Python 3.5.2, multidict-2.0.0 from PyPI, following code fails with SIGSEGV:

from multidict import MultiDict
set() - MultiDict().keys()

If i understood correctly, error happens in PyAnySet_Check call, since set substraction is basically difference method call with type check:

if (!PyAnySet_Check(so) || !PyAnySet_Check(other)) {
    Py_INCREF(Py_NotImplemented);
    return Py_NotImplemented;
}
return set_difference(so, other);

It also can be reproduced with dict and list objects, so i think problem may be related to type checking.