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

Nick Coghlan ncoghlan at gmail.com
Tue Oct 8 14:49:28 CEST 2013

On 8 Oct 2013 22:31, "Antoine Pitrou" <solipsis at pitrou.net> wrote:

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 :-)

We did ask the same question about those (except the Weak* variants, simply due to age).

Each time, the point that each new dict variant would be used to justify yet more variants was a cause for concern.

defaultdict made it through because it's just a convenience API for the underlying "key missing" protocol, while OrderedDict spent time maturing outside the standard library first.

> 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.

Yet composing ChainMap with defaultdict still breaks, because it's a conceptually incoherent thing to do.

"Composition doesn't work with some mappings" isn't an adequate answer to the criticism that a composition based design could work with more mappings than just the builtin dict.

> 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)

Except the one who wanted to combine it with OrderedDict. Users won't be able to combine it with ChainMap either.

The concrete container vs container-wrapper design decision is a fundamental one and I don't believe the current PEP adequately makes the case in favour of the former.

Cheers, Nick.

Regards Antoine.

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