code: def GetStructFormatString(self):
      
      if self._format_string is None and self._data_type_maps:
        format_strings = []
        for member_data_type_map in self._data_type_maps:
          if member_data_type_map is None:
            return None
  
          member_format_string = member_data_type_map.GetStructFormatString()
          if member_format_string is None:
            return None
  
          format_strings.append(member_format_string)
  
        self._format_string = ''.join(format_strings)
  
      return self._format_string
docs: Retrieves the Python struct format string.
  
      Returns:
        str: format string as used by Python struct or None if format string
            cannot be determined.