Error Handling with Exceptions

Errors are normal. A missing file, bad input or network failure should produce a clear message, not a crash.

try and except

Put risky code inside try. If it raises an exception, the matching except block runs and the program continues.

try:
    age = int(input("Your age: "))
    print(f"You will be {age + 1} next year.")
except ValueError:
    print("Please enter a number.")

Specific Exceptions

Catch the exception you expect. A bare except: hides bugs, including spelling mistakes in your own code.

try:
    with open("scores.txt") as file:
        scores = [int(line) for line in file]
except FileNotFoundError:
    scores = []
    print("No scores file yet. Starting empty.")
except ValueError:
    print("The file contains something that is not a number.")

Raising Errors

Use raise when a caller passes something invalid. The message should tell the caller what to fix.

def set_score(score):
    if not 0 <= score <= 100:
        raise ValueError("Score must be between 0 and 100")
    return score

try:
    set_score(140)
except ValueError as error:
    print("Rejected:", error)

finally

finally always runs, whether or not an error occurred. Use it to close files or release resources.

Practice

1. Ask for a number and handle invalid input.
2. Open a file that may not exist and provide a fallback.
3. Raise a ValueError for an empty name.

Hands-on exercise

Write a function safe_divide(a, b).
1. Return a / b.
2. If b is zero, return None and print a clear message.
3. Test it with 10 / 2 and 10 / 0.

Recap

Catch only what you expect, give useful messages and raise errors early when input is invalid.