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On 8 Oct 2013 22:31, "Antoine Pitrou" <solipsis@pitrou.net> wrote:
\>
\> Le Tue, 8 Oct 2013 22:12:02 +1000,
\> Nick Coghlan <ncoghlan@gmail.com> a �crit :
\> >
\> > 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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