Copy of my comment on the PR.

https://github.com/python/cpython/pull/1218#issuecomment-352372211

>  Merged from master... Again... Hopefully this won't end up missing 3.7 entirely... 😔

Oops sorry, I wanted this feature but I didn't follow closely the PR.

I don't know well the Windows API, so I didn't want to take the responsability of reviewing (approving) such PR. But I see that @zooba and @gpshead approved it, so I'm now confortable to merge it :-) Moreover, AppVeyor validated the PR, so let me merge it.

I prefer to merge the PR right now to not miss the Python 3.7 feature freeze, and maybe fix issues later if needed, before 3.7 final.

Thank you @segevfiner for this major subprocess enhancement. I really love to see close_fds default changing to True on Windows. It will help to fix many corner cases which are very tricky to debug.

Sorry for the slow review, but the subprocess is a critical module of Python, and we lack of Windows developers to review changes specific to Windows.