Antoine Pitrou writes:
ExitStack was quite a new thing, API-wise. The proposal here is to generalize something which already exists in various forms all over the Internet, and respecting a well-known API, MutableMapping.
What Nick said was "I was too close to the design, and time and a very few good comments greatly improved the product." That's worth considering as generic advice rather than for the specifics of the case he faced compared to yours.
Which modules in the stdlib would benefit from rewriting using "transformdict"? How about on PyPI?