Passing arguments to a function from list using Threads in python

Question

I am trying to pass arguments to a function from a list using threads in python (Note:The values to the function should not be hard-coded and all the elements of the list will be passed.) Please look at the sample code here:

from threading import Thread
list=['john','doe','srav','dev','app']
def m1(name,prefix):
    for ele in range(2):
        print(name +prefix)

def main1():
    t1=Thread(target=m1,args=('abcd','john'))
    t2 = Thread(target=m1, args=('abcd', 'doe'))
    t1.start()
    t2.start()

if __name__ == '__main__':
    main1()

Here i hard-coded the values to the function ('john','doe')instead of that pass from the list and all the elements will be passed.

Looks like you want an executor. It will let you map the function to each list element in a separate thread. — Mad Physicist
– Mad Physicist, Commented Nov 8, 2017 at 7:39
Mad, can you provide sample code to do that i am new to python — user8904897
– user8904897, Commented Nov 8, 2017 at 7:43

Mad Physicist · Accepted Answer · 2017-11-08 19:16:58Z

Your tasks are well suited for use with the concurrent.futures module. In particular, Executor.map applies a function to the elements of an iterable.

You can use something similar to the ThreadPoolExecutor example in the docs:

from concurrent.futures import ThreadPoolExecutor
from itertools import repeat

names = ['john', 'doe', 'srav', 'dev', 'app']
def m1(name, prefix):
    for _ in range(2):
        print(name + prefix)

with ThreadPoolExecutor(2) as executor:
    executor.map(m1, repeat('abcd', len(names)), names)

If you find the repeat syntax to be awkward, you have a couple of alternatives:

with ThreadPoolExecutor(2) as executor:
    for name in names:
        executor.submit(m1, 'abcd', name)

OR

with ThreadPoolExecutor(2) as executor:
    executor.map(lambda name: m1('abcd', name), names)

In all cases, the with block will implicitly call executor.shutdown, which will wait for all the tasks to complete.

As a rule, don't call a variable list: it will shadow the name of the builtin class.

xpa1492 · Accepted Answer · 2017-11-08 07:56:38Z

I am not sure if I completely understand what you are looking to achieve. My understanding is that you want to start a separate thread for each value in your list, so each value gets processed "in parallel". I modified your main1 function to do so:

def main1():
  threads = [Thread(target=m1,args=('abcd',x)) for x in list]
  for thread in threads: thread.start()
  for thread in threads: thread.join()

threads = [Thread(target=m1,args=('abcd',x)) for x in list] creates a separate thread for each value in the list.

for thread in threads: thread.start() starts each thread.

for thread in threads: thread.join() makes sure each thread is finished before returning from the main1 function (if you would rathee return immediately, just delete this line).

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