Python - Validation of an integer - except or except ValueError?

Question

I've created the following test in Python 3.x:

answer = int(input("What's 5 x 8?"))
if answer == 40:
    print("Correct!")
else:
    print("Incorrect!")

answer = int(input("What's 4 + 7?"))
if answer == 11:
    print("Correct!")
else:
    print("Incorrect!")

answer = int(input("What's 9 - 4?"))
if answer == 5:
    print("Correct!")
else:
    print("Incorrect!")

but I don't know whether I should use except or except ValueError to validate each answer.

Using except :

while True:
    try:
        answer = int(input("What's 5 x 8?"))
        if answer == 40:
            print("Correct!")
        else:
            print("Incorrect!")
        break
    except:
        print("That's not a valid answer. Enter an integer.")
#This would be repeated for the two other questions

Using except ValueError :

while True:
    try:
        answer = int(input("What's 5 x 8?"))
        if answer == 40:
            print("Correct!")
        else:
            print("Incorrect!")
        break
    except ValueError:
        print("That's not a valid answer. Please enter an integer.")
#This would be repeated for the two other questions

I only want integers to be accepted (eg '400', '3', etc.). Which one is better to use in my case?

Answer 1

Bare except is pretty much never the right thing, so you probably want to be catching the ValueError .

The problem with bare except is that it'll catch anything including things like KeyboardInterrupt , SystemExit , etc. Even if you become a little more restrictive and do:

try:
    ...
except Exception:
    ...

You can still end up glossing over all sorts of programming errors (eg if you accidentally misspelled an attribute and the code was raising AttributeError -- You'd be catching that exception and tracking down the resulting bugs would be harder).

My rule is to only catch exceptions if you know how to handle them . In this case, you know how to handle a ValueError from that block of code, but you don't know how to handle TypeError , or AttributeError or pretty much anything else. So, you should handle the ValueError and nothing else.

Answer 2

如果要验证整数，则应使用ValueError。

Answer 3

Do not use bare except in the context you want to catch the exception. If you want to catch all kinds of exception not related to program exit , which is true for most of the cases, use except Exception . But it is always a good idea to be specific.

And in the while loop you are break ing out abruptly. If you want to break out only when the correct answer is recieved, then:

while True:
    try:
        x = int(input("8 + 21 :"))
    except ValueError:
        print("Enter a valid number")
    else:
        if x == 29:
            break
        else:
            continue  # this is not required, just being explicit

See if it works.