At some point in the recent past, I found myself enjoying the process of creating my own exceptions for specific business logic needs at my Python day job. So, naturally, I thought: what if you could use exceptions for all control flow in Python?

smasher applications have two main components: Actor objects that communicate with each other by way of raised exceptions, called Atoms, and the global Atoms namespace.

Looking at an example, we see:

class SimpleGame(Actor):
    atoms = ['Correct', 'Wrong', 'Win', 'KeepGoing', 'Done']

Actor classes declare their own atoms, which are instantiated as subclasses of the Atom exception class. Actors catch atoms using the dispatch decorator:

@dispatch({('Wrong', 'ValueError', 'Again'): 'take_a_guess',
           'Correct': 'correct',
           'KeepGoing': 'play_a_game',
           'Guess': ('evaluate', int)})
def take_a_guess(self):
    self.server.get_guess()

Here we see that the main locus for control flow is the take_a_guess method. If any of the Wrong, ValueError, or Again exceptions are raised, we recurse; the Correct and KeepGoing atoms are routed to the appropriate methods; finally, we see that the Guess atom is expected to contain a single element, an int, which will be passed to the evaluate method. Atoms can also be dispatched to raw lambdas or even other atoms, though in practice it will usually suffice simply to catch the original signal further up.