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Tue Sep 10 14:25:51 CEST 2013 · https://mail.python.org/pipermail/python-dev/2013-September/128504.html

On 10 September 2013 13:24, Paul Moore <p.f.moore at gmail.com> wrote:
> 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'm using the case where the transform is str.lower here - sorry I
wasn't explicit.
Paul

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