Just look at the bytecode. >>> import dis >>> def increment_counter(): ... counter = counter + 1 >>> dis.dis(increment_counter) 2 0 L

Just look at the bytecode. >>> import dis >>> def increment_counter(): ... counter = counter + 1 >>> dis.dis(increment_counter) 2 0 LOAD_FAST 0 (counter) 2 LOAD_CONST 1 (1) 4 BINARY_ADD 6 STORE_FAST 0 (counter) 8 LOAD_CONST 0 (None) 10 RETURN_VALUE A data race looks like LOAD_FAST from thread 1 begins STORE_FAST from thread 2 begins memory is partially overwritten LOAD_FAST from thread 1 finishes (got partially written garbage) STORE_FAST from thread 2 finishes This is not a data race: LOAD_FAST from thread 1 LOAD_FAST from thread 2 STORE_FAST from thread 1 STORE_FAST from thread 2 There's no LOAD_FAST + STORE_FAST "transaction" here, GIL has no such guarantee. Edit: You also need to realize that in Python, that BINARY_ADD can be anything: import time class SlowCounter(int): def __add__(self, other): time.sleep(1) return SlowCounter(super().__add__(other)) counter = SlowCounter() counter += 1 The GIL isn't going to wait around for the __add__.