Python multimethods with arbitrary dispatch

mm_example.py

from multimethod import defmulti, defmethod

do_work = defmulti(type)

@defmethod(do_work, str)
def do_str_work(s):
    return "a string"

@defmethod(do_work, int)
def do_int_work(i):
    return "an int"

@defmethod(do_work, list)
def do_list_work(l):
    return "a list"

@defmethod(do_work, tuple)
def do_tuple_work(t):
    return "a tuple"

@defmethod(do_work, dict)
def do_dict_work(d):
    return "a dict"

multimethod.py

class DispatchError(Exception): pass

class defmulti(object):
    def __init__(self, dispatch_fn):
        self.dispatch_fn = dispatch_fn
        self.implementations = {}

    def __call__(self, *args, **kwargs):
        dispatch = self.dispatch_fn(*args, **kwargs)
        try:
            impl = self.implementations[dispatch]
        except KeyError:
            raise DispatchError("no implementation found for dispatch value: "
                                + str(dispatch))
        return impl(*args, **kwargs)

    def respond_to(self, dispatch, fn):
        self.implementations[dispatch] = fn

def defmethod(mm, dispatch):
    def register_method(fn):
        mm.respond_to(dispatch, fn)
        return fn
    return register_method