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Tue Sep 10 14:35:58 CEST 2013 · https://mail.python.org/pipermail/python-dev/2013-September/128505.html

Le Tue, 10 Sep 2013 13:24:29 +0100,
Paul Moore <p.f.moore at gmail.com> a écrit :
> On 10 September 2013 13:00, Nick Coghlan <ncoghlan at gmail.com> wrote:
> > Is this just syntactic sugar for recursive lookup of a transformed
> > version in __missing__? Or a way of supplying a custom "key"
> > function to a dictionary?
> 
> Not quite, because the dict should preserve the originally entered
> key.
> 
> >>> td['FOO'] = 42
> >>> td['bar'] = 1
> >>> td['foo']
> 42
> >>> td['BAR']
> 1
> >>> list(td.keys())
> ['FOO', 'bar']
> 
> This actually prompts the question, what should the following produce:
> 
> >>> td['FOO'] = 42
> >>> td['foo'] = 32
> >>> list(td.keys())
> 
> ['FOO'] or ['foo']? Both answers are justifiable. Both are possibly
> even useful depending on context...

I think it would be best to leave it as an implementation detail,
because whichever is easiest to implement depends on the exact
implementation choices (e.g. C vs. Python).

(my prototypes right now conserve the last __setitem__ key, not the
first)

Regards

Antoine.

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