code: def GetGtfsClassByFileName(self, filename):
      
      if filename not in self._file_mapping:
        return None
      mapping = self._file_mapping[filename]
      class_list = mapping['classes']
      if len(class_list) > 1:
        raise problems.NonStandardMapping(filename)
      else:
        return self._class_mapping[class_list[0]]
docs: Returns the transitfeed class corresponding to a GTFS file.
  
      Args:
        filename: The filename whose class is to be returned
  
      Raises:
        NonStandardMapping if the specified filename has more than one
            corresponding class