Would you have us construct two related classes each time we find ourselves in such a situation and copy an object from one class to the other depending on the circumstances?
Python itself seems to think so, given the pairings of set/frozenset & list/tuple.
Using genuinely immutable objects as dictionary keys is much easier than saying "while this object is part of a dictionary, don't alter it's hash value or comparison results". Instead, the immutable version is provided to say "alterations are not allowed on this copy"
You can certainly *do* the former (using __hash__ and appropriate comparison overrides), but it isn't particularly easy to do correctly, and hence usually isn't a great idea unless copies are *really* expensive (and even then, a shallow copy approach can often suffice).
Cheers, Nick.
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Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia
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http://boredomandlaziness.skystorm.net
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