def save(self, filename=None):
        filename = Path(filename) if filename is not None else self.filename
        with filename.open("w") as f:
            f.write(self.yaml)
def test_brewster_angle():
    m1 = Medium('m1', n=1)
    m2 = Medium('m2', n=1.33)
    assert ae(brewster_angle(m1, m2), 0.93, 2)
    m1 = Medium('m1', permittivity=e0, n=1)
    m2 = Medium('m2', permittivity=e0, n=1.33)
    assert ae(brewster_angle(m1, m2), 0.93, 2)
    assert ae(brewster_angle(1, 1.33), 0.93, 2)