code: def testGetEnvelope(self):
      for dtype in datatypes:
          try:
              envelope = dtype.Get_envelope()
          except NotImplementedError:
              self.skipTest("mpi-type-get_envelope")
          if (
              "LAM/MPI" == MPI.get_vendor()[0]
              and "COMPLEX" in dtype.name
          ):
              continue
          ni, na, nc, nd, combiner = envelope
          self.assertEqual(combiner, MPI.COMBINER_NAMED)
          self.assertEqual(ni, 0)
          self.assertEqual(na, 0)
          self.assertEqual(nc, 0)
          self.assertEqual(nd, 0)
          self.assertEqual(dtype.envelope, envelope)
          self.assertEqual(dtype.combiner, combiner)
          self.assertTrue(dtype.is_named)
          self.assertTrue(dtype.is_predefined)
          otype, combiner, params = dtype.decode()
          self.assertIs(dtype, otype)
          self.assertEqual(combiner, "NAMED")
          self.assertEqual(params, {})
  
imports: from mpi4py import MPI
  import mpiunittest
  import struct
label_desc: Use image node_id 7 for calling the TestDatatype obj's underlying member method code with example usage: obj.testGetEnvelope() without return types