Python Custom Exceptions
Define and Use Your Own Errors

Introduction to Python Custom Exceptions

In Python, not every error fits into the neat boxes of ValueError, IndexError, or TypeError. Sometimes, your program has its own rules — and when those are broken, you want your own error messages. That’s where custom exceptions come in.

Defining your own exception class helps you raise meaningful, specific errors that are easier to debug and maintain. This tutorial walks you through everything you need to know — from syntax to output — about creating and using custom exceptions in Python.

Why Use Custom Exceptions?

• Readability: Custom exception names clearly explain what went wrong.
• Modularity: Group specific behaviors under logical exception types.
• Control: Handle different error types more granularly during exception handling.

How to Define a Custom Exception in Python

To define a custom exception, we subclass the built-in Exception class. Here's the basic syntax:

class MyCustomError(Exception):
    pass

This creates a new exception type called MyCustomError. Now, you can raise it using the raise keyword.

raise MyCustomError("Something went wrong in your custom logic!")

Basic Example: Custom Validation Exception

Let’s say you’re writing a function that only accepts even numbers. If the input is odd, you want to raise your own error.

class NotEvenNumberError(Exception):
    def __init__(self, number):
        super().__init__(f"The number {number} is not even.")

def check_even(n):
    if n % 2 != 0:
        raise NotEvenNumberError(n)
    return f"{n} is even."

# Test
print(check_even(10))
print(check_even(7))

10 is even.
Traceback (most recent call last):
  ...
NotEvenNumberError: The number 7 is not even.

How to Customize Exception Messages

You can go beyond basic text by passing arguments and overriding the __str__ method:

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

    def __str__(self):
        return f"Transaction failed: Balance (${self.balance}) is less than amount (${self.amount})."

def withdraw(balance, amount):
    if amount > balance:
        raise LowBalanceError(balance, amount)
    return balance - amount

# Test
print(withdraw(100, 50))
print(withdraw(100, 150))

50
Traceback (most recent call last):
  ...
LowBalanceError: Transaction failed: Balance ($100) is less than amount ($150).

Important Guidelines When Creating Custom Exceptions

• Always inherit from Exception or a subclass of it — never from BaseException unless you have a rare system-level reason.
• Use descriptive names like UserNotFoundError or ConfigFileMissingError.
• Consider using docstrings in your custom exception classes if they’ll be reused in larger applications.

Organizing Multiple Custom Exceptions

If you're building a large application, group related exceptions under a base class:

class AppError(Exception):
    """Base class for all application-specific errors."""
    pass

class ConfigError(AppError):
    pass

class DatabaseError(AppError):
    pass

When to Raise Custom Exceptions

Use them when your application rules are broken. Examples:

• File not found in a specific directory even though the path exists
• Invalid user input beyond what's handled by ValueError
• When you need user-specific error feedback

How to Verify Custom Exceptions Work

You can test them using try/except blocks:

try:
    withdraw(20, 50)
except LowBalanceError as e:
    print("Caught:", e)

Caught: Transaction failed: Balance ($20) is less than amount ($50).

Conclusion

Custom exceptions give your code clarity and precision. Whether you’re building libraries, APIs, or full-stack apps, defining your own exceptions helps express intent and manage errors gracefully.

In the next lesson, we’ll explore how to handle both built-in and custom exceptions using try, except, and related keywords.