code: def test_add_edge(self):
      G = self.Graph()
      G.add_edge(0, 1)
      assert G.adj == {0: {1: {0: {}}}, 1: {0: {0: {}}}}
      G = self.Graph()
      G.add_edge(*(0, 1))
      assert G.adj == {0: {1: {0: {}}}, 1: {0: {0: {}}}}
      G = self.Graph()
      with pytest.raises(ValueError):
          G.add_edge(None, "anything")
  
imports: from collections import UserDict
  import pytest
  import networkx
  from networkx.utils import edges_equal
  from .test_graph import BaseAttrGraphTester
  from .test_graph import TestGraph
label_desc: Use image node_id 9 for calling the TestMultiGraph obj's underlying member method code with example usage: obj.test_add_edge() without return types