The gather function performs the inverse of the scatter functionality. In this case, all processes send data to a root process that collects the data received. The gather function implemented in mpi4py is, as follows:
recvbuf = comm.gather(sendbuf, rank_of_root_process)
Here, sendbuf is the data that is sent and rank_of_root_process represents the process receiver of all the data:
Gathering data from processes 1, 2, 3, 4
In the following example, we wanted to represent just the condition shown in the preceding figure. Each process builds its own data that is to be sent to the root processes that are identified with the rank zero:
from mpi4py import MPI
comm = MPI.COMM_WORLD
size = comm.Get_size()
rank = comm.Get_rank()
data = (rank+1)**2
data = comm.gather(data, root=0)
if rank == 0:
print ("rank = %s " %rank +\
"...receiving data to other process")
for i in range(1,size):
data[i] = (i+1)**2
value = data[i]
...