[Python-Dev] PEP 455: TransformDict (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Tue Oct 8 14:30:02 CEST 2013

Le Tue, 8 Oct 2013 22:12:02 +1000, Nick Coghlan <ncoghlan at gmail.com> a écrit :

It's OK if the key transforming API has to constrain the behaviour of the underlying mapping or require an appropriately designed transform function to handle more esoteric containers. Either would still be better than categorically disallowing composition to the point where you can't even compose it with OrderedDict.

Well, you could ask the same question about OrderedDict, defaultdict or Weak*Dictionary since neither of them use composition :-)

ChainMap doesn't compose sensibly with arbitrary mappings like defaultdict, but composition is still the right design choice because it works well with most custom mappings.

ChainMap is easy to compose since it doesn't have to keep any data-driven internal state.

It's not that I think this is necessarily a bad idea (although the composability problem gives me grave doubts), it's that I think it's not an urgent idea, so why rush to review and include it in the weeks remaining before the beta, when the option of publishing it as a recipe or a PyPI module remains available?

It's just that I disagree we're rushing anything. The feature is fairly simple, many people have already had a need for something like that. (and amongst those people absolutely zero have said the design of the feature proposal is inadequate)

Regards

Antoine.

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