On 8 Oct 2013 22:31, "Antoine Pitrou" <solip...@pitrou.net> wrote: > > Le Tue, 8 Oct 2013 22:12:02 +1000, > Nick Coghlan <ncogh...@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. > > > _______________________________________________ > Python-Dev mailing list > Python-Dev@python.org > https://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: https://mail.python.org/mailman/options/python-dev/ncoghlan%40gmail.com
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