Nick,I think we've gone through enough clarifications of the PEP now to be clear on the proposal. I saw in one of your earliest replies (right after Chris posted his first draft) that you're hesitant to support the PEP because of what would have to change to contextlib. What I couldn't quite read is whether you think that the proposal by itself is not an improvement, or whether you're just worried about compatibility.
Apparently you know of a large group of users who use an older 3rd party version of contextlib, and for whom that older, 3rd party contextlib should keep working with future versions of Python 3 without updating their version of contextlib -- did I get that right? What exactly is the constraint there that makes their version of contextlib immutable even though the version of Python they are using may move forward?
Separate from this special case, I am also worried about backward compatibility, and I have yet to get a good idea for how widespread code is that depends on StopIteration bubbling out from generators. I also don't have a good idea how often this issue bites users, but I have a feeling it does bite. E.g. this quote from c.l.py (https://mail.python.org/pipermail/python-list/2014-November/680775.html):
I did find it annoying occasionally that raising StopIteration inside a
generator expression conveys a different behavior than elsewhere. It did
take me quite a while to understand why that is so, but after that it
did not cause me much of a headache anymore.