Docserver: there is only one.

chrome/common/extensions/docs/server2/app_yaml_helper.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import logging
 
 _APP_YAML_CONTAINER = '''
 application: chrome-apps-doc
 version: %s
 runtime: python27
 api_version: 1
 threadsafe: false
 '''
 
 class AppYamlHelper(object):
   '''Parses the app.yaml file, and is able to step back in the host file
   system's revision history to find when it changed to some given version.
   '''
   class Delegate(object):
     def GetHostFileSystemForRevision(self, revision):
       '''Revision may not be None.
       '''
-      raise NotImplementedError()
+      raise NotImplementedError(self.__class__)
 
   def __init__(self,
                app_yaml_path,
                file_system_at_head,
                delegate,
                object_store_creator):
     self._app_yaml_path = app_yaml_path
     self._file_system_at_head = file_system_at_head
     self._delegate = delegate
     self._store = object_store_creator.Create(
@@ skipping 88 matching lines @@
       # Back up a revision then find when app.yaml was last updated before then.
       if found == 0:
         logging.warning('All revisions are greater than %s' % app_version)
         return 0
       next_file_system = self._delegate.GetHostFileSystemForRevision(
           found - 1)
 
     if found is None:
       raise ValueError('All revisions are less than %s' % app_version)
     return found