This code snippet shows how to wrap a concurrent.futures.Executor class to provide a limited queue size.

max_queue_size_pool.py

from threading import BoundedSemaphore
from concurrent.futures import ProcessPoolExecutor


class MaxQueuePool:
    """This Class wraps a concurrent.futures.Executor
    limiting the size of its task queue.

    If `max_queue_size` tasks are submitted, the next call to submit will block
    until a previously submitted one is completed.

    """
    def __init__(self, executor, max_queue_size, max_workers=None):
        self.pool = executor(max_workers=max_workers)
        self.pool_queue = BoundedSemaphore(max_queue_size)

    def submit(self, function, *args, **kwargs):
        """Submits a new task to the pool, blocks if Pool queue is full."""
        self.pool_queue.acquire()

        future = self.pool.submit(function, *args, **kwargs)
        future.add_done_callback(self.pool_queue_callback)

        return future

    def pool_queue_callback(self, _):
        """Called once task is done, releases one queue slot."""
        self.pool_queue.release()


if __name__ == '__main__':
    pool = MaxQueuePool(ProcessPoolExecutor, 8)
    f = pool.submit(print, "Hello World!")
    f.result()

@noxdafox great, thank you!

@noxdafox could you explain some point, pls? I need to do some work upon the result of each future. What is the best way for doing this: to add extra callable to future.add_done_callback in the custom submit method or to use smth like as_completed() in the main pipeline? Is it possible to make it non blocking the queue filling?

Sorry but this is not a place for such discussions.

Please use stackoverflow for that. Create a question stating what you are trying to achieve, what you have been doing so far and where you are having troubles/being stuck. Please tag it as multiprocessing, I usually am pretty active there.