Extending built-in exceptions in Python - Mini Project: Build & Apply

Extending built-in exceptions

📖 Scenario: Imagine you are building a simple banking app. You want to handle errors when someone tries to withdraw more money than they have.

🎯 Goal: Create a custom error by extending Python's built-in Exception class. Use it to show a clear message when a withdrawal is too large.

📋 What You'll Learn

Create a custom exception class called InsufficientFundsError that extends Exception

Add an __init__ method to InsufficientFundsError that takes balance and amount as parameters

Store balance and amount as attributes in InsufficientFundsError

Override the __str__ method to return a message like: 'Cannot withdraw {amount}, only {balance} available.'

Create a variable balance set to 100

Create a variable withdraw_amount set to 150

Write code that raises InsufficientFundsError if withdraw_amount is greater than balance

Use a try-except block to catch InsufficientFundsError and print the error message

💡 Why This Matters

🌍 Real World

Custom exceptions help make error messages clearer and easier to understand in real applications like banking or shopping apps.

💼 Career

Knowing how to extend exceptions is useful for writing clean, maintainable code and handling errors in professional software development.

1

Create the custom exception class

Create a class called InsufficientFundsError that extends Exception. Add an __init__ method that takes balance and amount as parameters and stores them as attributes.

Python

# Your code here

Need a hint?

Remember to use class InsufficientFundsError(Exception): to extend the built-in Exception class.

2

Add the __str__ method to show the error message

Add a __str__ method to InsufficientFundsError that returns the string: f"Cannot withdraw {self.amount}, only {self.balance} available."

Python

class InsufficientFundsError(Exception):
    def __init__(self, balance, amount):
        self.balance = balance
        self.amount = amount

# Your code here

Need a hint?

The __str__ method should return a formatted string with self.amount and self.balance.

3

Set up balance and withdrawal amount variables

Create a variable called balance and set it to 100. Create another variable called withdraw_amount and set it to 150.

Python

class InsufficientFundsError(Exception):
    def __init__(self, balance, amount):
        self.balance = balance
        self.amount = amount
    def __str__(self):
        return f"Cannot withdraw {self.amount}, only {self.balance} available."

# Your code here

Need a hint?

Use simple assignment to create balance and withdraw_amount.

4

Raise and catch the custom exception

Write a try-except block. Inside try, raise InsufficientFundsError(balance, withdraw_amount) if withdraw_amount is greater than balance. In except InsufficientFundsError as e, print e.

Python

class InsufficientFundsError(Exception):
    def __init__(self, balance, amount):
        self.balance = balance
        self.amount = amount
    def __str__(self):
        return f"Cannot withdraw {self.amount}, only {self.balance} available."

balance = 100
withdraw_amount = 150

# Your code here

Need a hint?

Use raise InsufficientFundsError(balance, withdraw_amount) inside the try block and catch it with except InsufficientFundsError as e.