The mid index computation in _bisectmodule.c in both internal_bisect_right and internal_bisect_left is done with:

mid = (lo + hi) / 2; // all three variables Py_ssize_t

which is  susceptible to overflows for large arrays, which would lead to undefined behavior (and in practice almost certainly a crash with a negative index)

The fix is trivial - mid = lo + (hi - lo) / 2; - but since I'm just starting to look into the code base I may be missing some undocumented assertions that guarantee this can't happen.