python - AttributeError: Can't get attribute 'cube' on <module '__main__' (built-in)> when using pool.apply function from multiprocessing module

Question

I was running this code piece in python terminal, and it gave me the following error: AttributeError: Can't get attribute 'cube' on <module '__main__' (built-in)> The code and the error message are as the following:

[Clang 10.0.0 ] :: Anaconda, Inc. on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def cube(x):
...     return x**3
... 
>>> import multiprocessing as mp
>>> pool = mp.Pool(processes=4)
>>> res = [pool.apply(cube, args=(x,)) for x in range(1,7)]
Process SpawnPoolWorker-1:
Traceback (most recent call last):
  File "/opt/anaconda3/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
    self.run()
  File "/opt/anaconda3/lib/python3.8/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/opt/anaconda3/lib/python3.8/multiprocessing/pool.py", line 114, in worker
    task = get()
  File "/opt/anaconda3/lib/python3.8/multiprocessing/queues.py", line 358, in get
    return _ForkingPickler.loads(res)
AttributeError: Can't get attribute 'cube' on <module '__main__' (built-in)>

Now, after searching this link: multiprocessing error "AttributeError: Can't get attribute 'testfuncxx' on <module '__main__", I update the above code as the following and run it through the text editor Atom, import multiprocessing, time

def cube(num):
    time.sleep(num)
    print(num**3)

def apply_async_callback():
    pool = multiprocessing.Pool(processes=4)
    for i in range(10):
        pool.apply_async(cube, args=(i,))
    pool.close()   # close the use of pool, to stop inserting processes into this pool
    pool.join()    # continue processing each subprocesses, until this subprocess has been finished

if __name__=='__main__':
    apply_async_callback()

It works. I was wondering, could anyone please explain to me why is it? Moreover, did I understand the difference between pool.close() and pool.join() function correctly?

Answer 1

等待大神答复