'NoneType'对象在Python中不可迭代的错误。

Question

我试图在路径列表里面找到最短的列表，但是我收到一个错误('NoneType'对象在Python中不是可迭代的错误)。这个函数是用于 "图模式 "或 "搜索算法 "寻找通往目标的最短路径。 请让我知道我在这里做错了什么。 谢谢！我正试图找到最短的路径。

graph = {'A':['B','C'],'B':['F','D','I'],'C':['D','E','G'],'D':['H'],'F':['I'],'E':['G'],'I':['H'],'D':['H'],'H':['G']} 




def find_all_paths(graph, start, end, path=[]):

        path = path + [start]
        if start == end:
            return [path]
        if start not in graph:
            return []
        paths = []
        for node in graph[start]:
            if node not in path:
                newpaths = find_all_paths(graph, node, end, path)
                for newpath in newpaths:
                    paths.append(newpath)

        minList=min(paths, key= len)
        print(minList)

Answer 1

顶点 G 遗漏在您的邻接列表中。在修改您的邻接列表并返回 paths 的函数，你就能得到正确的结果。

graph = {'A':['B','C'],'B':['F','D','I'],'C':['D','E','G'],'D':['H'],'F':['I'],'E':['G'],'I':['H'],'D':['H'],'H':['G'], 'G':[]} 

def find_all_paths(graph, start, end, path=[]):
    path = path + [start]
    if start == end:
        return [path]
    paths = []
    for node in graph[start]:
        if node not in path:
            newpaths = find_all_paths(graph, node, end, path)
            if newpaths:
                for newpath in newpaths:
                    paths.append(newpath)
    return paths

graph

{'A': ['B', 'C'],
 'B': ['F', 'D', 'I'],
 'C': ['D', 'E', 'G'],
 'D': ['H'],
 'F': ['I'],
 'E': ['G'],
 'I': ['H'],
 'H': ['G'],
 'G': []}

paths = find_all_paths(graph,'A', 'H',[])

输出。

paths 

[['A', 'B', 'F', 'I', 'H'],
 ['A', 'B', 'D', 'H'],
 ['A', 'B', 'I', 'H'],
 ['A', 'C', 'D', 'H']]

为了找到一条最短路径，你可以使用

min(paths, key=len)