code: def test_add_edge_string(self):
      self.graph.add_edge("d", "e")
      self.assertListEqual(sorted(self.graph.nodes()), ["d", "e"])
      self.assertListEqual(list(self.graph.edges()), [("d", "e")])
      self.graph.add_nodes_from(["a", "b", "c"])
      self.graph.add_edge("a", "b")
      self.assertListEqual(
          hf.recursive_sorted(self.graph.edges()),
          [["a", "b"], ["d", "e"]],
      )
  
imports: import unittest
  import networkx
  import numpy
  import pgmpy.tests.help_functions
  from pgmpy.base import DAG, PDAG
label_desc: Use image node_id 10 for calling the TestDAGCreation obj's underlying member method code with example usage: obj.test_add_edge_string() without return types