code: def __parameter_default(self, final_subfield):
      
      if final_subfield.default:
        if isinstance(final_subfield, messages.EnumField):
          return final_subfield.default.name
        else:
          return final_subfield.default
docs: Returns default value of final subfield if it has one.
  
      If this subfield comes from a field list returned from __field_to_subfields,
      none of the fields in the subfield list can have a default except the final
      one since they all must be message fields.
  
      Args:
        final_subfield: A simple field from the end of a subfield list.
  
      Returns:
        The default value of the subfield, if any exists, with the exception of an
            enum field, which will have its value cast to a string.