How can I add exception for when user inputs a negative number

Question

I'm trying to add an exception to recognise when someone inputs a negative number, and respond with something to say you can only input a positive number

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.')

At the moment it will respond to the user inputting a string instead of an integer but I want it to be able to respond to the user inputting something like -4 by printing 'You cant use negative numbers'.

Totally new to Python, so any advice on how to add this in would be very much appreciated, thank you.

Answer 1

只是

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

Answer 2

Define your own exception class, which you can choose to catch or not:

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.")

Answer 3

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.')