Best practices for custom exceptions in Python - Mini Project: Build & Apply

Best practices for custom exceptions

📖 Scenario: Imagine you are building a simple banking application. You want to handle errors clearly when users try to withdraw more money than they have in their account.

🎯 Goal: Create a custom exception called InsufficientFundsError to handle withdrawal errors. Use it in a function that withdraws money from an account balance.

📋 What You'll Learn

Create a custom exception class named InsufficientFundsError that inherits from Exception.

Add a constructor to InsufficientFundsError that accepts a message and passes it to the base class.

Create a variable balance with the value 100.

Create a function withdraw(amount) that raises InsufficientFundsError if amount is greater than balance.

Use a try-except block to call withdraw(150) and print the error message if caught.

💡 Why This Matters

🌍 Real World

Custom exceptions help you handle specific errors in your programs clearly, like when a bank account has insufficient funds.

💼 Career

Understanding custom exceptions is important for writing robust, maintainable code in software development jobs.

1

Create the custom exception class

Create a custom exception class called InsufficientFundsError that inherits from Exception. Add an __init__ method that takes a message parameter and passes it to super().__init__(message).

Python

# Your code here

Need a hint?

Remember to inherit from Exception and call super().__init__(message) inside the constructor.

2

Create the balance variable

Create a variable called balance and set it to 100.

Python

class InsufficientFundsError(Exception):
    def __init__(self, message):
        super().__init__(message)

# Your code here

Need a hint?

Just create a variable named balance and assign it the value 100.

3

Create the withdraw function with exception

Create a function called withdraw that takes a parameter amount. Inside the function, check if amount is greater than balance. If yes, raise InsufficientFundsError with the message "Not enough funds to withdraw {amount}". Otherwise, subtract amount from balance.

Python

class InsufficientFundsError(Exception):
    def __init__(self, message):
        super().__init__(message)

balance = 100

# Your code here

Need a hint?

Use raise InsufficientFundsError(...) to throw the error when amount is too big.

4

Use try-except to handle the exception

Write a try-except block that calls withdraw(150). Catch the InsufficientFundsError as e and print e.

Python

class InsufficientFundsError(Exception):
    def __init__(self, message):
        super().__init__(message)

balance = 100

def withdraw(amount):
    global balance
    if amount > balance:
        raise InsufficientFundsError(f"Not enough funds to withdraw {amount}")
    balance -= amount

# Your code here

Need a hint?

Use try to call withdraw(150) and except InsufficientFundsError as e to catch and print the error.