mm module (by Guido van Rossum)

mm.py

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)
        function = self._typemap.get(types)
        if function is None:
            raise TypeError("No match")
        return function(*args)
    def register(self, types, function):
        if types in self._typemap:
            raise TypeError("Duplicate registration")
        self._typemap[types] = function

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