[Python-Dev] PEP 455: TransformDict (original) (raw)

Joao S. O. Bueno jsbueno at python.org.br
Sat Sep 14 06:53:20 CEST 2013

On 13 September 2013 22:40, Ethan Furman <ethan at stoneleaf.us> 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.getcanonical(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. ;)

I hope you are aware that this pattern does not help when one wants one canonical key having a non-canonical one, besides having to linearly walk through all keys and check the "transform" to each one.

I mean - given no function to retrieve the canonical key, one would have to resort to:

my_key = data.transform(given_key) for key, value in data.items(): if data.transform(key) == my_key: ....

WHich would defeat not only the purpose of a Transform dict, but the purpose of a dict alltogether.

(of course, the one obvious way to do it would be to have the original key stored along the value - which is just a bit less silly than the example above)

OTOH, having a get_canonical_key method or similar seens trivial enough- if the only provided method retrieves the value as well, it would not be that bad.

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, .transformkey, then problem solved.

-- Ethan

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