Catching errors with try

When programs crash

So far every error was the end of your program. Try it and type a word instead of a number:

age = int(input("How old are you? "))
print(f"In 10 years you'll be {age + 10}.")

Python stops with a ValueError because “hello” can’t be turned into a number. In a real program (or on a robot!) that shouldn’t happen.

try and except

With try you say: “Give this a go.” If something goes wrong, Python jumps into the except part instead of crashing:

try:
    age = int(input("How old are you? "))
    print(f"In 10 years you'll be {age + 10}.")
except ValueError:
    print("Sorry, that wasn't a number.")

print("The program keeps running!")

After except you write which error you want to catch. You already know the names from the error messages.

Several kinds of errors

You can handle different errors differently by writing several except blocks below each other:

numbers = [10, 20, 30]

try:
    position = int(input("Which position (0–2)? "))
    print(f"There you'll find {numbers[position]}.")
except ValueError:
    print("Please enter a number.")
except IndexError:
    print("That position doesn't exist.")

Ask until it fits

With while True and break you keep asking until a valid input arrives. break ends the loop immediately:

while True:
    try:
        number = int(input("Number between 1 and 10: "))
        if 1 <= number <= 10:
            break
        print("Not between 1 and 10.")
    except ValueError:
        print("That's not a number.")

print(f"Thanks, you chose {number}.")

Watch out: Never write just except: without an error name. Then you also catch errors you didn’t expect at all – and you can no longer find typos in your code.

Quick check

Before you start the task: what does this program print? Type your guess – then Python works it out.

try:
    number = int("12a")
    print("works")
except ValueError:
    print("not a number")
print("carry on")