I want a set module, but if I finish Greg's abandoned work I want sets of sets too. Sets don't have "keys", they're conceptually collections of values, and it would be as odd not to allow sets containing sets as not to allow lists containing lists, or to ban dicts as dict values.
IMO it's no odder than disallowing dicts as dict keys: it's a hack that allows a much faster implementation.
That is, like sets of sets in Icon too, this is a notion of inclusion by object identity (although Icon does that on purpose, while the BTree-based set mostly inherits it from that BTrees don't implement any comparison slots). That's very easy to implement. It's braindead if you think of sets as collections of values, but that's what taking pain too seriously leads to.
I don't think it is acceptable to have sets-of-sets but test for membership (in that case) by object identity.
If you really think object identity is all that's needed, I suggest we stick to disallowing sets of sets; algorithms needing sets-of-set-object-identities can use id() on the inner sets.
--Guido van Rossum (home page: http://www.python.org/%7Eguido/)