Custom error messages in Python - Mini Project: Build & Apply

Custom error messages

📖 Scenario: You are creating a simple program that asks a user to enter their age. You want to make sure the age is a positive number. If the user enters a wrong value, you will show a clear custom error message to help them understand the mistake.

🎯 Goal: Build a program that checks the user's age input and shows a custom error message if the input is not a positive number.

📋 What You'll Learn

Create a variable called age_input with the value '-5' (a string).

Create a variable called error_message with the value 'Age must be a positive integer.'.

Write a try block that converts age_input to an integer called age.

If age is less than or equal to zero, raise a ValueError with the error_message.

Write an except block that catches ValueError and prints the error_message.

Print age if no error occurs.

💡 Why This Matters

🌍 Real World

Custom error messages help users understand what went wrong and how to fix it, improving user experience in apps and websites.

💼 Career

Many programming jobs require writing clear error handling to make software reliable and user-friendly.

1

Create the input variable

Create a variable called age_input and set it to the string '-5'.

Python

# Create a variable called age_input with the value '-5'
age_input = ''  # Your code here

Need a hint?

Use age_input = '-5' to create the variable.

2

Create the error message variable

Create a variable called error_message and set it to the string 'Age must be a positive integer.'.

Python

age_input = '-5'
# Create a variable called error_message with the value 'Age must be a positive integer.'
error_message = ''  # Your code here

Need a hint?

Use error_message = 'Age must be a positive integer.' to create the variable.

3

Add the try block and check age

Write a try block that converts age_input to an integer called age. Then check if age is less than or equal to zero. If yes, raise a ValueError with the error_message.

Python

age_input = '-5'
error_message = 'Age must be a positive integer.'

# Write a try block to convert age_input to int and check if age <= 0
try:
    pass  # Your code here

Need a hint?

Use age = int(age_input) inside the try block. Then check if age <= 0 and raise ValueError(error_message).

4

Add except block and print age

Add an except ValueError block that prints the error_message. After the try-except, print the variable age if no error occurs.

Python

age_input = '-5'
error_message = 'Age must be a positive integer.'

try:
    age = int(age_input)
    if age <= 0:
        raise ValueError(error_message)
except ValueError:
    pass  # Your code here

# Print age if no error

Need a hint?

Use except ValueError: and inside it print(error_message). Use else: to print age if no error.