正则匹配时间格式失败引发AttributeError而非ValueError的原因

问题描述 #

我希望convert函数能验证格式为“9 AM to 5 PM”或“9:00 AM to 5:00 PM”的字符串，当时间与AM/PM之间无空格（例如“9AM to 5PM”）或格式错误时，抛出ValueError。但实际输入“9AM to 5PM”时，却触发了AttributeError，提示'NoneType'对象没有'group'属性，请问这是为什么？

现有代码如下：

import re
import sys

def main():
    try:
        print(convert(input("Hours: ")))
    except ValueError:
        print("Invalid format. Please use '9:00 AM to 5:00 PM' or '9 AM to 5 PM'.")

def convert(s):
    matches = re.search(r"^([0-9]+):?([0-9]+)? (AM|PM) to ([0-9]+):?([0-9]+)? (AM|PM)$", s)

    hour1 = int(matches.group(1))
    minute1 = int(matches.group(2) or 0)
    hour2 = int(matches.group(4))
    minute2 = int(matches.group(5) or 0)
    am_pm1 = matches.group(3).upper()
    am_pm2 = matches.group(6).upper()

    if matches:
        if hour1 > 12 or hour2 > 12 or minute1 > 59 or minute2 > 59:
            raise ValueError
        if hour1 == 12 and am_pm1 == "AM":
            hour1 = 0
        elif hour1 != 12 and am_pm1 == "PM":
            hour1 += 12
        if hour2 == 12 and am_pm2 == "AM":
            hour2 = 0
        elif hour2 != 12 and am_pm2 == "PM":
            hour2 += 12
    else:
        raise ValueError

    time1 = f"{hour1:02}:{minute1:02}"
    time2 = f"{hour2:02}:{minute2:02}"
    
    return time1 + " to " + time2

if __name__ == "__main__":
    main()

错误原因 #

核心问题是正则匹配失败后直接调用了matches.group()方法：

• 当输入“9AM to 5PM”时，你的正则表达式要求时间数字与AM/PM之间必须有空格，但输入中没有这个空格，导致re.search()无法匹配，返回None。
• 代码中先执行了matches.group(1)等分组提取操作，之后才检查matches是否有效。此时matches是None，调用group()方法自然会触发AttributeError，而不是预期的ValueError。

修复方案 #

调整代码执行顺序，先验证正则匹配结果，再提取分组内容：

1. 在调用任何matches.group()之前，先判断matches是否不为None，如果匹配失败直接抛出ValueError。
2. 移除原代码中冗余的else分支，提前的判断已经覆盖了匹配失败的场景。

修正后的完整代码 #

import re
import sys

def main():
    try:
        print(convert(input("Hours: ")))
    except ValueError:
        print("Invalid format. Please use '9:00 AM to 5:00 PM' or '9 AM to 5 PM'.")

def convert(s):
    matches = re.search(r"^([0-9]+):?([0-9]+)? (AM|PM) to ([0-9]+):?([0-9]+)? (AM|PM)$", s)

    # 先检查匹配结果，失败直接抛出ValueError
    if not matches:
        raise ValueError

    # 匹配成功后再提取分组内容
    hour1 = int(matches.group(1))
    minute1 = int(matches.group(2) or 0)
    hour2 = int(matches.group(4))
    minute2 = int(matches.group(5) or 0)
    am_pm1 = matches.group(3).upper()
    am_pm2 = matches.group(6).upper()

    # 验证时间数值的合法性
    if hour1 > 12 or hour2 > 12 or minute1 > 59 or minute2 > 59:
        raise ValueError
    if hour1 == 12 and am_pm1 == "AM":
        hour1 = 0
    elif hour1 != 12 and am_pm1 == "PM":
        hour1 += 12
    if hour2 == 12 and am_pm2 == "AM":
        hour2 = 0
    elif hour2 != 12 and am_pm2 == "PM":
        hour2 += 12

    time1 = f"{hour1:02}:{minute1:02}"
    time2 = f"{hour2:02}:{minute2:02}"
    
    return time1 + " to " + time2

if __name__ == "__main__":
    main()

内容的提问来源于stack exchange，提问作者testtest