Docserver: there is only one.

chrome/common/extensions/docs/server2/empty_dir_file_system.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.
 
 from file_system import FileNotFoundError, FileSystem, StatInfo
 from future import Future
 
 class EmptyDirFileSystem(FileSystem):
   '''A FileSystem with empty directories. Useful to inject places to disable
   features such as samples.
   '''
   def Read(self, paths, binary=False):
     result = {}
     for path in paths:
       if not path.endswith('/'):
         raise FileNotFoundError('EmptyDirFileSystem cannot read %s' % path)
       result[path] = []
     return Future(value=result)
 
   def Stat(self, path):
     if not path.endswith('/'):
       raise FileNotFoundError('EmptyDirFileSystem cannot stat %s' % path)
     return StatInfo(0, child_versions=[])
+
+  def GetIdentity(self):
+    return self.__class__.__name__