Sorry for the delay. I understood that the change targets the proactor event loop, and I was busy to fix annoying random bugs in this code (it's not done yet, see for example the issue #23095 for the most recent bug). Windows is not my favorite OS, I am less intersted to fix Windows specific bugs, but sometimes I try to fix them.

I still don't understand if the change is (or should be) specific to the proactor event loop or not. Antoine, can you please elaborate the rationale of your patch?

Is the "legacy" code only used on Python 3.4 and older? Is ssl.MemoryBIO always present in Python 3.5 and newer?

I would like to see benchmarks of memory BIO vs current code on Linux and Windows. Even if it would make the code simpler to always use memory BIO, I care of network performances. Maybe we may only use memory BIO for the proactor event loop?

Do you know if other applications use memory BIO for networking with SSL?

Did you try your patch on Python 3.3? On Linux and Windows?

Anyway, great job!