[Python-Dev] PEP 455: TransformDict

MRAB python at mrabarnett.plus.com
Sat Sep 14 04:39:18 CEST 2013

On 14/09/2013 02:40, Ethan Furman wrote:
> On 09/13/2013 06:25 PM, MRAB wrote:
>> On 14/09/2013 01:49, Steven D'Aprano wrote:
>>> Is it more common to want both the canonical key and value at the same
>>> time, or to just want the canonical key? My gut feeling is that I'm
>>> likely to have code like this:
>>> d = TransformDict(...)
>>> for key in data:
>>>      key = d.get_canonical(key)
>>>      value = d[key]
>>>      print("{}: {}".format(key, value))
>> I think I must be missing something. I thought that iterating over the
>> dict would yield the original keys, so if you wanted the original key
>> and value you would write:
>> for key, value in data.items():
>>      print("{}: {}".format(key, value))
> Well, that's certainly how I would do it.  ;)
>> and if you wanted the transformed key you would apply the transform
>> function to the key.
> Indeed.  The question is:  how?  It is entirely possible that your function has a TransformDict alone, and no memory of
> the transform function used to create the dict...
> If the key transform function were saved directly on the TransformDict instance as, say, .transform_key, then problem
> solved.
defaultdict has .default_factory, so having something like
.transform_key would have the added advantage of consistency.

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