[issue18986] Add a case-insensitive case-preserving dict

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Here is an alternative C implementation. It adds to the dict class support of the __transform__() method. If this method is defined in dict subclass it used to transforming keys. collections.TransformDict is just utilizes this feature as collections.defaultdict utilizes __missing__(). This patch changes twice less C code than previous one (227 vs 474 lines).

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Added file: http://bugs.python.org/file31749/dict__transform__.patch

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