old_contents: 
  
  class Actions(object):
      def terminate_instance(self):
          pass
  
      def alert(self):
          pass
  
  
new_contents: 
  
  class Actions(object):
      def terminate_instance(self):
          raise NotImplementedError
  
      def alert(self):
          raise NotImplementedError
  
  
subject: raise NotImplementedError on unimplemnted actions instead of ignoring the call