Python multiprocessing Queue NotImplementedError macOS

Question

System information

• python 3.8.7
• OS 11.1 (Big Sur)
• Python installed via brew install python@3.8

To reproduce on Big Sur and most probably older versions:

import multiprocessing as mp


if __name__ == '__main__':
    exp_queue = mp.Queue()
    print(exp_queue.qsize())

Results in:

  File "/Users/username/Library/Application Support/JetBrains/PyCharm2020.3/scratches/scratch.py", line 5, in <module>
    print(exp_queue.qsize())
  File "/usr/local/Cellar/python@3.8/3.8.7/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/queues.py", line 120, in qsize
    return self._maxsize - self._sem._semlock._get_value()
NotImplementedError

It looks like whoever wrote this in multiprocessing/queues.py line 120 is aware of the issue, but I can't find a solution somewhere:

def qsize(self):
    # Raises NotImplementedError on Mac OSX because of broken sem_getvalue()
    return self._maxsize - self._sem._semlock._get_value()

Answer 1

As Víctor Terrón has suggested in a GitHub discussion, you can use his implementation:

https://github.com/vterron/lemon/blob/d60576bec2ad5d1d5043bcb3111dff1fcb58a8d6/methods.py#L536-L573

According to the doc:

A portable implementation of multiprocessing.Queue. Because of multithreading / multiprocessing semantics, Queue.qsize() may raise the NotImplementedError exception on Unix platforms like Mac OS X where sem_getvalue() is not implemented. This subclass addresses this problem by using a synchronized shared counter (initialized to zero) and increasing / decreasing its value every time the put() and get() methods are called, respectively. This not only prevents NotImplementedError from being raised, but also allows us to implement a reliable version of both qsize() and empty().

Answer 2

Simplest solution : use multiprocessing.Manager . It provides a Queue class with qsize() implemented. You don't have to copypasta GPL-licensed code. Quick example:

import multiprocessing
import time

def worker(x, que):
    que.put(x**2)

if __name__ == '__main__':
    inputs = list(range(1000))

    pool = multiprocessing.Pool(processes=5)
    m = multiprocessing.Manager()
    q = m.Queue()
    workers = [pool.apply_async(worker, (i, q)) for i in inputs]
    while q.qsize() < len(inputs):
        time.sleep(1)

    results = [q.get() for _ in range(q.qsize())]
    assert len(results) == len(inputs)

I'm running Ventura 13.0.1 (macOS) and it works fine.