Lab Activity: Practice Error Handling and Debugging

Objectives

• Learn how to use try/except for handling errors.

• Create a custom exception (NegativeNumberError).

• Build a simple input loop that only accepts positive integers.

• Show friendly error messages when input is wrong.

• Practice debugging with breakpoint().

• (Optional) Use logging to track invalid inputs.

Step 1 — Create Your File

Create a new file called app.py.

Step 2 — Custom Exception

Add a custom exception for negative numbers:

# app.py

class NegativeNumberError(Exception):
    """Raised when the user enters a negative number."""
    pass

Step 3 — Input Loop with try/except

Write a loop that keeps asking for a number until the user enters a valid one:

while True:
    try:
        raw = input("Enter a positive integer: ")
        number = int(raw)  # may raise ValueError

        if number < 0:
            # our custom exception
            raise NegativeNumberError()

        if number == 0:
            print("Zero is not allowed, please enter a number greater than zero.")
            continue

        # Valid input
        print(f"Great, you entered {number}. Its square is {number * number}.")
        break  # exit the loop

    except ValueError:
        print("That's not a valid number, please try again.")

    except NegativeNumberError:
        print("Please enter a positive number, negative is not allowed.")

Step 4 — Test Your Program

Run it several times with these inputs:

• hello → “That’s not a valid number…”

• 12.3 → “That’s not a valid number…”

• -5 → “Please enter a positive number…”

• 0 → “Zero is not allowed…”

• 7 → “Great, you entered 7. Its square is 49.”

Step 5 — (Optional) Add Logging

At the very top of your file:

import logging
logging.basicConfig(filename="errors.log", level=logging.WARNING)

Inside the except blocks, add:

logging.warning("Invalid input: %s", raw)

Now invalid attempts are saved to errors.log.

Outcome

By the end of this lab you’ll know how to:

• Handle bad input with try/except.

• Use a custom exception for special error cases.

• Write clear messages for users.

• Debug your code with breakpoint().

• (Optionally) log errors for later review.