"""
Process class can also be subclassed just like threading.Thread;
Queue works like queue.Queue but for cross-process, not cross-thread
"""
import os, time, queue
from multiprocessing import Process, Queue  # process-safe shared queue
# queue is a pipe + locks/semas
class Counter(Process):
    label = ' @'

    def __init__(self, start, queue):  # retain state for use in run
        self.state = start
        self.post = queue
        Process.__init__(self)

    def run(self):  # run in newprocess on start()
        for i in range(3):
            time.sleep(1)
            self.state += 1
            print(self.label, self.pid, self.state)  # self.pid is this child's pid
            self.post.put([self.pid, self.state])  # stdout file is shared by all
        print(self.label, self.pid, '-')


if __name__ == '__main__':
    print('start', os.getpid())
    expected = 9
    post = Queue()
    p = Counter(0, post)  # start 3 processes sharing queue
    q = Counter(100, post)  # children are producers
    r = Counter(1000, post)
    p.start()
    q.start()
    r.start()
    while expected:  # parent consumes data on queue
        print('left------------>', expected)
        time.sleep(0.5)  # this is essentially like a GUI,
        try:  # though GUIs often use threads
            data = post.get(block=False)
        except queue.Empty:
            print('no data...')
        else:
            print('posted:', data)
            expected -= 1
    p.join()
    q.join()
    r.join()  # must get before join putter
    print('finish', os.getpid(), r.exitcode)  # exitcode is child exit status