I like this change, and think we should go ahead with it, but just wanted to note that I suspect it may make the "Badly timed signals may lead to __exit__ being skipped even after __enter__ succeeded" problem slightly easier to hit: https://bugs.python.org/issue29988

That's not a new problem though, and this change should make it easier to apply the conventional solutions (i.e. only checking for signals when execution jumps backwards within a function, as well as in function pre-or-post-ambles)