code: def testUnion(self):
      group = MPI.Group.Union(MPI.GROUP_EMPTY, self.GROUP)
      self.assertEqual(MPI.Group.Compare(group, self.GROUP), MPI.IDENT)
      group.Free()
      group = MPI.Group.Union(self.GROUP, MPI.GROUP_EMPTY)
      self.assertEqual(MPI.Group.Compare(group, self.GROUP), MPI.IDENT)
      group.Free()
      group = MPI.Group.Union(self.GROUP, self.GROUP)
      self.assertEqual(MPI.Group.Compare(group, self.GROUP), MPI.IDENT)
      group.Free()
  
imports: from mpi4py import MPI
  import mpiunittest
label_desc: Use image node_id 4 for calling the BaseTestGroup obj's underlying member method code with example usage: obj.testUnion() without return types