code: def testCartcommZeroDim(self):
      comm = self.COMM
      topo = comm.Create_cart([])
      if topo == MPI.COMM_NULL:
          return
      self.assertEqual(topo.dim, 0)
      self.assertEqual(topo.dims, [])
      self.assertEqual(topo.periods, [])
      self.assertEqual(topo.coords, [])
      rank = topo.Get_cart_rank([])
      self.assertEqual(rank, 0)
      inedges, outedges = topo.inoutedges
      self.assertEqual(inedges, [])
      self.assertEqual(outedges, [])
      topo.Free()
  
imports: from mpi4py import MPI
  import mpiunittest
label_desc: Use image node_id 3 for calling the BaseTestTopo obj's underlying member method code with example usage: obj.testCartcommZeroDim() without return types