More specific the Decimal class is mutable and usable as dict key.
It's *meant* to be immutable though. The fact that we used __slots__ instead of __setattr__ to implement the immutability, so you *can* overwrite the slot variables if you really want to is merely an artifact of the current Python implementation.
The limited mutability bug will disappear in Python 2.5, so it's not a good example of a 'mutable' dict key (especially given that the only way to mutate it is to modify private variables directly).
And, as I've stated previously, if the issue of sane mutable keys in dictionaries and sets really bugs you so much - implement identity_dict and identity_set in C and lobby for their inclusion in the collections module.
On the more general point of "don't use mutable objects with non-identity based comparisons as dictionary keys", try teaching students for a while (or listen to those who have):
When stating useful general principles, it is never, ever worth it to get into the quibbly little details about exceptions to the principle. If students ask, admit that they exist, but point out that the exceptions are rare, and not worth worrying about at that point in their learning.
Only when one is aware of the reasons for a principle, can one be aware of good reasons not to follow it :)
Cheers, Nick.
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Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia
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http://boredomandlaziness.skystorm.net
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