Using concurrent.futures with infinite iterator and stopping criteria

Question

I am trying to parallelize a loop which uses an infinite generator as input to collect some data and stops when a certain amount of data has been received.

My implementation is something like this.

class A:
  def __iter__(self):
    i = 0
    while True:
       yield i 
       i += 1


def procpar(x):
    r = random.random()
    print('Computing x =', x)
    
    if r > 0.5
        return [2 * x]
    else:
        return [2 * x, x ** 2]


with ProcessPoolExecutor(4) as pool:
  out = []
  x = A()
  
  for res in pool.map(procpar, x):
    out.extend(res)
    if len(out) > 100:
      break

Now, when I run it, I do get this output, after which it just hangs and nothing happend.

Computing x = 1
Computing x = 6
Computing x = 2
Computing x = 3
Computing x = 4
Computing x = 5

Looking into whats going on, is that the map method is trying to unroll and generate data from the iterator x = A(), so it is stuck in an infinite loop.

Any suggestions how to avoid being stuck in infinite loop. Ofcorse, I could call the iterator x in chunks before feeding them to the process pool, but looking if someone may have better or more straightforward solution.

Answer 1

Try using multiprocessing.pool.imap instead:

from multiprocessing import Pool
import random


class A:
    def __iter__(self):
        i = 0
        while True:
            yield i
            i += 1


def procpar(x):
    r = random.random()
    print('Computing x =', x)

    if r > 0.5:
        return [2 * x]
    else:
        return [2 * x, x ** 2]


# Required for Windows:
if __name__ == '__main__':
    with Pool(4) as pool:
        out = []
        x = A()

        for res in pool.imap(procpar, x):
            out.extend(res)
            if len(out) > 100:
                break
    print(out)

Prints:

Computing x = 0
Computing x = 1
Computing x = 2
Computing x = 3
Computing x = 4
Computing x = 5
Computing x = 6
Computing x = 7
Computing x = 8
Computing x = 9
Computing x = 10
Computing x = 11
Computing x = 12
Computing x = 13
Computing x = 14
Computing x = 15
Computing x = 16
Computing x = 17
Computing x = 18
Computing x = 19
Computing x = 20
Computing x = 21
Computing x = 22
Computing x = 23
Computing x = 24
Computing x = 25
Computing x = 26
Computing x = 27
Computing x = 28
Computing x = 29
Computing x = 30
Computing x = 31
Computing x = 32
Computing x = 33
Computing x = 34
Computing x = 35
Computing x = 36
Computing x = 37
Computing x = 38
Computing x = 39
Computing x = 40
Computing x = 41
Computing x = 42
Computing x = 43
Computing x = 44
Computing x = 45
Computing x = 46
Computing x = 47
Computing x = 48
Computing x = 49
Computing x = 50
Computing x = 51
Computing x = 52
Computing x = 53
Computing x = 54
Computing x = 55
Computing x = 56
Computing x = 57
Computing x = 58
Computing x = 59
Computing x = 60
Computing x = 61
Computing x = 62
Computing x = 63
Computing x = 64
[0, 2, 1, 4, 4, 6, 8, 16, 10, 12, 14, 49, 16, 64, 18, 20, 22, 24, 144, 26, 28, 196, 30, 225, 32, 256, 34, 289, 36, 38, 361, 40, 400, 42, 441, 44, 484, 46, 529, 48, 576, 50, 625, 52, 54, 56, 58, 60, 900, 62, 961, 64, 66, 1089, 68, 1156, 70, 1225, 72, 1296, 74, 1369, 76, 78, 1521, 80, 1600, 82, 1681, 84, 1764, 86, 88, 90, 2025, 92, 2116, 94, 96, 2304, 98, 100, 102, 2601, 104, 2704, 106, 2809, 108, 110, 3025, 112, 3136, 114, 116, 3364, 118, 3481, 120, 3600, 122]