code: def test_single_node_graph(self):
      G = nx.DiGraph()
      G.add_node(0)
      assert nx.johnson(G) == {0: {0: [0]}}
  
imports: import pytest
  import networkx
  from networkx.utils import pairwise
label_desc: Use image node_id 1 for calling the TestJohnsonAlgorithm obj's underlying member method code with example usage: obj.test_single_node_graph() without return types