code: def testAppNum(self):
      appnum = MPI.COMM_WORLD.Get_attr(MPI.APPNUM)
      if appnum is not None:
          self.assertTrue(appnum == MPI.UNDEFINED or appnum >= 0)
  
imports: from mpi4py import MPI
  import mpiunittest
  import os
label_desc: Use image node_id 4 for calling the TestWorldAttrs obj's underlying member method code with example usage: obj.testAppNum() without return types