class_docstr: Tests specific to dict-of-dict-of-dict digraph data structure
class_docstr_tok: ["Tests","specific","to","dict-of-dict-of-dict","digraph","data","structure"]
code: def test_add_edge(self):
      G = self.Graph()
      G.add_edge(0, 1)
      assert G.adj == {0: {1: {}}, 1: {}}
      assert G.succ == {0: {1: {}}, 1: {}}
      assert G.pred == {0: {}, 1: {0: {}}}
      G = self.Graph()
      G.add_edge(*(0, 1))
      assert G.adj == {0: {1: {}}, 1: {}}
      assert G.succ == {0: {1: {}}, 1: {}}
      assert G.pred == {0: {}, 1: {0: {}}}
      with pytest.raises(ValueError, match="None cannot be a node"):
          G.add_edge(None, 3)
  
imports: import pytest
  import networkx
  from networkx.utils import nodes_equal
  from .test_graph import BaseAttrGraphTester, BaseGraphTester
  from .test_graph import TestEdgeSubgraph
  from .test_graph import TestGraph
label_desc: Use image node_id 3 for calling the TestDiGraph obj's underlying member method code with example usage: obj.test_add_edge() without return types