I'm having trouble with the Scatter function in the MPI4Py Python module. My assumption is that I should be able to pass it a single list for the sendbuffer. However, I'm getting a consistent error message when I do that, or indeed add the other two arguments, recvbuf and root:

  File "code/step3.py", line 682, in subbox_grid
    i = mpi_communicator.Scatter(station_range, station_data)
  File "Comm.pyx", line 427, in mpi4py.MPI.Comm.Scatter (src/
mpi4py_MPI.c:44993)
  File "message.pxi", line 321, in mpi4py.MPI._p_msg_cco.for_scatter
(src/mpi4py_MPI.c:14497)
  File "message.pxi", line 232, in mpi4py.MPI._p_msg_cco.for_cco_send
(src/mpi4py_MPI.c:13630)
  File "message.pxi", line 36, in mpi4py.MPI.message_simple (src/
mpi4py_MPI.c:11904)
ValueError: message: expecting 2 or 3 items

Here is the relevant code snipped, starting a few lines above 682 mentioned above.

for station in stations
        #snip--do some stuff with station
        station_data = []
        station_range = range(1,len(station))
        mpi_communicator = MPI.COMM_WORLD
        i = mpi_communicator.Scatter(station_range, nsm)
        #snip--do some stuff with station[i]
        nsm = combine(avg, wt, dnew, nf1, nl1, wti[i], wtm, station[i].id)
        station_data = mpi_communicator.Gather(station_range, nsm)

I've tried a number of combinations initializing station_range, but I must not be understanding the Scatter argument types properly.

Does a Python/MPI guru have a clarification this?