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

[Lukas Lueg]

Should'nt the key'ing behaviour be controlled by the type of the key
instead of the type of the container?


2013/9/10 MRAB <python at mrabarnett.plus.com>

> On 10/09/2013 20:08, Paul Moore wrote:
>
>> On 10 September 2013 19:31, Antoine Pitrou <solipsis at pitrou.net> wrote:
>>
>>> I think it would be a flaw to have this detail implementation-defined.
>>>> This would be like saying that it is implementation-defined which
>>>> of A,B,C is returned from "A and B and C" if all are true.
>>>>
>>>
>>> Ok, it seems everyone (except me :-)) agrees that it should return the
>>> first key value, so that's how it will be.
>>>
>>
>> If you retain the first key value, it's easy enough for the
>> application to implement "retain the last" semantics:
>>
>> try:
>>      del d[k]
>> finally:
>>      d[k] = v
>>
>>  That would raise a KeyError is the key was missing. A better way is:
>
> d.pop(k, None)
>
> d[k] = v
>
>  If you provide "retain the last", I can't see any obvious way of
>> implementing "retain the first" in application code without in effect
>> reimplementing the class.
>>
>>  "Retain the first" does feel more natural to me.
>
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