app: Add BaseApplication class

The `Application` class is now a sub-class of `BaseApplication`.
Applications that require more control of the framework's behavior can
extend `BaseApplication` instead of `Application`. The `BaseApplication`
class also provides some new features:

* `BaseApplication.setup_routes` is always called when an instance is
  created, and can be used to configure the request dispatcher
* `BaseApplication.update_config` updates the application configuration
  from a configuration file

The major difference between `BaseApplication` and `Application` is that
the latter requires a request dispatcher, while the former does not.
This pattern allows simple applications to construct a `Router` or root
object and then initialize the `Application` without needing to define a
sub-class.

src/milla/app.py

@@ -21,40 +21,33 @@ Please give me a docstring!
 :Updater: $Author$
 '''
 
+from milla import util
 from milla.controllers import FaviconController
-from milla.util import asbool
 from webob.exc import HTTPNotFound, WSGIHTTPException, HTTPMethodNotAllowed
 import milla.dispatch.traversal
+import os
 import sys
 
 
-__all__ = ['Application']
+__all__ = [
+    'Application',
+    'BaseApplication',
+]
 
 
-class Application(object):
-    '''Represents a Milla web application
+class BaseApplication(object):
+    '''Base class for Milla applications
 
-    Constructing an ``Application`` instance needs a dispatcher, or
-    alternatively, a root object that will be passed to a new
-    :py:class:`milla.dispatch.traversal.Traverser`.
-
-    :param obj: An object implementing the dispatcher protocol, or an
-       object to be used as the root for a Traverser
-
-    ``Application`` instances are WSGI applications.
-
-    .. py:attribute:: config
-
-       A mapping of configuration settings. For each request, the
-       configuration is copied and assigned to ``request.config``.
+    This class can be used by applications that need to customize the
+    behavior of the framework. In most cases, :py:class:`Application`
+    instances can be created directly and a sublcass is not necessary.
     '''
 
-    def __init__(self, obj):
-        if not hasattr(obj, 'resolve'):
-            # Object is not a dispatcher, but the root object for traversal
-            obj = milla.dispatch.traversal.Traverser(obj)
-        self.dispatcher = obj
-        self.config = {'milla.favicon': True}
+    DEFAULT_CONFIG = {}
+
+    def __init__(self):
+        self.config = self.DEFAULT_CONFIG.copy()
+        self.setup_routes()
 
     def __call__(self, environ, start_response):
         start_response = StartResponseWrapper(start_response)
@@ -127,6 +120,32 @@ class Application(object):
                 return self._find_attr(obj.func, attr)
             raise
 
+    def update_config(self, filename):
+        '''Update application configuration from a file
+
+        :param filename: Path to configuration file
+
+        This method will update the application configuration using
+        the values found in the specified configuration file. If the
+        specified file does not exist or is not accessible, no
+        changes will be made.
+
+        The configuration file will be read using
+        :py:func:`milla.util.read_config`.
+        '''
+
+        if filename and os.access(filename, os.R_OK):
+            self.config.update(util.read_config(filename))
+
+    def setup_routes(self):
+        '''Initialize the request dispatcher
+
+        The default implementation of this method does nothing, but it
+        is called when an instance of :py:class:`BaseApplication` is
+        created, and can be used by subclasses to set up the request
+        dispatcher.
+        '''
+
     def make_request(self, environ):
         '''Create a :py:class:`~milla.Request` from a WSGI environment
 
@@ -162,7 +181,7 @@ class Application(object):
             return self.dispatcher.resolve(path_info)
         except milla.dispatch.UnresolvedPath:
             if (path_info == '/favicon.ico' and
-                    asbool(self.config.get('milla.favicon'))):
+                    util.asbool(self.config.get('milla.favicon'))):
                 return FaviconController()
             else:
                 return path_not_found
@@ -182,6 +201,36 @@ class Application(object):
         raise
 
 
+class Application(BaseApplication):
+    '''Represents a Milla web application
+
+    Constructing an ``Application`` instance needs a dispatcher, or
+    alternatively, a root object that will be passed to a new
+    :py:class:`milla.dispatch.traversal.Traverser`.
+
+    :param obj: An object implementing the dispatcher protocol, or an
+       object to be used as the root for a Traverser
+
+    ``Application`` instances are WSGI applications.
+
+    .. py:attribute:: config
+
+       A mapping of configuration settings. For each request, the
+       configuration is copied and assigned to ``request.config``.
+    '''
+
+    DEFAULT_CONFIG = {
+        'milla.favicon': True,
+    }
+
+    def __init__(self, obj):
+        super(Application, self).__init__()
+        if not hasattr(obj, 'resolve'):
+            # Object is not a dispatcher, but the root object for traversal
+            obj = milla.dispatch.traversal.Traverser(obj)
+        self.dispatcher = obj
+
+
 class StartResponseWrapper():
 
     def __init__(self, start_response):