Well I didn't think of this myself. It's literally what the python official doc says: > A global interpreter lock (GIL) is used internally to ensure that only

Well I didn't think of this myself. It's literally what the python official doc says: > A global interpreter lock (GIL) is used internally to ensure that only one thread runs in the Python VM at a time. In general, Python offers to switch among threads only between bytecode instructions; how frequently it switches can be set via sys.setswitchinterval(). Each bytecode instruction and therefore all the C implementation code reached from each instruction is therefore atomic from the point of view of a Python program. https://docs.python.org/3/faq/library.html#what-kinds-of-global-value-mutation-are-thread-safe https://docs.python.org/3/faq/library.html#what-kinds-of-glo... If this is not the case please let the official python team know their documentation is wrong. It indeed does state that if Py_DECREF is invoked the bets are off. But a ton of operations never do that.