'Test len'

def test_len(self):

    test_son = SON()
    self.assertEqual(0, len(test_son))
    test_son = SON([(1, 100), (2, 200), (3, 300)])
    self.assertEqual(3, len(test_son))
    test_son.popitem()
    self.assertEqual(2, len(test_son))
