code: def test_singleton(self):
      G = nx.DiGraph()
      G.add_node(0)
      assert nx.dominance_frontiers(G, 0) == {0: set()}
      G.add_edge(0, 0)
      assert nx.dominance_frontiers(G, 0) == {0: set()}
  
imports: import pytest
  import networkx
label_desc: Use image node_id 2 for calling the TestDominanceFrontiers obj's underlying member method code with example usage: obj.test_singleton() without return types