code: def test_cycle(self):
      n = 5
      G = nx.cycle_graph(n, create_using=nx.DiGraph())
      assert nx.dominance_frontiers(G, 0) == {
          i: set() for i in range(n)
      }
  
imports: import pytest
  import networkx
label_desc: Use image node_id 4 for calling the TestDominanceFrontiers obj's underlying member method code with example usage: obj.test_cycle() without return types