'NoneType' object is not iterable error in Python

Question

我试图在路径列表中找到最短的列表，但我收到一个错误（'NoneType' object 在 Python 中是不可迭代的错误）。 此 function 用于“图形模式”或“搜索算法”，用于寻找到达目标的最短路径。 请让我知道我在这里做错了什么。 谢谢！

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已从您的邻接列表中丢失。 修改邻接列表并从 function 返回paths后，您将能够得到正确的结果

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',[])

Output：

paths 

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

为了找到最短路径之一，您可以使用

min(paths, key=len)