Python multiprocessing - return values from 3 different functions [duplicate]

Question

I need to execute 3 functions in parallel and retrieve a value from each of them.

Here my code:

def func1():
   ...
    return x

def func2():
   ...
    return y

def func3():
   ...
    return z


p1 = Process(target=func1)
first = p1.start()
p2 = Process(target=func2)
second= p2.start()
p3 = Process(target=func3)
third = p3.start()
p1.join()
p2.join()
p3.join()

but first, second and third seems to be 'NoneType' objects.

What's wrong in my code?

Thanks

Answer 1

There are a few different ways to solve this. The simplest one is to use a multiprocessing.Pool and the apply_async function:

from multiprocessing import Pool

def func1():
    x = 2
    return x

def func2():
    y = 1
    return y

def func3():
    z = 5
    return z

if __name__ == '__main__':
    with Pool(processes=3) as pool:
        r1 = pool.apply_async(func1, ())
        r2 = pool.apply_async(func2, ())
        r3 = pool.apply_async(func3, ())

        print(r1.get(timeout=1))
        print(r2.get(timeout=1))
        print(r3.get(timeout=1))

The multiprocessing.Pool is a rahter helpful construct that takes care of the underlying communication between processes, by setting up pipes and queues and what else is needed. The most common use case is to use it together with different data to the same function (distributing the work) using the .map function. However, it can also be used for different functions, by e.g. the .apply_async construct like I am doing here.

This, however, does not work from the interpreter but must be stored as as .py file and run using python filename.py.