Throwing frozensets out, too. If regular sets aren't obscure enough, frozensets might be your thing. It looks like a set, it acts like a set, but its... hashable (for indexing) and (immutable.) Why use this? For algorithms that rely on combinations (not permutations), frozensets can be very useful. E.g. NOT this -- (0, 1) (1, 0) (both distinct using tuples) vs frozenset([0, 1]) ([1, 0] or [0, 1] have the same identity / frozenset.) You can use this for indexing algorithms and things like that. Sometimes, sets are very convenient because they naturally 'normalise' entries into a fixed order. This can simply a lot of code.