import sys, os, time
from subprocess import Popen, list2cmdline

def cpu_count():
    ''' Returns the number of CPUs in the system
    '''
    num = 1
    if sys.platform == 'win32':
        try:
            num = int(os.environ['NUMBER_OF_PROCESSORS'])
        except (ValueError, KeyError):
            pass
    elif sys.platform == 'darwin':
        try:
            num = int(os.popen('sysctl -n hw.ncpu').read())
        except ValueError:
            pass
    else:
        try:
            num = os.sysconf('SC_NPROCESSORS_ONLN')
        except (ValueError, OSError, AttributeError):
            pass

    return num

def exec_commands(cmds):
    ''' Exec commands in parallel in multiple process 
    (as much as we have CPU)
    '''
    if not cmds: return # empty list

    def done(p):
        return p.poll() is not None
    def success(p):
        return p.returncode == 0
    def fail():
        sys.exit(1)

    max_task = cpu_count()
    processes = []
    while True:
        while cmds and len(processes) < max_task:
            task = cmds.pop()
            print list2cmdline(task)
            processes.append(Popen(task))

        for p in processes:
            if done(p):
                if success(p):
                    processes.remove(p)
                else:
                    fail()

        if not processes and not cmds:
            break
        else:
            time.sleep(0.05)

commands = [
    ['curl', 'http://www.reddit.com/'],
    ['curl', 'http://en.wikipedia.org/'],
    ['curl', 'http://www.google.com/'],
    ['curl', 'http://www.yahoo.com/'],
    ['curl', 'http://news.ycombinator.com/']
]
exec_commands(commands)