doc doc - 1 year ago 89
Python Question

Multiprocessing using map

I have a list of strings and on every string I am doing some changes that you can see in

. Now, to speed this up, I split up the list into sublists using
(the number of sublists is the number of CPU cores - 1). That way I get lists that look like

What I try to achieve:

I want to do
on every of these sublists simultaneously, returning the sublists as separate lists. I want to wait until all processes finish and then join these sublists into one list. The approach below does not work.

import multiprocessing
from multiprocessing import Pool
from contextlib import closing

def readFiles():
words = []
with open("somefile.txt") as f:
w = f.readlines()
words = words + w
return words

def chunked(words, num_cpu):
avg = len(words) / float(num_cpu)
out = []
last = 0.0
while last < len(words):
out.append(words[int(last):int(last + avg)])
last += avg
return out

def wordify(chunk,wl):
wl.append([chunk[word].split(",", 1)[0] for word in range(len(chunk))])
return wl

if __name__ == '__main__':
num_cpu = multiprocessing.cpu_count() - 1
words = readFiles()
chunked = chunked(words, num_cpu)
wordlist = []
wordify(words, wordlist) # works
with closing(Pool(processes = num_cpu)) as p:, chunked, wordlist) # fails

Answer Source

You have write your code so that you're just passing a single function to map; it's not smart enough to know that your hoping it passes wordlist into the second argument of your function.

TBH partial function application is a bit clunky in Python, one way would be to use a lambda: x: wordify(x, wordlist), chunked)

Or you could use functools.partial if you prefer that syntax:

from functools import partial, wordlist), chunked)
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