Terry Reedy wrote: >> In this model, a >> get() method, or even a __getitem__ with s[k] is k, is only natural. > > No, if key and value were the same thing, the get method would be > nonesense, as Guido said. But since they are not, since the implict key > is an abstract class, retrieving the representative, perhaps canonical > object given *any* member *is* natural. Being able to do so is also a > standard practice in mathematics. To formalise this invariant to some degree: given a set "s" and two items "k1" and "k2", such that "k1 in s" and "k2 in s" and "k1 == k2", there is a single item "k" in s such that "k1 == k" and "k2 == k". If __getitem__ were to be provided by sets, then the last part of that invariant could be written "s[k1] is s[k2]". If you actually care about that aspect of the semantics for a particular application, it would be far clearer to use a full dict() and live with the additional memory usage. While I can see how saving that pointer-per-entry might make sense in some circumstances, for most I would see it as a needlessly confusing micro-optimisation. Cheers, Nick. -- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia ---------------------------------------------------------------
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