Hello,
Following the python-dev discussion, I've written a PEP to recap the
proposal and the various arguments. It's inlined below, and it will
probably appear soon at http://www.python.org/dev/peps/pep-0455/, too.
Whenever I read a discussion about the dict, I always wonder whether
the same thing applies to a set. Have you considered the utility of
a TransformSet? Or is it YAGNI?