def UpdateClass(self, class_name, gtfs_class):
    """Updates an entry in the list of known classes.

    Args:
        class_name: A string with the class name that is to be updated.
        gtfs_class: The new class
    Raises:
        NonexistentMapping if there is no class with the specified class_name.
    """
    if class_name not in self._class_mapping:
      raise problems.NonexistentMapping(class_name)
    self._class_mapping[class_name] = gtfs_class
Updates an entry in the list of known classes.

    Args:
        class_name: A string with the class name that is to be updated.
        gtfs_class: The new class
    Raises:
        NonexistentMapping if there is no class with the specified class_name.