code: def testPair(self):
      for mt in mpipairtypes:
          with self.subTest(datatype=mt.name):
              dt = tonumpy(mt)
              if np_dtype is not None:
                  self.assertTrue(dt.isalignedstruct)
                  self.assertEqual(dt.itemsize, mt.extent)
      integral = "bhilqpBHILQP"
      floating = "fdg"
      vtypes = integral + floating
      itypes = integral
      for vcode, icode in itertools.product(vtypes, itypes):
          value = MPI.Datatype.fromcode(vcode)
          index = MPI.Datatype.fromcode(icode)
          pair = MPI.Datatype.Get_value_index(value, index)
          if pair == MPI.DATATYPE_NULL:
              continue
          vt, it, pt = map(tonumpy, (value, index, pair))
          dt = (f"{vt},{it}", {"align": True})
          if np_dtype is not None:
              dt = np_dtype(dt[0], **dt[1])
          self.assertEqual(pt, dt)
  
imports: from mpi4py import MPI
  from mpi4py.util.dtlib import from_numpy_dtype
  from mpi4py.util.dtlib import to_numpy_dtype
  import sys, os
  import itertools
label_desc: Use image node_id 20 for calling the TestUtilDTLib obj's underlying member method code with example usage: obj.testPair() without return types