def test_directed_edge_v_from_and_v_to(self):
""" Get a directed edge's v_from and v_to properties """
v0 = DirectedVertex(val='v0')
v1 = DirectedVertex(val='v1')
v2 = DirectedVertex(val='v2')
v3 = DirectedVertex(val='v3')
e01 = DirectedEdge((v0, v1))
self.assertEqual(e01.v_from, v0)
self.assertEqual(e01.v_to, v1)
with self.assertRaises(AttributeError):
e01.v_from = v2
with self.assertRaises(AttributeError):
e01.v_to = v3
