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

def GetGtfsClassByFileName(self, filename):
    """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
    """
    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]]