Creating Custom Exceptions in Python

Python

Learn Python Programming: A Beginner’s Guide to Coding, Automation, and Data Science

 

Why Create Custom Exceptions? 

Python has many built-in exceptions (like ValueError, TypeError), but sometimes you need to define your own!

Imagine you’re building a banking app, and you want a custom exception for insufficient funds. Instead of using a generic ValueError, why not create a meaningful error like InsufficientFundsError? 

Defining a Custom Exception

Custom exceptions in Python are created by subclassing Exception.

class InsufficientFundsError(Exception):
    """Custom exception for handling insufficient balance."""
    def __init__(self, balance, amount):
        self.balance = balance
        self.amount = amount
        super().__init__(f"Withdrawal of {amount} failed! Available balance: {balance}")

Raising a Custom Exception

You can raise your custom exception using raise when something goes wrong.

def withdraw_money(balance, amount):
    if amount > balance:
        raise InsufficientFundsError(balance, amount)
    balance -= amount
    return balance

try:
    new_balance = withdraw_money(100, 200)  # Not enough funds! 
except InsufficientFundsError as e:
    print(f"Error: {e}")

Output:

Error: Withdrawal of 200 failed! Available balance: 100

Catching Multiple Exceptions

You can handle both built-in and custom exceptions together.

try:
    new_balance = withdraw_money(50, 100)  # Another insufficient funds error!
except InsufficientFundsError as e:
    print(f"Custom Exception Caught: {e}")
except Exception as e:
    print(f"General Exception: {e}")

Summary 

Step	Action
Define	Create a class inheriting from Exception
Raise	Use raise to trigger the exception
Catch	Handle it using try-except

With custom exceptions, your error messages become clearer, your code is easier to debug, and your programs are more professional!