  This also
+happens to be true for ``repoze.bfg``::
+
+  Django appears to be a MVC framework, but you call the Controller
+  the "view", and the View the "template". How come you don’t use the
+  standard names?
+
+  Well, the standard names are debatable.
+
+  In our interpretation of MVC, the "view" describes the data that
+  gets presented to the user. It’s not necessarily how the data looks,
+  but which data is presented. The view describes which data you see,
+  not how you see it. It’s a subtle distinction.
+
+  So, in our case, a "view" is the Python callback function for a
+  particular URL, because that callback function describes which data
+  is presented.
+
+  Furthermore, it’s sensible to separate content from presentation —
+  which is where templates come in. In Django, a "view" describes
+  which data is presented, but a view normally delegates to a
+  template, which describes how the data is presented.
+
+  Where does the "controller" fit in, then? In Django’s case, it’s
+  probably the framework itself: the machinery that sends a request to
+  the appropriate view, according to the Django URL configuration.
+
+  If you’re hungry for acronyms, you might say that Django is a "MTV"
+  framework — that is, "model", "template", and "view." That breakdown
+  makes much more sense.
+
+Jargon
+------
+
+The following jargon is used casually in descriptions of
+``repoze.bfg`` operations.
+
+mapply
+
+  code which dynamically ("magically") determines which arguments to
+  pass to a view based on environment and request parameters.
+
+request
+
+  A ``WebOb`` request object.
+
+response
+
+  An object that has three attributes: app_iter (representing an
+  iterable body), headerlist (representing the http headers sent
+  upstream), and status (representing the http status string).  This
+  is the interface defined for ``WebOb`` response objects.
+
+view
+
+  A callable that accepts arbitrary values (mapped into it by
+  "mapply") and which returns a response object.
+
+view constructor
+
+  A callable which returns a view object.  It should accept two
+  values: context and request.
+
+model
+
+  An object representing data in the system.  A model is part of the
+  object graph traversed by the system.  Models are traversed to
+  determine a context.
+
+context
+
+  A model in the system that is the subject of a view.
+
+view registry
+
+  A registry which maps a context and view name to a view constructor.
+
+template
+
+  A file that is capable of representing some text when rendered.
+
+interface
+
+  An attribute of a model object that determines its type.
+
+How ``repoze.bfg`` Processes a Request
+--------------------------------------
+
+When a user requests a page from your ``repoze.bfg`` -powered
+application, the system uses this algorithm to determine which Python
+code to execute:
+
+ 1.  The request for the page is presented to ``repoze.bfg``'s
+     "router" in terms of a standard WSGI request, which is
+     represented by a WSGI environment and a start_response callable.
+
+ 2.  The router creates a `WebOb <http://pythonpaste.org/webob/>`_
+     request object based on the WSGI environment.
+
+ 3.  The router uses the WSGI environment's ``PATH_INFO`` variable 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 ``['a', 'b', 'c']``.
+
+ 4.  Traversal begins at the root object.  For the traversal sequence
+     ``['a', 'b', '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.
+
+ 5.  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").
+
+ 6.  When traversal ends for any of the reasons in the previous step,
+     the the last object found during traversal is deemed to be the
+     "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 subseqent path elements after the view name are
+     deemed the "subpath".  For instance, if ``PATH_INFO`` was
+     ``/a/b`` and the root returned an "A" object, and the "A" object
+     returned a "B" object, 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']``.
+
+ 7.  Armed with the context, the view name, and the subpath, the
+     router performs a view lookup.  It attemtps to look up a view
+     constructor from the ``repoze.bfg`` view registry using the view
+     name and the context.  If a view constructor is found, it is
+     converted into a WSGI application: it is "wrapped in" ( aka
+     "adapted to") a WSGI application using mapply.  The WSGI adapter
+     uses mapply to map request and environment variables into the
+     view when it is called.  If a view constructor is not found, a
+     generic WSGI ``NotFound`` application is constructed. 
+
+In either case, the resulting WSGI application is called.  The WSGI
+application's return value is an iterable.  This is returned upstream
+to the WSGI server.  The WSGI application also calls start_response
+with a status code and a header list.
