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

Glenn Linderman v+python at g.nevcal.com
Thu Sep 12 21:15:14 CEST 2013

On 9/12/2013 8:40 AM, Antoine Pitrou wrote:
> Le Thu, 12 Sep 2013 08:05:44 -0700,
> Ethan Furman <ethan at stoneleaf.us> a écrit :
>> On 09/12/2013 07:43 AM, Antoine Pitrou wrote:
>>> Yeah, so this is totally silly. What you're basically saying is "we
>>> don't need TransformDict since people can re-implement it
>>> themselves".
>> No, what I'm saying is that the "case-preserving" aspect of
>> transformdict is silly.  The main point of transformdict is to
>> enable, for example, 'IBM', 'Ibm', and 'ibm' to all match up as the
>> same key.  But why?  Because you don't trust the user data.  And if
>> you don't trust the user data you have to add the correct version of
>> the key yourself before you ever process that data, which means you
>> already have the correct version stored somewhere.
> That's assuming there is an a priori "correct" version. But there might
> not be any. Keeping the original key is important for different reasons
> depending on the use case:
> - for case-insensitive dicts, you want to keep the original key for
>    presentation, logging and debugging purposes (*)
> - for identity dicts, the original key is mandatory because the id()
>    value in itself is completely useless, it's just used for matching

  - for dict with non-hashable key, but a transform function that can 
derive a hashable key from it, the presentation key value may be much 
more complex than the actual key value.
> (*) For a well-known example of such behaviour, think about Windows
> filesystems.
> Regards
> Antoine.

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