Here's a reworking which returns iterators. I had to decide what to do if the user tries to access things out of order; I raise an exception. Anything else would complicate the code quite a lot I think. def groupby(key, iterable): it = iter(iterable) value = it.next() # If there are no items, this takes an early exit oldkey = [key(value)] cache = [value] lock = [] def grouper(): yield cache.pop() for value in it: newkey = key(value) if newkey == oldkey[0]: yield value else: oldkey[0] = newkey cache.append(value) break del lock[0] while 1: if lock: raise LookupError, "groups accessed out of order" if not cache: break lock.append(1) yield grouper() --Greg Ball