code: def lookup_descriptor(self, definition_name):
          
          try:
              return self.__descriptors[definition_name]
          except KeyError:
              pass
  
          if self.__descriptor_loader:
              definition = self.__descriptor_loader(definition_name)
              self.__descriptors[definition_name] = definition
              return definition
          else:
              raise messages.DefinitionNotFoundError(
                  'Could not find definition for %s' % definition_name)
docs: Lookup descriptor by name.
  
          Get descriptor from library by name.  If descriptor is not found will
          attempt to find via descriptor loader if provided.
  
          Args:
            definition_name: Definition name to find.
  
          Returns:
            Descriptor that describes definition name.
  
          Raises:
            DefinitionNotFoundError if not descriptor exists for definition name.