Right now the backends check during set_inner whether type(incoming_value) == type([]) for Multilinks. Really any finite interator would work; it looks like all roundup does is "for x in incoming_value". We could replace the type check with "hasattr(x, '__iter__') and let any iterator through. Then we wouldn't have to enumerate all the things that are allowed (set, frozenset, list, tuple, etc).

I don't know the hyperdb well enough yet to know if there is a good reason to limit it to just lists. Maybe Richard can comment. If he's okay with it, I'll change backend_anydbm.py, backend_metakit.py, rdbms_common.py.

As for returning sets, did you have any specific use cases where returning a set would be a big win over getting a list back?