如何在用户输入负数时添加异常答案

【问题标题】：How can I add exception for when user inputs a negative number如何在用户输入负数时添加异常
【发布时间】：2020-06-01 13:57:10
【问题描述】：

我正在尝试添加一个例外来识别某人何时输入负数，并回复说您只能输入正数

print('How many cats do you have?')
numCats = input()
try: 
    if int(numCats) >=4:
        print('Thats a lot of cats.')
    else:
        print('Thats not that many cats.')
except ValueError: 
    print('You did not enter a number.')

目前它会响应用户输入字符串而不是整数，但我希望它能够通过打印“您不能使用负数”来响应用户输入 -4 之类的内容。

对 Python 完全陌生，因此非常感谢任何有关如何添加它的建议，谢谢。

【问题讨论】：

您已经知道如何针对恒定阳性进行测试。那么负数有什么问题呢？
你可能想阅读python教程...docs.python.org/3/tutorial/errors.html#raising-exceptions

标签： python exception except

【解决方案1】：

只是

raise ValueError("you must give a positive number")

【讨论】：

【解决方案2】：

print('How many cats do you have?')
try: 
    numCats = int(input()) #Moving the int() around input() means we aren't calling int() for every if branch.
    #Also, need to move that in here, so it gets caught by the try/except
    if numCats >= 4:
        print('Thats a lot of cats.')
    elif numCats < 0:
        print('You need a positive amount of cats.') #just printing instead of using a raise statement, an exception is unnecessary
    else:
        print('Thats not that many cats.')
except ValueError: 
    print('You did not enter a number.')

【讨论】：

【解决方案3】：

定义你自己的异常类，你可以选择捕获或不捕获：

class NegativeNumberException(Exception):
    pass

print('How many cats do you have?')
try:
    numCats = int(input())
    if numCats >=4:
        print('Thats a lot of cats.')
    elif numCats < 0:
        raise NegativeNumberException()
    else:
        print('Thats not that many cats.')
except ValueError:
    print('You did not enter a number.')
except NegativeNumberException as e:
    print("You entered a negative number.")

【讨论】：