How to make sure you run the same number of function asynchronically, adding a new one when an old one finishes.

fixed_async_funcs.py

import concurrent.futures
from concurrent.futures import FIRST_COMPLETED, ALL_COMPLETED
import time
from random import random

def do_magic(title):
    time.sleep(random() * 3)
    print("Bye", title)


class MagicDoer:

    def __init__(self):
        self.futures = []
        self.executor = concurrent.futures.ThreadPoolExecutor(max_workers=4)
        self.number = 0


    def add_magic(self):
        self.number += 1
        f = self.executor.submit(do_magic, self.number)
        self.futures.append(f)


    def run(self):
        # Startup
        for x in range(4):
            self.add_magic()

        # Loop, create new ones until death
        while self.number < 10:
            waited = concurrent.futures.wait(self.futures, timeout=30, return_when=FIRST_COMPLETED)
            for d in waited.done:
                self.futures.remove(d)  # remove old

            self.add_magic()

        # Wait on the remaining Futures
        concurrent.futures.wait(self.futures, timeout=30, return_when=ALL_COMPLETED)

        print('OK')


def main():
    MagicDoer().run()


if __name__ == "__main__":
    main()