It depends what you mean by 'undefined behavior'. The GIL makes operations atomic on the bytecode instruction level. Critically, this includes loading and stori

It depends what you mean by 'undefined behavior'. The GIL makes operations atomic on the bytecode instruction level. Critically, this includes loading and storing of objects, meaning that refcounting is atomic. However, this doesn't extend to most other operations, which generally need to pull an object onto the stack, manipulate it, and store it back in separate opcodes. So with Python concurrency, you can get unpredictable behavior (such as two threads losing values when incrementing a counter), but not undefined behavior in the C sense, such as use-after-free.