Multithreading / Multiprocessing solution using concurrent.futures

Question

Hi I'm referencing the following question because it's similar to what I'm trying to achieve, however, I'm getting an error that I can't seem to figure out so looking for some help

Combining multithreading and multiprocessing with concurrent.futures

Here's my test code:

from concurrent.futures import ThreadPoolExecutor, ProcessPoolExecutor
import numpy as np
from os import cpu_count
from functools import partial

num_list = range(0,1000)
  
def test(x):
    x**2
             
def multithread(f, lst):
    print('Thread running')
    with ThreadPoolExecutor() as thread_executor:
        thread_executor.map(f, lst)

def multiprocesser(lst, f, n_processors=cpu_count()//2):
    chunks = np.array_split(lst, n_processors)
    with ProcessPoolExecutor(max_workers=n_processors) as mp_executor:
        mp_executor.map(partial(multithread, f), chunks)

if __name__ == '__main__':
    multiprocesser(num_list, test)

Process SpawnProcess-31:
Traceback (most recent call last):
  File "C:\Users\Test_user\Anaconda3\envs\test_env\lib\multiprocessing\process.py", line 315, in _bootstrap
    self.run()
  File "C:\Users\Test_user\Anaconda3\envs\test_env\lib\multiprocessing\process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Users\Test_user\Anaconda3\envs\test_env\lib\concurrent\futures\process.py", line 237, in _process_worker
    call_item = call_queue.get(block=True)
  File "C:\Users\Test_user\Anaconda3\envs\test_env\lib\multiprocessing\queues.py", line 122, in get
    return _ForkingPickler.loads(res)
AttributeError: Can't get attribute 'multithread' on <module '__main__' (built-in)>
Process SpawnProcess-32:
Traceback (most recent call last):
  File "C:\Users\Test_user\Anaconda3\envs\test_env\lib\multiprocessing\process.py", line 315, in _bootstrap
    self.run()
  File "C:\Users\Test_user\Anaconda3\envs\test_env\lib\multiprocessing\process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Users\Test_user\Anaconda3\envs\test_env\lib\concurrent\futures\process.py", line 237, in _process_worker
    call_item = call_queue.get(block=True)
  File "C:\Users\Test_user\Anaconda3\envs\test_env\lib\multiprocessing\queues.py", line 122, in get
    return _ForkingPickler.loads(res)
AttributeError: Can't get attribute 'multithread' on <module '__main__' (built-in)>

So I didn't specify number of threads (don't see a reason to for the threadpool executor). Having trouble understanding what the error actually means and how I can fix it. Any help would be appreciated.

Answer 1

The error probably stems from the fact that multithread() is being called incorrectly.

Try this:

from concurrent.futures import ThreadPoolExecutor, ProcessPoolExecutor
import numpy as np
from os import cpu_count
from functools import partial

num_list = range(0,1000)

def test(x):
    x**2
               
def multithread(f, lst):
    print('Thread running')
    with ThreadPoolExecutor() as thread_executor:
        thread_executor.map(f, lst)

def multiprocesser(lst, f, n_processors=cpu_count()//2):
    chunks = np.array_split(lst, n_processors)
    with ProcessPoolExecutor(max_workers=n_processors) as mp_executor:
        mp_executor.map(partial(multithread, f), chunks)

if __name__ == '__main__':
    multiprocesser(num_list, test)

Answer 2

Missing if __name__ == '__main__'

if __name__ == '__main__':
    multiprocesser(num_list, test)

Unintended recursion

When you don't block out the call to multiprocessor() , you have recursion when the subprocess runs the python script.

Safe importing of main module

The following is an example of the same type of problem from the multiprocessing docs:

https://docs.python.org/3/library/multiprocessing.html?highlight=multiprocess#the-spawn-and-forkserver-start-methods

Make sure that the main module can be safely imported by a new Python interpreter without causing unintended side effects (such a starting a new process).

For example, using the spawn or forkserver start method running the following module would fail with a RuntimeError :

 multiprocessing import Process def foo(): print('hello') p = Process(target=foo) p.start()

Instead one should protect the “entry point” of the program by using if __name__ == '__main__': as follows:

 from multiprocessing import Process, freeze_support, set_start_method def foo(): print('hello') if __name__ == '__main__': freeze_support() set_start_method('spawn') p = Process(target=foo) p.start() ```