+
+A Sample Application
+--------------------
+
+A typical simple ``repoze.bfg`` application consists of four things:
+
+  1. A ``views.py`` module, which contains view code.
+
+  2. A ``models.py`` module, which contains model code.
+
+  3. A ``configure.zcml`` file which maps view names to model types.
+     This is also known as the "view registry", although it also
+     often contains non-view-related declarations.
+
+  4. A "templates" directory, which is full of zc3.pt templates.
+
+An application must be a Python package (meaning it must have an
+__init__.py and it must be findable on the PYTHONPATH).
+
+views.py
+~~~~~~~~
+
+A views.py module might look like so::
+
+  from webob import Response
+  from repoze.bfg.template import TemplateView
+
+  class MyHelloView(object):
+      def __init__(self, context, request):
+          self.context = context
+          self.request = request
+
+      def __call__(self):
+          response = Response('Hello from %s @ %s' % (self.context.__name__, 
+                                                      self.request['PATH_INFO'])
+          return response
+
+  class MyTemplateView(TemplateView):
+       def getInfo(self):
+           return {'name':self.context.__name__}
+
+models.py
+~~~~~~~~~
+
+A models.py might look like so::
+
+  from UserDict import UserDict
+
+  from zope.interface import implements
+  from zope.interface import Attribute
+  from zope.interface import Interface
+
+  class IMyModel:
+      __name__ = Attribute('Name of the model instance')
+
+  class MyModel(UserDict):
+      implements(IMyModel)
+      def __init__(self, name):
+          self.__name__ = name
+          UserDict.__init__(self, {})
+
+  # model instance info would typically be stored in a database of some
+  # kind; here we put it at module scope for demonstration purposes.
+
+  root = Model('root')
+  root['a'] = Model('a')
+  root['b'] = Model('b')
+
+  def get_root(environ):
+      return root
+    
+configure.zcml
+~~~~~~~~~~~~~~
+
+A view registry might look like so::
+
+  <configure xmlns="http://namespaces.zope.org/zope"
+      xmlns:browser="http://namespaces.repoze.org/browser"
+  	           i18n_domain="repoze.bfg">
+
+    <!-- this must be included for the view declarations to work -->
+    <include package="repoze.bfg" />
+
+    <!-- the default view for a MyModel -->
+    <browser:page
+        for=".models.IMyModel"
+        class=".views.MyHelloView"
+        permission="repoze.view"
+        />
+
+    <!-- the templated view for a MyModel -->
+    <browser:page
+        for=".models.IMyModel"
+        class=".views.MyTemplateView"
+        template="templates/my.pt"
+        name="templated.html"
+        permission="repoze.view"
+        />
+
+  </configure>
+
+templates/my.pt
+~~~~~~~~~~~~~~~
+
+A template that is used by a view might look like so::
+
+  <html xmlns="http://www.w3.org/1999/xhtml"
+       xmlns:tal="http://xml.zope.org/namespaces/tal">
+  <head></head>
+  <body tal:define="info view.getInfo()">
+    <h1>My template viewing ${info.name}</h1>
+  </body>
+  </html>
+
+Running the Application
+-----------------------
+
+To run the application above, the simplest method is to run it
+directly from a starter script (although you might also use Paste to
+perform this task)::
+
+  from paste import httpserver
+
+  from repoze.bfg import make_app
+  from myapp.models import get_root
+  import myapp
+
+  app = make_app(myapp.get_root, myapp)
+  httpserver.serve(app, host='0.0.0.0', port='5432')
+  
+Viewing the Application
+-----------------------
+
+Visit http://localhost:5432/ in your browser.  You will see::
+
+  Hello from root @ /
+
+Visit http://localhost:5432/a in your browser.  You will see::
+
+  Hello from a @ /a
+
+Visit http://localhost:5432/b in your browser.  You will see::
+
+  Hello from b @ /b
+
+Visit http://localhost:5432/templated.html in your browser.  You will
+see::
+
+  My template viewing root
+
+
+Visit http://localhost:5432/a/templated.html in your browser.  You
+will see::
+
+  My template viewing a
+
+Visit http://localhost:5432/b/templated.html in your browser.  You
+will see::
+
+  My template viewing b