How to get the result of multiprocessing.Pool.apply_async

Question

I want to get the result of the function run by Pool.apply_async in Python.

How to assign the result to a variable in the parent process? I tried to use callback but it seems complicated.

Answer 1

The solution is very simple:

import multiprocessing

def func():
    return 2**3**4

p = multiprocessing.Pool()
result = p.apply_async(func).get()
print(result)

Since Pool.apply_async() returns an AsyncResult, you can simply get the result from the AsyncResult.get() method.

Hope this helps!

Answer 2

Well an easy way would be to have a helper class like this:

class Result():
    def __init__(self):
        self.val = None

    def update_result(self, val):
        self.val = val

result = Result()

def f(x):
    return x*x

pool.apply_async(f, (10,), callback=result.update_result)

When the thread runs and calculates the result it will call your callback which will update the result.val.

In any case you need to check that the thread has finished.