Custom Exceptions

Python's built-in exceptions cover many cases, but complex applications often require domain-specific error handling. Creating custom exceptions allows you to communicate what went wrong in the language of your specific domain, rather than the language of the Python interpreter.

Inheriting from Exception

All custom user-defined exceptions should inherit from the built-in Exception class (or a subclass of it), not BaseException. This ensures they are caught by standard try...except Exception blocks but allows system-exiting exceptions (like KeyboardInterrupt) to pass through.

Exception Hierarchies

Grouping exceptions creates a hierarchy that simplifies error handling. Callers can catch the base class to handle any error from your module, or specific subclasses for granular control.

Adding Context

Custom exceptions are classes, meaning they can store state. You can pass metadata (error codes, specific values, contextual state) into the __init__ method to help the catcher debug or recover from the error.