Mike> Tuples have the problem that they are immutable, except when Mike> they're not (or for proper values of immutable, your Mike> choice). They're hashable, except when they're not. Or Mike> equivalently, they can be used as dictionary keys - or set Mike> elements - except when they can't.
For those of us not following this thread closely, can you identify cases where tuples are mutable, not hashable or can't be used as dictionary keys? I've never encountered any such cases. Skip -- http://mail.python.org/mailman/listinfo/python-list