On 9/10/2013 2:46 PM, Antoine Pitrou wrote:

> >> Which reminds one - this class should obviously have a method for
> >> retrivieng the original key value, given a matching key -
> >>
> >> d.canonical('foo') -> 'Foo'

> >
> > I don't know. Is there any use case?
> > (sure, it is trivially implemented)

> 
> 
> Well, I'd expect it to simply be there. I had not thought of
> other usecases for the transformdict itself -

Well, it is not here for dict, set, etc.

But they don't change the keys (although numbers have different representations on occasion).

One use of transformdict might be to allow use of non-hashable items as keys, by extracting an actual key from the internals of the non-hashable item. The key may be sufficiently unique to enable use of the dict structure for lookups, but it would certainly be handy to obtain the actual item again. Without a canonical lookup feature, one would be forced to also include the key as part of the value, or some such hack.

I also thought João's example was a very practical reason to have the canonical lookup feature, by some name or another.