为何if语句正常运行，else语句触发时出现NameError错误？

错误原因 #

• 当你在第一个输入中选择**非"left"**的选项时，程序执行第一个else分支，打印"Game over"，但此时swim_or_wait变量根本没被定义。
• 程序不会自动停止，会继续执行后面的if swim_or_wait.lower() == "wait"语句，找不到变量就抛出NameError。

修复方案 #

方案1：游戏结束后直接退出程序

在每个Game Over的分支里加入退出逻辑，避免程序继续执行后续未定义变量的代码：

import sys

left_or_right = input("Choose which way you want to go. Left or right? ")

if left_or_right.lower() == "left":
  swim_or_wait = input("Do you want to swim or wait? ")
else:
    print("Wrong way. You fell into a hole. Game over.")
    sys.exit()  # 退出程序，不再执行后续代码

if swim_or_wait.lower() == "wait":
  which_door = input("Choose which door you want to go through? Red, blue, or yellow? ")
else:
  print("You've been attacked by a trout. Game Over.")
  sys.exit()  # 退出程序

if which_door.lower() == "red":
  print("Burned by fire. Game Over.")
elif which_door.lower() == "blue":
  print("Eaten by beasts. Game Over.")
elif which_door.lower() == "yellow":
  print("Congratulations!!! The game is yours. You Win!")
else:
  print("Wrong door. Game Over.")

方案2：嵌套代码分支

把依赖前面变量的代码放到对应的if分支里，确保只有变量被定义后才会执行：

left_or_right = input("Choose which way you want to go. Left or right? ")

if left_or_right.lower() == "left":
  swim_or_wait = input("Do you want to swim or wait? ")
  if swim_or_wait.lower() == "wait":
    which_door = input("Choose which door you want to go through? Red, blue, or yellow? ")
    if which_door.lower() == "red":
      print("Burned by fire. Game Over.")
    elif which_door.lower() == "blue":
      print("Eaten by beasts. Game Over.")
    elif which_door.lower() == "yellow":
      print("Congratulations!!! The game is yours. You Win!")
    else:
      print("Wrong door. Game Over.")
  else:
    print("You've been attacked by a trout. Game Over.")
else:
    print("Wrong way. You fell into a hole. Game over.")

内容的提问来源于stack exchange，提问作者IniaPM