How can I recover the return value of a function passed to multiprocessing.Process? How can I recover the return value of a function passed to multiprocessing.Process? python python

How can I recover the return value of a function passed to multiprocessing.Process?


Use shared variable to communicate. For example like this:

import multiprocessingdef worker(procnum, return_dict):    """worker function"""    print(str(procnum) + " represent!")    return_dict[procnum] = procnumif __name__ == "__main__":    manager = multiprocessing.Manager()    return_dict = manager.dict()    jobs = []    for i in range(5):        p = multiprocessing.Process(target=worker, args=(i, return_dict))        jobs.append(p)        p.start()    for proc in jobs:        proc.join()    print(return_dict.values())


I think the approach suggested by @sega_sai is the better one. But it really needs a code example, so here goes:

import multiprocessingfrom os import getpiddef worker(procnum):    print('I am number %d in process %d' % (procnum, getpid()))    return getpid()if __name__ == '__main__':    pool = multiprocessing.Pool(processes = 3)    print(pool.map(worker, range(5)))

Which will print the return values:

I am number 0 in process 19139I am number 1 in process 19138I am number 2 in process 19140I am number 3 in process 19139I am number 4 in process 19140[19139, 19138, 19140, 19139, 19140]

If you are familiar with map (the Python 2 built-in) this should not be too challenging. Otherwise have a look at sega_Sai's link.

Note how little code is needed. (Also note how processes are re-used).


For anyone else who is seeking how to get a value from a Process using Queue:

import multiprocessingret = {'foo': False}def worker(queue):    ret = queue.get()    ret['foo'] = True    queue.put(ret)if __name__ == '__main__':    queue = multiprocessing.Queue()    queue.put(ret)    p = multiprocessing.Process(target=worker, args=(queue,))    p.start()    p.join()    print(queue.get())  # Prints {"foo": True}

Note that in Windows or Jupyter Notebook, with multithreading you have to save this as a file and execute the file. If you do it in a command prompt you will see an error like this:

 AttributeError: Can't get attribute 'worker' on <module '__main__' (built-in)>


matomo