User-defined exceptions

https://docs.python.org/3/tutorial/errors.html#tut-userexceptions

INTRODUCTION

Programs may name their own exceptions by creating a new exception class. Exceptions should typically be derived from the 'Exception' class, either directly or indirectly.

All exceptions must be instances of a class that derives from 'BaseException'.

# Syntax.

class SomeError(Exception):   # a name with 'Error' if we want to raport errors
    docstring   # optional
    statements

class MyError(Exception):
    pass   # the default __init__() is used

raise MyError("message")   # raising the exception
exception = MyError("a", "b")   # creating an instance of the exception class
print(exception.args)   # ('a', 'b'), the tuple is created by the constructor
print(exception)   # ('a', 'b'), str() uses exception.args