code: def test_onion_digraph(self):
      G = nx.complete_graph(3)
      G = nx.DiGraph(G)
      G.add_edge(2, 1)
      with pytest.raises(
          nx.NetworkXNotImplemented,
          match="not implemented for directed type",
      ):
          nx.onion_layers(G)
  
imports: import pytest
  import networkx
  from networkx.utils import nodes_equal
label_desc: Use image node_id 24 for calling the TestCore obj's underlying member method code with example usage: obj.test_onion_digraph() without return types