[Python-Dev] PEP 455: TransformDict

[Nick Coghlan]

On 8 Oct 2013 18:36, "Antoine Pitrou" <solipsis at pitrou.net> wrote:
>
> Le Tue, 8 Oct 2013 08:21:43 +0200,
> Antoine Pitrou <solipsis at pitrou.net> a écrit :
> >
> > > I think there *is* a potentially worthwhile generalisation here,
> > > but I'm far from sure "is-a-dict" is the right data model - for
> > > composability reasons, it feels like a "has-a" relationship with an
> > > underlying data store may make more sense.
> >
> > It doesn't work. Your "underlying mapping" can show too much variation
> > for the wrapper/proxy to have sane semantics. For example, how do you
> > combine with a defaultdict or a WeakKeyDictionary, knowing that the
> > wrapper/proxy has to have its own internal mapping as well?
>
> (for the record, I'm saying that because I did try the composition
> route.)

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. 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.

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?

Enums eventually made it in because we wanted to *use* them to dramatically
improve error messages from other stdlib modules. What's the comparable end
user payoff for including TransformDict in 3.4 rather than 3.5 (or never)?

Cheers,
Nick.

>
> Regards
>
> Antoine.
>
>
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