Not sure why this was flagged dead. If you look at many stackoverflow answers, around threading, many explicitly rely on the GIL, quoting source/documentation “proving” that it’s safe, avoiding the use of threading.Lock() and the like.

As an early python programmer, I copy pasted these types of answers. My old threaded code absolutely has these bugs. I’ve even seen code in production that has these bugs, because they’re sometimes dumb performance enhancements, rather than bugs, unless you happen to use a Python interpreter without a GIL, especially one that that doesn’t exist yet.

Great care would have to be taken to make sure the GIL was not disabled by default, for anything an existing thread touches (or some super, dynamic aware, smarts to know if it can be disabled).