Multiprocessing AsyncResult.get() hangs in Python 3.7.2 but not in 3.6

Question

I'm trying to port some code from Python 3.6 to Python 3.7 on Windows 10. I see the multiprocessing code hang when calling .get() on the AsyncResult object. The code in question is much more complicated, but I've boiled it down to something similar to the following program.

import multiprocessing


def main(num_jobs):
    num_processes = max(multiprocessing.cpu_count() - 1, 1)
    pool = multiprocessing.Pool(num_processes)

    func_args = []
    results = []

    try:
        for num in range(num_jobs):
            args = (1, 2, 3)
            func_args.append(args)
            results.append(pool.apply_async(print, args))

        for result, args in zip(results, func_args):
            print('waiting on', args)
            result.get()
    finally:
        pool.terminate()
        pool.join()


if __name__ == '__main__':
    main(5)

This code also runs in Python 2.7. For some reason the first call to get() hangs in 3.7, but everything works as expected on other versions.

Answer 1

I think this is a regression in Python 3.7.2 as described here. It seems to only affect users when running in a virtualenv.

For the time being you can work-around it by doing what's described in this comment on the bug thread.

import _winapi
import multiprocessing.spawn
multiprocessing.spawn.set_executable(_winapi.GetModuleFileName(0))

That will force the subprocesses to spawn using the real python.exe instead of the one that's in the virtualenv. So, this may not be suitable if you're bundling things into an exe with PyInstaller, but it works OK when running from the CLI with local Python installation.