This is related to bpo-37417: os.sched_setaffinity doesn't properly handle errors that arise during iteration of the mask argument: Python 3.9.0a0 (heads/master:d52a83a, Jun 26 2019, 15:13:41) [GCC 5.4.0 20160609] on linux Type "help", "copyright", "credits" or "license" for more information. >>> import os >>> bad_iter = map(int, "0X") >>> os.sched_setaffinity(0, bad_iter) ValueError: invalid literal for int() with base 10: 'X' The above exception was the direct cause of the following exception: Traceback (most recent call last): File "<stdin>", line 1, in <module> SystemError: <built-in function sched_setaffinity> returned a result with an error set It looks like this bug is also present on all versions of Python 3. I've attached a patch with a fix and a regression test.