code: def appnum():
      if MPI.APPNUM == MPI.KEYVAL_INVALID:
          return None
      return MPI.COMM_WORLD.Get_attr(MPI.APPNUM)
  
file_path: mpi4py/test/test_dynproc.py
imports: from mpi4py import MPI
  import mpiunittest
label_desc: Use image node_id 3 for calling a global function with example usage: appnum() and returns: MPI, None