The GIL doesn't really help Python code though, because the interpreter may switch threads between any two opcodes. It only protects the state of the Python in

The GIL doesn't really help Python code though, because the interpreter may switch threads between any two opcodes. It only protects the state of the Python interpreter and that of C/Cython extension modules. Though even there, you can have unexpected thread switches, e.g. in Cython `self.obj = None` can result in a thread switch if the value previously stored in `self.obj` had a `__del__` method implemented in Python. And AFAIK pretty much any Python object allocation can trigger the cycle collector which can trigger `__del__` on (completely unrelated) objects in reference cycles, so it's pretty much impossible to rely on the GIL to keep any non-trivial code block atomic.