Question

Im trying to run a single external program with multiple arguments in python. For that I use something like

for i in range(10):
    subprocess.Popen(["./foo", i])

The processes are completly independent. Nevertheless, they run only on a single core on my Mac OS X. Is there a simple way to distribute the processes over all cores (4 in my case)?

Answer 1

Below is an example of a program that uses multiprocessing to convert several pictures in parallel;

"""Convert DICOM files to PNG format, remove blank areas. The blank erea
   removal is based on the image size of a Philips flat detector. The image
   goes from 2048x2048 pixels to 1574x2048 pixels."""

import os
import sys
import subprocess
from multiprocessing import Pool, Lock

globallock = Lock()

def checkfor(args):
    """Make sure that a program necessary for using this script is
    available."""
    if isinstance(args, str):
        args = args.split()
    try:
        f = open('/dev/null')
        subprocess.call(args, stderr=subprocess.STDOUT, stdout=f)
        f.close()
    except:
        print "Required program '{}' not found! exiting.".format(args[0])
        sys.exit(1)

def processfile(fname):
    """Use the convert(1) program from the ImageMagick suite to convert the
       image and crop it."""
    size = '1574x2048'
    args = ['convert', fname, '-units', 'PixelsPerInch', '-density', '300',
            '-crop', size+'+232+0', '-page', size+'+0+0', fname+'.png']
    rv = subprocess.call(args)
    globallock.acquire()
    if rv != 0:
        print "Error '{}' when processing file '{}'.".format(rv, fname)
    else:
        print "File '{}' processed.".format(fname)
    globallock.release()

def main(argv):
    """Main program.

    Keyword arguments:
    argv -- command line arguments
    """
    if len(argv) == 1:
        path, binary = os.path.split(argv[0])
        print "Usage: {} [file ...]".format(binary)
        sys.exit(0)
    checkfor('convert')
    p = Pool()
    p.map(processfile, argv[1:])
    p.close()


## This is the main program ##
if __name__ == '__main__':
    main(sys.argv)