Example of an exception class in Python

If I'm going to suggest that you define your own exceptions, I really ought to give an example:

class PlayerError(Exception):
    pass

class PlayerIsNotPlayingInExpectedDirectionError(PlayerError):
    def __init__(self, direction):
        """direction parameter should be 'forwards' or 'in reverse'"""
        if 'rev' in direction : direction = 'in reverse'
        self.direction = direction

    def __str__(self):
        return "Player was not playing %s as expected." % self.direction