code: def test_postorder_nodes(self):
      assert list(nx.dfs_postorder_nodes(self.G, source=0)) == [
          4,
          2,
          3,
          1,
          0,
      ]
      assert list(nx.dfs_postorder_nodes(self.D)) == [1, 0, 3, 2]
      assert list(nx.dfs_postorder_nodes(self.D, source=0)) == [1, 0]
  
imports: import networkx
label_desc: Use image node_id 3 for calling the TestDFS obj's underlying member method code with example usage: obj.test_postorder_nodes() without return types