def __parameter_enum(self, param):
if isinstance(param, messages.EnumField):<EOL><INDENT>return [enum_entry[<NUM_LIT:0>] for enum_entry in sorted(<EOL>param.type.to_dict().items(), key=lambda v: v[<NUM_LIT:1>])]<EOL><DEDENT>
Returns enum descriptor of a parameter if it is an enum.

        An enum descriptor is a list of keys.

        Args:
          param: A simple field.

        Returns:
          The enum descriptor for the field, if it's an enum descriptor, else
              returns None.