How to Create a Custom Exception in Python — Minimal Example

A minimal Python example demonstrating how to define, raise, and catch a custom exception class by subclassing Exception.

Overview

This script shows the pattern for creating a user-defined exception in Python by subclassing the built-in Exception class. It demonstrates how to raise the custom exception under a specific condition and catch it with a try/except block.

How It Works

  1. CustomException is defined as a subclass of Exception, accepting a message parameter.
  2. some_function() raises CustomException when a condition is met (x < y).
  3. The exception is caught in a try/except block, and the message is printed.

Code Example

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

def some_function():
    x = 5
    y = 6
    if x < y:
        raise CustomException("Something went wrong")

try:
    some_function()
except CustomException as e:
    print(e.message)

Usage

From the Console_Code folder:

python CustomException.py

Output (confirmed by running the script):

Something went wrong

Notes

  • No external dependencies required — uses only Python built-ins.
  • The script includes a discussion in the docstring about when it is appropriate to make a custom exception serializable (relevant for distributed systems where exceptions may be passed across process boundaries).

File Structure

Custom Exception/
└── Console_Code/
    └── CustomException.py    # Main script

FAQ

Do I need to store message on the exception? No — str(e) already returns the message passed to super().__init__(message). Storing it as self.message simply makes it available as a named attribute, which is handy when you add more fields (such as an error code).

Should custom exceptions inherit from Exception or BaseException? From Exception (or one of its subclasses). BaseException is reserved for exceptions such as KeyboardInterrupt and SystemExit that ordinary except Exception blocks should not catch.

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