'NoneType' object is not iterable error in Python

Question

I am trying to find the shortest list inside of the Paths list but I receive an error ('NoneType' object is not iterable error in Python). This function is for "graph pattern" or "searching algorithm" finding the shortest path to a goal. Please let me know what I am doing wrong here. Thanks!

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

The vertex G has been missed out from your adjacency list. After modifying your adjacency list and returning paths from the function, you would be able to get the correct result

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']]

For finding one of the shortest paths, you can use

min(paths, key=len)