from mpi4py import MPI

comm = MPI.COMM_WORLD
rank = comm.Get_rank()
size = comm.Get_size()

# each rank gets a number
val = rank * 10
print ("Rank %d/%d has value %d" %(rank, size, val))

# rank zero get the sum
sum = comm.reduce(val, op=MPI.SUM, root=0)
if rank==0:
    print ("Rank 0 got the sum, the total is %d" %sum)


"""outout
$ mpirun -n 4 python reduce.py
Rank 3/4 has value 30
Rank 1/4 has value 10
Rank 2/4 has value 20
Rank 0/4 has value 0
Rank 0 got the sum, the total is 60
"""