Python try and except: how to handle errors

Wrap the risky line in try: and put the recovery in except, naming the error you expect. Python runs the except block instead of crashing.

Retry on bad input, handle a missing file

while True:
    try:
        quantity = int(input("How many mugs? "))
        break
    except ValueError:
        print("Please type a whole number, like 3.")

try:
    with open("orders.txt", encoding="utf-8") as file:
        print(file.read())
except FileNotFoundError:
    print("No orders yet.")

print(f"You ordered {quantity} mugs.")

Run it, type three, press Enter, then type 3 (and no orders.txt exists):

How many mugs? three
Please type a whole number, like 3.
How many mugs? 3
No orders yet.
You ordered 3 mugs.

How it works

• int("three") raises a ValueError. Python skips the rest of the try block and jumps to the matching except ValueError.
• If int() works, break ends the loop. If it fails, break is skipped, the message prints, and the loop asks again.
• Opening a file that doesn't exist raises FileNotFoundError. The second except catches it, so the program goes on.
• If nothing goes wrong, the except block is skipped completely.

You can stack several except blocks under one try to handle different errors differently.

Common mistake

Using a bare except: with no name:

try:
    quantity = int(input("How many mugs? "))
except:
    print("Something went wrong.")

This catches every error, including typos in your own code, and hides what really happened. Name the error instead: except ValueError:.

Also keep the try block short. Put only the lines that might fail inside it, so an except can't accidentally cover a bug elsewhere.

Takeaway

Put risky code in try, handle a specific named error in except, and add a while True: loop with break when you want to ask again.