def test_append(self):
tree = BinaryTree(1)
self.assertIsNone(tree.left)
tree.append_node(2)
self.assertEqual(tree.left.key, 2)
tree.append_node(3)
self.assertEqual(tree.right.key, 3)
tree.append_node(2)
list_ = [1, 2, 3]
self.assertEqual([node.key for node in tree.breadth_first_traversal()],
list_)
