Recursive code returns None [duplicate]

问题:

This question already has an answer here:

• Why does my function return None? 4 answers

I really do not understand, why the code

def isIn(char, aStr): 
    ms = len(aStr)/2
    if aStr[ms] == char:
        print \'i am here now\'
        return True
    elif char>aStr[ms] and not ms == len(aStr)-1:
        aStr = aStr[ms+1:]
    elif char <aStr[ms] and not ms == 0:
        aStr = aStr[0:ms]
    else:
        return False
    isIn(char, aStr)

print isIn(\'a\', \'ab\')

does keep on returning None. it prints \'i am here now\', but it does not return True, just as the next line says. Why?

回答1:

You probably want a return on the last line:

return isIn(char, aStr)

Without it, the function simply returns None when it terminates without seeing a return.