Global Interpreter Lock - GIL
This is Python’s solution do dealing with the fact that the C libraries they embed into their language are not thread safe. You cannot use two python interpreters in the same multi-threaded application at one time.
It’s a horrible horrible hack.
And Python continues this fine legacy.
Not super impressed with Guido the architect of Python. His answer for this is use fork and lots of processes but that doesn’t work with windows.
Another good reason to get rid of it.