python - AttributeError：“节点”对象没有属性“is_goal”UCS Python

Question

from ..problem import Problem
from ..datastructures.priority_queue import PriorityQueue
import queue

def get_solver_mapping():
    return dict(ucs=UCS)


    class UCS(object):
        # TODO, excercise 1:
        # - implement Uniform Cost Search (UCS), a variant of Dijkstra's Graph Search
        # - use the provided PriorityQueue where appropriate
        # - to put items into the PriorityQueue, use 'pq.put(<priority>, <item>)'
        # - to get items out of the PriorityQueue, use 'pq.get()'
        # - store visited nodes in a 'set()'
        def solve(self, problem: Problem):
            node = problem.get_start_node()
            visited = set()
            pq = queue.PriorityQueue()
            pq.put((0, node, [node]))
    
            while not pq.empty():
                f, current_node, path = pq.get()
                visited.add(current_node)
                if current_node.is_goal:
                    return path 
                else:
                    for edge in current_node.out_edges:
                        child = edge.to()
                    if child not in visited:
                        pq.put((current_node_priority + edge.weight, child, path + [child]))
            return None