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# to run: mpirun -np 4 python mpi_example.py

from mpi4py import MPI

import numpy as np

from netCDF4 import Dataset

rank = MPI.COMM_WORLD.rank # The process ID (integer 0-3 for 4-process run)

nc = Dataset('parallel_test.nc', 'w', parallel=True, comm=MPI.COMM_WORLD,

info=MPI.Info())

# below should work also - MPI_COMM_WORLD and MPI_INFO_NULL will be used.

#nc = Dataset('parallel_test.nc', 'w', parallel=True)

d = nc.createDimension('dim',4)

v = nc.createVariable('var', np.int, 'dim')

v[rank] = rank

# switch to collective mode, rewrite the data.

v.set_collective(True)

v[rank] = rank

nc.close()

# reopen the file read-only, check the data

nc = Dataset('parallel_test.nc', parallel=True, comm=MPI.COMM_WORLD,

info=MPI.Info())

assert rank==nc['var'][rank]

nc.close()

# reopen the file in append mode, modify the data on the last rank.

nc = Dataset('parallel_test.nc', 'a',parallel=True, comm=MPI.COMM_WORLD,

info=MPI.Info())

if rank == 3: v[rank] = 2*rank

nc.close()

# reopen the file read-only again, check the data.

# leave out the comm and info kwargs to check that the defaults

# (MPI_COMM_WORLD and MPI_INFO_NULL) work.

nc = Dataset('parallel_test.nc', parallel=True)

if rank == 3:

assert 2*rank==nc['var'][rank]

else:

assert rank==nc['var'][rank]

nc.close()