Extract of the crashing test: def test_cmp(self): ... class Exc(Exception): pass class BadCmp(object): def __eq__(self, other): raise Exc s1 = slice(BadCmp()) s2 = slice(BadCmp()) self.assertEqual(s1, s1) self.assertRaises(Exc, lambda: s1 == s2) # <==== CRASH HERE ... The latest change in test_slice.py has been made in 2016: 3 years ago. Latest changes in Objects/sliceobject.c were refactoring related to header files (november 2018). The latest "significant" change was done in April 2018: "bpo-33012: Fix invalid function cast warnings with gcc 8 for METH_NOARGS". I bet more on a recent change which is not directly related to slices... like the implementation of the PEP 572? It would help to know when the crash started to occur exactly (date + time).