如何在Python多层if语句中使用input函数并解决变量未定义问题

问题原因 #

你遇到的报错本质是变量未在所有代码执行路径中被定义，具体问题出在原有代码的逻辑结构上：

• level_2变量仅在level_1 == "left"的条件分支中被赋值创建，当你输入right时，这个分支不会执行，level_2自然不存在，但你后续代码直接全局执行level_2.lower()以及level_2的取值判断，就会触发变量未定义的报错。
• 同理，原有代码中level_3仅在level_2 == "upstairs"的分支中被创建，如果选了basement，后续直接判断level_3的值也会触发同类报错，只是你之前没触发这个场景而已。
• 补充一个原有代码的隐藏问题：level_1.lower()、level_2.lower()这两行写法是无效的，Python的字符串是不可变类型，调用lower()方法不会修改原变量，必须赋值回原变量才会生效，比如level_1 = level_1.lower()，不然用户输入大写的Right、LEFT这类内容时，判断逻辑会直接失效。

修正逻辑说明 #

你调整后的缩进和嵌套if写法是正确的：

• 用elif替代平级if，避免不必要的分支判断
• 把level_2、level_3的相关逻辑全部嵌套在上一级条件分支内，只有上一级条件满足、变量被成功创建后，才会执行后续相关操作，从根源上避免了访问未定义变量的问题。
• 小优化点：修正后的代码开头重复写了两次欢迎语，删除重复的两行即可，不影响核心逻辑。

原问题代码

print("Welcome to the dungeon.")
print("Your mission is to find the ursine sword.") 

level_1 = input("there is two doors infront of you type 'right' for the right door and 'left' for the left door.\n" )
level_1.lower()
if level_1 == "right" :
  print("this door is closed your level is too low to open it.\n")
if level_1 == "left" :
 level_2 = input("you found a two ladder one leads to upstairs type 'upstairs' and another leads to the basement type 'basement'.\n ")
level_2.lower()

if level_2 == "basement" :
   print("hisssssss ,  you got bitten by snake game over")
if level_2 == "upstairs" :
  level_3 =int(input("it seems like the final room thre is three swords infront of you two of them must be traps chose '1','2' or '3' "))
if level_3 == 1 :
  print("you released a powerful ghost you'r dead")
elif level_3 == 3 :
  print("you opened a portal to a cater you're dead")
elif level_3 == 2 :
  print("congratulations you found the ursine sword")
else :
  print("game over.")

修正后代码

print("Welcome to the dungeon.")
print("Your mission is to find the ursine sword.") 

level_1 = input("there is two doors infront of you type 'right' for the right door and 'left' for the left door.\n").lower()
if level_1== "right":
  print("this door is closed your level is too low to open it.\n")
elif level_1== "left":
  level_2 =input("you found a two ladder one leads to upstairs type 'upstairs' and another leads to the basement type 'basement'.\n ").lower()
  if level_2== "basement":
    print("hisssssss ,  you got bitten by snake game over")
  elif level_2== "upstairs":
    level_3 = int(input("it seems like the final room thre is three swords infront of you two of them must be traps chose '1','2' or '3' "))
    if level_3 == 1 :
     print("you released a powerful ghost you'r dead")
    elif level_3 == 3 :
     print("you opened a portal to a cater you're dead")
    elif level_3 == 2 :
     print("congratulaitons you found the ursine sword")
else :
  print("game over.")

内容的提问来源于stack exchange，提问作者Loay Alhefny