Multimethods in Python

mm.py

# Example usage of https://www.artima.com/weblogs/viewpost.jsp?thread=101605
# with added commutativity

import itertools

registry = {}

class MultiMethod(object):
    def __init__(self, name):
        self.name = name
        self.typemap = {}

    def __call__(self, *args):
        types = tuple(arg.__class__ for arg in args) # a generator expression!
        function = self.typemap.get(types)
        if function is None:
            raise TypeError(f"no match for {types}")
        return function(*args)

    def register(self, types, function):
        if types in self.typemap:
            raise TypeError("duplicate registration")
        for p in itertools.permutations(types):
            self.typemap[p] = function

def multimethod(*types):
    def register(function):
        name = function.__name__
        mm = registry.get(name)
        if mm is None:
            mm = registry[name] = MultiMethod(name)
        mm.register(types, function)
        return mm
    return register

class Rock():
    pass

class Paper():
    pass

class Scissors():
    pass

def diag(t, ob1, ob2):
    print(f'{ob1.__class__.__name__} vs {ob2.__class__.__name__}: {t} wins')

@multimethod(Paper, Rock)
def play(ob1, ob2):
    diag('Paper', ob1, ob2)

@multimethod(Scissors, Paper)
def play(ob1, ob2):
    diag('Scissors', ob1, ob2)

@multimethod(Rock, Scissors)
def play(ob1, ob2):
    diag('Rock', ob1, ob2)

if __name__ == '__main__':
    r = Rock()
    p = Paper()
    s = Scissors()
    play(p, r)
    play(r, p)
    play(s, p)
    play(r, s)