Writing a Custom Exception Class

This lesson covers how to create custom exception classes in Python, so you can clearly distinguish between different kinds of errors. It's written for anyone searching "Python custom exception tutorial" or "Python Exception inheritance".

Inheriting from Exception lets you create your own dedicated exception type. Giving each kind of error its own class name makes it much easier to tell what went wrong when you catch it with except. Once you can distinguish "invalid input" from "network failure," you can write tailored handling for each in your code.

The sample code defines a class called ValidationError that inherits from Exception, with just a pass statement in its body. Inside the check_age function, an invalid age raises this exception with raise ValidationError(...), and except ValidationError as e: catches it and displays the error message.

A common beginner mistake is not knowing what to put inside a custom exception class. In fact, even an empty class with just pass works fine as a custom exception, as long as it inherits from Exception. If you want to carry more detailed information, you can override __init__ to add extra attributes.

Larger applications often define a custom exception class for each kind of error that can occur. Including specific, useful information in the error message makes later investigation much easier.