code: def test_loop(self):
      g = nx.DiGraph()
      g.add_edges_from([("a", "b"), ("b", "c"), ("b", "a")])
      df = nx.dominance_frontiers(g, "a")
      assert df == {"a": set(), "b": set(), "c": set()}
  
imports: import pytest
  import networkx
label_desc: Use image node_id 11 for calling the TestDominanceFrontiers obj's underlying member method code with example usage: obj.test_loop() without return types