old_contents: from __future__ import absolute_import, print_function
  
  class Handler(object):
      """Subtypes of Handler describe a way to modify a Document."""
  
      def __init__(self, *args, **kwargs):
          self._failed = False
          self._error = None
          self._error_detail = None
  
      def url_path(self):
          """Returns a default URL path if the spelling specified one."""
          return None
  
      def static_path(self):
          """Returns a path to app-specific static resources, if applicaple."""
          return None
  
      def modify_document(self, doc):
          """Modifies the application document however the spelling specifies."""
          pass
  
      @property
      def failed(self):
          """True if the handler failed to modify the doc"""
          return self._failed
  
      @property
      def error(self):
          """Error message if the handler failed"""
          return self._error
  
      @property
      def error_detail(self):
          """Traceback or other details if the handler failed"""
          return self._error_detail
  
      def on_server_loaded(self, server_context):
          pass
  
      def on_server_unloaded(self, server_context):
          pass
  
      def on_session_created(self, session_context):
          pass
  
      def on_session_destroyed(self, session_context):
          pass
  
new_contents: from __future__ import absolute_import, print_function
  
  class Handler(object):
      """Subtypes of Handler describe a way to modify a Document."""
  
      def __init__(self, *args, **kwargs):
          self._failed = False
          self._error = None
          self._error_detail = None
  
      def url_path(self):
          """Returns a default URL path if the spelling specified one."""
          return None
  
      def static_path(self):
          """Returns a path to app-specific static resources, if applicable."""
          return None
  
      def modify_document(self, doc):
          """Modifies the application document however the spelling specifies."""
          pass
  
      @property
      def failed(self):
          """True if the handler failed to modify the doc"""
          return self._failed
  
      @property
      def error(self):
          """Error message if the handler failed"""
          return self._error
  
      @property
      def error_detail(self):
          """Traceback or other details if the handler failed"""
          return self._error_detail
  
      def on_server_loaded(self, server_context):
          pass
  
      def on_server_unloaded(self, server_context):
          pass
  
      def on_session_created(self, session_context):
          pass
  
      def on_session_destroyed(self, session_context):
          pass
  
subject: Update handler.py