Spawning multiple processes with Python

Question

Earlier I tried to use the threading module in python to create multiple threads. Then I learned about the GIL and how it does not allow taking advantage of multiple CPU cores on a single machine. So now I'm trying to do multiprocessing (I don't strictly need seperate threads).

Here is a sample code I wrote to see if distinct processes are being created. But as can be seen in the output below, I'm getting the same process ID everytime. So multiple processes are not being created. What am I missing?

import multiprocessing as mp
import os

def pri():
    print(os.getpid())

if __name__=='__main__':

    # Checking number of CPU cores
    print(mp.cpu_count())

    processes=[mp.Process(target=pri()) for x in range(1,4)]

    for p in processes:
        p.start()

    for p in processes:
        p.join()

Output:

4
12554
12554
12554

Answer 1

The Process class requires a callable as its target.

Instead of running the function in the separate process, you are calling it and passing its result (None in this case) to the Process class.

Just change the following:

mp.Process(target=pri())

with:

mp.Process(target=pri)