Returning multiple lists from pool.map processes?

Question

Win 7, x64, Python 2.7.12

In the following code I am setting off some pool processes to do a trivial multiplication via the multiprocessing.Pool.map() method. The output data is collected in List_1.

NOTE: this is a stripped down simplification of my actual code. There are multiple lists involved in the real application, all huge.

import multiprocessing
import numpy as np

def createLists(branches):

    firstList = branches[:] * node

    return firstList


def init_process(lNodes):

    global node
    node = lNodes
    print 'Starting', multiprocessing.current_process().name


if __name__ == '__main__':

    mgr = multiprocessing.Manager()
    nodes = mgr.list()
    pool_size = multiprocessing.cpu_count()

    branches = [i for i in range(1, 21)]
    lNodes = 10
    splitBranches = np.array_split(branches, int(len(branches)/pool_size))

    pool = multiprocessing.Pool(processes=pool_size, initializer=init_process, initargs=[lNodes])
    myList_1 = pool.map(createLists, splitBranches)

    pool.close() 
    pool.join()  

I now add an extra calculation to createLists() & try to pass back both lists.

import multiprocessing
import numpy as np

def createLists(branches):

    firstList = branches[:] * node
    secondList = branches[:] * node * 2

    return firstList, secondList


def init_process(lNodes):
    global node
    node = lNodes
    print 'Starting', multiprocessing.current_process().name


if __name__ == '__main__':

    mgr = multiprocessing.Manager()
    nodes = mgr.list()
    pool_size = multiprocessing.cpu_count()

    branches = [i for i in range(1, 21)]
    lNodes = 10
    splitBranches = np.array_split(branches, int(len(branches)/pool_size))

    pool = multiprocessing.Pool(processes=pool_size, initializer=init_process, initargs=[lNodes])
    myList_1, myList_2 = pool.map(createLists, splitBranches)

    pool.close() 
    pool.join() 

This raises the follow error & traceback..

Traceback (most recent call last):

  File "<ipython-input-6-ff188034c708>", line 1, in <module>
    runfile('C:/Users/nr16508/Local Documents/Inter Trab Angle/Parallel/scratchpad.py', wdir='C:/Users/nr16508/Local Documents/Inter Trab Angle/Parallel')

  File "C:\Users\nr16508\AppData\Local\Continuum\Anaconda2\lib\site-packages\spyder\utils\site\sitecustomize.py", line 866, in runfile
    execfile(filename, namespace)

  File "C:\Users\nr16508\AppData\Local\Continuum\Anaconda2\lib\site-packages\spyder\utils\site\sitecustomize.py", line 87, in execfile
    exec(compile(scripttext, filename, 'exec'), glob, loc)

  File "C:/Users/nr16508/Local Documents/Inter Trab Angle/Parallel/scratchpad.py", line 36, in <module>
    myList_1, myList_2 = pool.map(createLists, splitBranches)

ValueError: too many values to unpack

When I tried to put both list into one to pass back ie...

return [firstList, secondList]
......
myList = pool.map(createLists, splitBranches)

...the output becomes too jumbled for further processing.

Is there an method of collecting more than one list from pooled processes?

Answer 1

This question has nothing to do with multiprocessing or threadpooling. It is simply about how to unzip lists, which can be done with the standard zip(*...) idiom.

myList_1, myList_2 = zip(*pool.map(createLists, splitBranches))