如果从函数内部执行，带有“ apply_async”的多处理池不执行任何操作

Question

I'm trying to use the multiprocessing module and more partuclarly the Pool.apply_async() function.

This code works well:

import multiprocessing

def do():
    print("Foobar", flush=True)

with multiprocessing.Pool(1) as pool:
    for i in range(2):
        pool.apply_async(do)

    pool.close()
    pool.join()

The "Foobar" string is printed twice.

However, if I put this code in a function and then call this function, nothing happens. No error nor "Foobar" , the program ends silently.

import multiprocessing

def test():

    def do():
        print("Foobar", flush=True)

    with multiprocessing.Pool(1) as pool:
        for i in range(5):
            pool.apply_async(do)

        pool.close()
        pool.join()

test()

Why that? I'm using Python 3.7.3 on Linux.

Answer 1

In order to retrieve your computation results do the following change to your code.

import multiprocessing

def test():

    def do():
        print("Foobar", flush=True)

    with multiprocessing.Pool(1) as pool:
        for i in range(5):
            result = pool.apply_async(do)

            result.get()

        pool.close()
        pool.join()

test()

You will see the reason why "nothing happens".

Traceback (most recent call last):
  File "/tmp/test.py", line 17, in <module>
    test()
  File "/tmp/test.py", line 12, in test
    result.get()
  File "/usr/lib/python3.5/multiprocessing/pool.py", line 608, in get
    raise self._value
  File "/usr/lib/python3.5/multiprocessing/pool.py", line 385, in _handle_tasks
    put(task)
  File "/usr/lib/python3.5/multiprocessing/connection.py", line 206, in send
    self._send_bytes(ForkingPickler.dumps(obj))
  File "/usr/lib/python3.5/multiprocessing/reduction.py", line 50, in dumps
    cls(buf, protocol).dump(obj)
AttributeError: Can't pickle local object 'test.<locals>.do'    

Python multiprocessing.Pool relies on the pickle protocol to serialize the data to be sent to the other process. The pickle protocol can serialize only top level functions and not nested ones.

To see what can be pickled and what cannot check the documentation .