python之return 语句返回 None 而不是 value

我遇到了一个奇怪的问题。此代码返回 None 而不是 True，即使它进入正确的分支并评估为 True:

edges = { (1, 'a') : [2, 3], 
          (2, 'a') : [2], 
          (3, 'b') : [4, 3], 
          (4, 'c') : [5] } 
accepting = [2, 5]  
loc = [] 
def nfsmsim(string, current, edges, accepting):  
 
    if string != "": 
        if ((current, string[0]) in edges.keys()): 
            global loc  
            loc = edges[(current, string[0])] 
            print "edge found:",loc 
 
    if (string == ""): 
        print "string is over",current,accepting 
        print type(current), type(accepting) 
        if current in accepting :  
            print "1" 
            return True 
        else:  
            print "2" 
            return 2 
    # fill in your code here  
    elif (current, string[0]) in edges.keys(): 
        global loc 
        string = string[1:] 
        nfsmsim(string, loc[0], edges, accepting) 
    elif len(loc)>1: 
        global loc 
        nfsmsim(string, loc[1], edges, accepting) 
 
 
# This problem includes some test cases to help you tell if you are on 
# the right track. You may want to make your own additional tests as well. 
print nfsmsim("abc", 1, edges, accepting) 

这个的输出是:

 string is over 5 [2, 5] 
<type 'int'> <type 'list'> 
1 
None (<< instead of True) 

请您参考如下方法:

这是一个递归函数。当您到达终端案例 (string == "") 时，您将返回 1 或 2。这会返回到调用函数——nfsmsim 的先前调用。但是 nfsmsim 的调用没有返回任何内容!您需要从 nfsmsim 的终端调用中获取值，并通过再次返回来传递它。

elif (current, string[0]) in edges.keys(): 
    global loc 
    string = string[1:] 
    nfsmsim(string, loc[0], edges, accepting) 
elif len(loc)>1: 
    global loc 
    nfsmsim(string, loc[1], edges, accepting)