Yes, you are right that the OS can schedule two Python threads to run at the same time, it's just that one of them will only run a few instructions and then block, just not any Python instructions. I hadn't really thought that through thoroughly, thanks for pointing it out.

Ah, I see now what you meant with constant locking. I interpreted your words as "constant" as in happening all the time as would be the case with fine grained locks instead of one long-lasting, global lock.