Parallel For Loops in Python: Examples with Joblib

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WIP Alert This is a work in progress. Current information is correct but more content may be added in the future.

Tested under Python 3.x

The Python Joblib.Parallel construct is a very interesting tool to spread computation across multiple cores.

It's in cases when you need to loop over a large iterable object (list, pandas Dataframe, etc) and you think that your taks is cpu-intensive.

In rough terms, it spawns multiple Python processes and handles each part of the iterable in a separate process. Then it joins everything at the end.

Simplest possible example

from math import sqrt
from joblib import Parallel, delayed

# single-core code
sqroots_1 = [sqrt(i ** 2) for i in range(10)]

# parallel code
sqroots_2 = Parallel(n_jobs=2)(delayed(sqrt)(i ** 2) for i in range(10))

A more complex example (process a large XML file)

The function must return a value

In order to update the example above to use any function, just define it and use its name:

# Python XML Processing Module
import xml.etree.ElementTree as ET

from joblib import Parallel, delayed

FILE = 'path/to/your/file'

tree = ET.parse(FILE)
dataset = tree.getroot()

def process_node(xml_node):
    # extract some information from
    # the xml node

    return 'node information'

# n_jobs=1 means: use all available cores
element_information = Parallel(n_jobs=-1)(delayed(process_node)(node) for node in dataset)


Dialogue & Discussion