meet it

and catch an error before it stops your program.

Some errors only happen while a program runs, often because of what someone typed. These are exceptions. int("ten") causes a , and without help your program stops dead.

Wrap the risky lines in a try block. If they work, Python skips the except block. If a ValueError happens, Python jumps straight to except ValueError:, runs those lines instead, and carries on below.

Name the error you expect. A bare except: catches everything, including mistakes you never meant to hide.

answers = ["12", "ten", "7"]
for answer in answers:
    try:
        number = int(answer)
        print(f"{number} doubled is {number * 2}")
    except ValueError:
        print(f"'{answer}' is not a whole number")

Lines after the error inside try are skipped, which is why nothing is doubled for ten.

guess first

Which lines run? What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

try:
    print("Start")
    age = int("twelve")
    print("Converted!")
except ValueError:
    print("That was not a number")
print("Finished")

your turn

Your turn. Keep asking until you get a number.

The loop asks for an age until the answer is a whole number. Fill in the error that int() causes for text like old. We will answer old, then 12.

Your code

press run to see what happens

fix it

The VAT calculator still crashes. Can you fix it?

Broken code to fix

press run to see what happens

Read the last line of the error. Which kind of error really happened?
float("free") causes a ValueError, but the except line is waiting for a TypeError, so it never catches it.
Change TypeError to ValueError on line 4, then run.

nailed it

you caught your first exception. :D

Put risky lines in try and the rescue plan in except.
Name the exact error you expect, like except ValueError:.
After except runs, the program carries on below.

new words in your collection

exception
try
except
ValueError

This lesson isn’t saved. A free account remembers every lesson you finish.

try and except catch an error before it stops your program.

Which lines run? What will this show?

Your turn. Keep asking until you get a number.

The VAT calculator still crashes. Can you fix it?

you caught your first exception. :D

and catch an error before it stops your program.