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

Antoine Pitrou solipsis at pitrou.net
Fri Sep 13 21:37:25 CEST 2013

On Fri, 13 Sep 2013 22:31:02 +0300 Serhiy Storchaka <storchaka at gmail.com> wrote:

13.09.13 21:40, Antoine Pitrou написав(ла): > Both are instances of a more general pattern, where a given > transformation function is applied to keys when looking them up: that > function being str.lower in the former example and the built-in > id function in the latter.

Please use str.casefold in examples. > >>> d = TransformDict(str.lower, [('Foo': 1)], Bar=2) {'Foo': 1} or [('Foo', 1)].

Ok, thanks.

> Providing a specialized container, not generic > ---------------------------------------------- > > It was asked why we would provide the generic TransformDict construct > rather than a specialized case-insensitive dict variant. The answer > is that it's nearly as cheap (code-wise and performance-wise) to provide > the generic construct, and it can fill more use cases.

Except lightweight IdentityDict which can be implemented more efficient than TransformDict(id). It doesn't need in calling the transform function, computing the hash of transformed key, comparing keys. But perhaps in many cases TransformDict(id) is enough.

That's true. But it's only important if TransformDict is the bottleneck. I doubt the memoizing dictionary is a bottleneck in e.g. the pure Python implementation of pickle or json.

> Python's own pickle module uses identity lookups for object > memoization: > http://hg.python.org/cpython/file/0e70bf1f32a3/Lib/pickle.py#l234

Also copy, json, cProfile, doctest and threadinglocal.

Thanks, will add them.

Regards

Antoine.

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