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