Python中输入不匹配时如何实现循环重新获取输入？

原代码的问题分析 #

• 第一段代码用if判断：输入不匹配时仅打印一次提示，程序不会重新获取输入，直接继续执行后续逻辑或终止，无法实现循环输入的需求。
• 第二段代码的while循环：循环体内没有重新调用input()更新buy的值，导致buy始终是最初的不匹配值，循环条件永远成立，因此会无限打印Please try again。

正确实现代码 #

写法一：在循环内重新获取输入更新变量

print('What would you like to purchase?')
buy = input('(Enter what you would like to purchase?)') 
while buy not in items_for_sale_today2: 
    print('Please try again')
    # 关键：循环内重新获取输入，更新buy的值
    buy = input('(Enter what you would like to purchase?)')
# 输入正确后，执行购买数量输入
purchase = input(f'How many {buy} would you like to purchase?')

写法二：用while True循环，符合条件时退出

print('What would you like to purchase?')
while True:
    buy = input('(Enter what you would like to purchase?)')
    if buy in items_for_sale_today2:
        # 输入正确，退出循环
        break
    print('Please try again')
purchase = input(f'How many {buy} would you like to purchase?')

注意点 #

原代码中的self.buy属于错误用法：如果这段代码不是在类的方法中，不需要使用self，直接用变量buy即可；如果是在类方法内，需要先将buy定义为实例属性（即self.buy = input(...)），但从代码结构看更偏向普通脚本，直接使用变量buy即可。另外用f-string格式化字符串比format()更简洁直观。

内容的提问来源于stack exchange，提问作者Kevin Koh