[Python-Dev] Add a "transformdict" to collections

[Antoine Pitrou]

Le Tue, 10 Sep 2013 07:18:41 -0700,
Eli Bendersky <eliben at gmail.com> a écrit :
> On Tue, Sep 10, 2013 at 5:22 AM, Antoine Pitrou <solipsis at pitrou.net>
> wrote:
> 
> > Le Tue, 10 Sep 2013 22:00:37 +1000,
> > Nick Coghlan <ncoghlan at gmail.com> a écrit :
> > > Is this just syntactic sugar for recursive lookup of a transformed
> > > version in __missing__?
> >
> > Nope. For one, it doesn't use __missing__ at all. I think
> > using __missing__ would be... missing the point, because it wouldn't
> > working properly if you have e.g. X != Y such that transform(X) == Y
> > and transform(Y) == X.
> >
> > (a simple example being transform = str.swapcase :-))
> >
> > > Or a way of supplying a custom "key" function
> > > to a dictionary?
> >
> > Probably, although I'm not entirely sure what you mean by that :-)
> >
> > > Any such proposal should consider how it composes with other dict
> > > variants like defaultdict, OrderedDict and counter.
> >
> > Well, one sticking point here is that those variants don't compose
> > with each other already :-)
> > But, yes, I may make another proposal with composition in mind.
> >
> 
> Does anyone have an idea how to make the existing variants more
> composable? It would be nice to get a better understanding of this
> before we add another variant. Conceptually, composability makes a
> lot of sense (what if we want this transformdict but also in
> insertion order...)

AFAICT, the main problem with composability is that the implementation
is less simple and it removes many opportunities for optimization. It
may or may not be a problem, depending on your use cases.

(I'm playing a bit with a TransformMap which is a composable version of
transformdict, and it's clear the implementation is less amenable to
optimization tweaks and shortcuts: in fact, I have to *add* some logic
to e.g. cater with defaultdict)

Regards

Antoine.