I lost David Eppstein's post, but I finally know what I want to say in response. David objected to the behavior of the groupby() subiterators to become invalidated when the outer iterator is moved on to the next subiterator. But I don't think there's a good use case for what he wants to do instead: save enough state so that the subiterators can be used in arbitrary order. An application that saves the subiterators for later will end up saving a copy of everything, so it might as well be written so explicitly, e.g.: store = {} for key, group in groupby(keyfunc, iterable): store[key] = list(group) # now access the groups in random order: for key in store: print store[key] I don't think the implementation should be complexified to allow leaving out the explicit list() call in the first loop. --Guido van Rossum (home page: http://www.python.org/~guido/)