code: def GetFormatStringAttributeNames(self):
      
      if self._format_string_attribute_names is None:
        self._format_string_attribute_names = []
        for format_string_piece in self.FORMAT_STRING_PIECES:
          attribute_names = self._FORMAT_STRING_ATTRIBUTE_NAME_RE.findall(
              format_string_piece)
  
          if attribute_names:
            self._format_string_attribute_names.extend(attribute_names)
  
      return set(self._format_string_attribute_names)
docs: Retrieves the attribute names in the format string.
  
      Returns:
        set(str): attribute names.