何も返されないのはなぜですか？

Question

私はプログラムを実行してもTrueまたはFalseの値を返しません。それはなぜ起こるのですか？

def esvocal(letter): 
vocal = "a","e","i","o","u" 
vocalup = "A","E","I","O","U" 

if letter == vocal and letter == vocalup: 
    return True 
else: 
    return False 
esvocal("s") 
esvocal("a") 

Answer 1

def esvocal(letter): 
    vocal =[ "a","e","i","o","u"] 
    vocalup = ["A","E","I","O","U"] 

    if letter in vocal and letter in vocalup: 
     return True 
    else: 
     return False 
esvocal("s") 
esvocal("a") 

1. Pythonは本当に代わりに==