Function returns None

问题

After making an error then doing recursion i get None.

def getplayerinput():
    a = ["rock","paper","scissors"]
    plin = raw_input("Choose %s/%s/%s: " %(a[0], a[1], a[2]))
    print plin,'-first print'
    if plin not in a:
        print "Wrong input"
        getplayerinput()
    else:
        print plin,'-second print'
        return plin

for i in range(0,11):
    print getplayerinput()

If you input first 'rock' then 'cat' then 'paper' you will get a 'None'.

回答1:

You do not return anything once the input is not valid. Do this instead:

def getplayerinput():
    a = ["rock","paper","scissors"]
    plin = raw_input("Choose %s/%s/%s: " %(a[0], a[1], a[2]))
    print plin,'-first print'
    if plin not in a:
        print "Wrong input"
        return getplayerinput() # <- added return
    else:
        print plin,'-second print'
        return plin

for i in range(0,11):
    print getplayerinput()

Otherwise the recursion call will return something to a top level call, but that one just swallows the return value from the recursion call, since it doesn't pass it on via return.

来源：https://stackoverflow.com/questions/32793924/function-returns-none