There are many C extensions written with that assumption, but if you were relying on the GIL to avoid Python level concurrency bugs you probably didn’t, they’ll

There are many C extensions written with that assumption, but if you were relying on the GIL to avoid Python level concurrency bugs you probably didn’t, they’ll just be slightly rarer and harder to reproduce. That was certainly my experience working on TruffleRuby, and I doubt Python is very different. We found a few bugs because we didn’t have a global lock, but they were often problems that showed up in a soak test on an implementation with a GIL, just not quite as often so they had escaped notice and were likely causing real failures in production somewhere.