Does this change break this function?
def subtle():
    one = {-0. : 'a', -1: 'b'}
    two = {0. : 'a', -1: 'b'}
    assert all(math.copysign(1, x) < 0 for x in one)
    assert any(math.copysign(1, x) > 0 for x in two)
Perhaps you should restrict yourself to strings...