def shortest_path(self, start: str, goal: str) -> List[str]:
        """
        Breadth-first search of the shortest checkpoints from start to goal.

        Args:
            start: starting node
            goal: goal node

        Returns:
            The shortest checkpoints from start to goal.
        """
        assert isinstance(goal, str)
        start_road_node = start[0]
        assert start != goal
        return next(self.bfs_paths(start_road_node, goal), [])

        Breadth-first search of the shortest checkpoints from start to goal.

        Args:
            start: starting node
            goal: goal node

        Returns:
            The shortest checkpoints from start to goal.
        
metadrive/component/road_network/node_road_network.py