What Am I Doing Wrong In This File With Pool.map Which Causes Nothing Appearing And I Have To Restart The Shell?

What am I missing or doing wrong in the following python file using multiprocessing? When I run it, nothing happens and I have to restart the shell! def f(x): lo=0 for i in

Solution 1:

Quoting from the multiprocessing docs:

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

In your case this looks like:

from multiprocessing import Pool

def f(x):
    lo = 0
    for i in range(x):
            lo += i
    return lo

def f_parallel(x1, x2, x3, x4):
    with Pool(processes=4) as pool:
        resulto_parallel = pool.map(f, [x1, x2, x3, x4])
    return resulto_parallel 

if __name__ == '__main__':    
    print(f_parallel(1, 2, 3, 4))

The key point is that the spawned processes will import your python script, which, if you didn't protect the call to f_parallel(1,2,3,4) would result in an endless spawning of subprocesses.

Note: Also added a print so that the result of the function call will actually be put to stdout

Solution 2:

You need to use the 'module name' idiom:

def f(x):
    lo = 0
    for i in range(x):
            lo += i
    return lo

from multiprocessing import Pool

def f_parallel(x1, x2, x3, x4):
    with Pool(processes=4) as pool:
        return pool.map(f,[x1, x2, x3, x4])

if __name__ == '__main__':
    print(f_parallel(1, 2, 3, 4))

Output:

[0, 1, 3, 6]

Without the if __name__ == '__main__': part above, multiprocessing re-reads the module (executing it) and gets into an infinite loop.