函数中的递归导致返回变量等于“无”[重复]

Question

我正在用 Python 制作凯撒密码。我创建了一个函数来返回用户想要加密的消息。然后我想对密码密钥做同样的事情。我希望函数只返回介于 1 到 26 之间的键，因为这是字母表的大小。它有效，但是当我故意输入一个大于 26 的数字时，输入一个介于 1 和 26 之间的数字；它显然返回“无”。这是我第一次使用递归。

def getKey():
    print("Enter the key you want to use to decrypt/encrypt the message")
    key = int(input()) # key input
    while True:
        if key >= 1 and key <= 26: #checking if key is in between range
            return key #giving back the key
        else: # this should run whenever the input is invalid
            print ("The key must be inbetween 1 and 26")
            break # to stop the loop
    getKey() # Recursion, to restart the function

key = getKey()
print(key) # this prints 'None' 

钥匙怎么了？去哪儿了！？

Answer 1

你的代码可以改写为：

def getKey():
    print("Enter the key you want to use to decrypt/encrypt the message")
    key = int(input()) # key input
    while True:
        if key >= 1 and key <= 26: #checking if key is in between range
            return key #giving back the key
        else: # this should run whenever the input is invalid
            print ("The key must be inbetween 1 and 26")
            break # to stop the loop
    getKey() # Recursion, to restart the function THEN THROW AWAY RESULT
    return None # This is what falling off the end of a function does.

key = getKey()
print(key) # this prints 'None' 

解决办法是：

def getKey():
    print("Enter the key you want to use to decrypt/encrypt the message")
    key = int(input()) # key input
    if key >= 1 and key <= 26: #checking if key is in between range
        return key #giving back the key
    else: # this should run whenever the input is invalid
        print ("The key must be inbetween 1 and 26")
        return getKey() # Recursion, to restart the function

请注意，我还删除了循环，因为如果你在递归，你只会循环一次。更好的解决方案是保留循环而不使用递归：

def getKey():
    print("Enter the key you want to use to decrypt/encrypt the message")
    while True:
        key = int(input()) # key input
        if key >= 1 and key <= 26: #checking if key is in between range
            return key #giving back the key
        else: # this should run whenever the input is invalid
            print ("The key must be inbetween 1 and 26")
            # Don't stop the loop.  Just try again.