Python: How can I check the number of pending tasks in a multiprocessing.Pool?

Question

I have a small pool of workers (4) and a very large list of tasks (5000~). I'm using a pool and sending the tasks with map_async(). Because the task I'm running is fairly long, I'm forcing a chunksize of 1 so that one long process can't hold up some shorter ones.

What I'd like to do is periodically check how many tasks are left to be submitted. I know at most 4 will be active, I'm concerned with how many are left to process.

I've googled around and I can't find anybody doing this.

Some simple code to help:

import multiprocessing
import time

def mytask(num):
    print('Started task, sleeping %s' % num)
    time.sleep(num)

pool = multiprocessing.Pool(4)
jobs = pool.map_async(mytask, [1,2,3,4,5,3,2,3,4,5,2,3,2,3,4,5,6,4], chunksize=1)
pool.close()

while True:
    if not jobs.ready():
        print("We're not done yet, %s tasks to go!" % <somethingtogettasks>)
        jobs.wait(2)
    else:
        break

Answer 1

You can check the number of pending jobs by seeing Pool._cache attribute assuming that you are using apply_async. This is where ApplyResult is stored until they are available and equals to the number of ApplyResults pending.

import multiprocessing as mp
import random
import time


def job():
    time.sleep(random.randint(1,10))
    print("job finished")

if __name__ == '__main__':
    pool = mp.Pool(5)
    for _ in range(10):
        pool.apply_async(job)

    while pool._cache:
        print("number of jobs pending: ", len(pool._cache))
        time.sleep(2)

    pool.close()
    pool.join()

Answer 2

Looks like jobs._number_left is what you want. _ indicates that it is an internal value that may change at the whim of the developers, but it seems to be the only way to get that info.