我不明白为什么这段代码不起作用（multiprocessing.Pipe）

Question

I'm studying Python multiprocessing Pipe. My goal is two make two independent processes, one of which sends the other a message for five time. I have no problem running it, but it just shows their PIDs and that's it. What did I wrong with this code? My environment is Windows 10(64bit) and Python 3.6.1(32bit).

import os
import multiprocessing as mp
import time

global sending_end, receiving_end 
sending_end, receiving_end = mp.Pipe()

def sender(sending_end=sending_end):
    print('SND PID: ', os.getpid() )
    for _ in range(5):
        sending_end.send('test')
        time.sleep(1)


class receiver(mp.Process):
    def __init__(self):
        mp.Process.__init__(self)

    def run(self, receiving_end=receiving_end):
        print('REC PID: ', os.getpid() )
        print( receiving_end.recv() )
        time.sleep(1)


if __name__ == '__main__':

    print('MAIN PID: ', os.getpid() )

    s = mp.Process( target = sender, args=(sending_end,) )
    s.start()

    r = receiver()
    r.start()     

    mp.freeze_support()

Answer 1

It seems you forget to call run() method of receiver class (CHILD) which inherits the multiprocessing.Process class (PARENT).

Since run() is not called explicitly, run() method of parent is called and it doesn't have your receiving value printing code. Therefore, it is giving feeling that code is not running.

Also some more things :

• Both the pipes need to be closed at the end like you close the file.
• Child class run() method need to called till the sending process is alive.

Please check the below code with above points incorporated.

Code:

import os
import multiprocessing as mp
import time

global sending_end, receiving_end
sending_end, receiving_end = mp.Pipe()


def sender(sending_end=sending_end):
    print('SND PID: ', os.getpid() )
    for i in range(5):
        sending_end.send('test_' + str(i) )
        time.sleep(1)
    print "Done from sender"
    #Closing sending pipe
    sending_end.close()


class receiver(mp.Process):
    def __init__(self):
        mp.Process.__init__(self)

    def run(self, receiving_end=receiving_end):
        print('REC PID: ', os.getpid() )
        print( "Dinesh - ",receiving_end.recv() )
        time.sleep(1)


if __name__ == '__main__':
    import sys
    print('MAIN PID: ', os.getpid() )

    s = mp.Process( target = sender, args=(sending_end,) )
    s.start()

    r = receiver()
    r.start()

    while True:
        #Checking sending process is alive or not
        if not s.is_alive():
            print "Sending process is done. Exiting"
            #Closing receiving end pipe
            receiving_end.close()
            #Closing receving process
            r.terminate()
            sys.exit()
        time.sleep(0.1)
        #Explicitly calling run method
        r.run()

    mp.freeze_support()

Output:

('MAIN PID: ', 16400)
('REC PID: ', 16400)
('REC PID: ', 12544)
('SND PID: ', 17744)
('Dinesh - ', 'test_0')
('REC PID: ', 16400)
('Dinesh - ', 'test_1')
('REC PID: ', 16400)
('Dinesh - ', 'test_2')
('REC PID: ', 16400)
('Dinesh - ', 'test_3')
('REC PID: ', 16400)
('Dinesh - ', 'test_4')
Done from sender
Sending process is done. Exiting