homepage

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()