I was about to say "yes, listiter.__next__ is atomic" (it should, really), but that's not true (Hint: there is a Py_DECREF in the code...).

The script below crashes with:
    Fatal Python error: Objects/listobject.c:2774 object at 0x7f66b7a6c600 has negative ref count -1
Attached patch fixes it.


import threading
class C:
    def __del__(self):
        print("DEL")
ITER = iter([C() for x in range(100)])
def f():
    for obj in ITER:
        pass
for x in range(10):
    t = threading.Thread(target=f)
    t.start()