def testMapValidAfterFieldCleared(self):
    # Map needs to work even if field is cleared.
    # For the C++ implementation this tests the correctness of
    # MapContainer::Release()
    msg = map_unittest_pb2.TestMap()
    int32_map = msg.map_int32_int32

    int32_map[2] = 4
    int32_map[3] = 6
    int32_map[4] = 8

    msg.ClearField('map_int32_int32')
    self.assertEqual(b'', msg.SerializeToString())
    matching_dict = {2: 4, 3: 6, 4: 8}
    self.assertMapIterEquals(int32_map.items(), matching_dict)