What the CPython GIL Actually Locks

The operating system scheduler is disabled while Python runs, so only one OS thread can exist in the process at all.

At most one thread executes Python bytecode at a time inside a given CPython process, even on a multi-core machine.

Any two threads that read and write the same variable are safe from race conditions, because the GIL serializes every access to it.

Every Python implementation must include a GIL, since it is defined by the language specification rather than by any single interpreter.