code: def test_get_roots(self):
      self.graph.add_edges_from(
          [
              ("A", "B"),
              ("B", "C"),
              ("B", "D"),
              ("D", "E"),
              ("D", "F"),
              ("A", "G"),
          ]
      )
      self.assertEqual(["A"], self.graph.get_roots())
      self.graph.add_edge("H", "G")
      self.assertEqual(
          sorted(["A", "H"]), sorted(self.graph.get_roots())
      )
  
imports: import unittest
  import networkx
  import numpy
  import pgmpy.tests.help_functions
  from pgmpy.base import DAG, PDAG
label_desc: Use image node_id 19 for calling the TestDAGCreation obj's underlying member method code with example usage: obj.test_get_roots() without return types