code: def test_add_nodes_from_string(self):
      self.graph = DAG()
      self.graph.add_nodes_from(["a", "b", "c", "d"])
      self.assertListEqual(
          sorted(self.graph.nodes()), ["a", "b", "c", "d"]
      )
      self.assertEqual(self.graph.latents, set())
  
      self.graph = DAG()
      self.graph.add_nodes_from(["a", "b", "c", "d"], latent=True)
      self.assertListEqual(
          sorted(self.graph.nodes()), ["a", "b", "c", "d"]
      )
      self.assertEqual(self.graph.latents, set(["a", "b", "c", "d"]))
  
      self.graph = DAG()
      self.graph.add_nodes_from(
          ["a", "b", "c", "d"], latent=[True, False, True, False]
      )
      self.assertListEqual(
          sorted(self.graph.nodes()), ["a", "b", "c", "d"]
      )
      self.assertEqual(self.graph.latents, set(["a", "c"]))
  
imports: import unittest
  import networkx
  import numpy
  import pgmpy.tests.help_functions
  from pgmpy.base import DAG, PDAG
label_desc: Use image node_id 6 for calling the TestDAGCreation obj's underlying member method code with example usage: obj.test_add_nodes_from_string() without return types