Only one process prints in unix, multiprocessing python

Question

I have a script where I'm loading a file which takes a while because there is quite much data to read and to prevent the user from terminating the process I want to show some kind of loading indication. I thought this was a good opportunity to learn how to use the multiprocessing module so I wrote this example to test the module:

import time, multiprocessing

def progress():
    delay = 0.5
    while True:
        print "Loading.",
        time.sleep(delay)
        print "\b.",
        time.sleep(delay)
        print "\b.",
        time.sleep(delay)
        print "\r                        \r",

    return

def loader(filename, con):
    # Dummy loader
    time.sleep(5)
    con.send(filename)
    con.close()
    return

if __name__ == "__main__":
    parrent_con, child_con = multiprocessing.Pipe()
    filename = "main.key"

    p1 = multiprocessing.Process(target=progress)
    p2 = multiprocessing.Process(target=loader, args=(filename, child_con))

    p1.start()
    p2.start()

    data = parrent_con.recv()
    p1.terminate()
    print "\n", data

It works as I expect when I run it in windows cmd, it prints "Loading" and sequentially adds dots until the loader is complete. But in unix where I need it to work I don't get any output from progress function, process p1.

I don't know the pythonic method, but the problem is buffering. By default, Linux buffers each line, and since you are not sending \n, nothing gets updated. Try running the program with \n instead of \r. If that works, you need to figure out how to disable line buffering in python. In C, the command is setvbuf — Mark Lakata
– Mark Lakata, Commented May 15, 2015 at 6:58
I was thinking of buffering as well. Does it work if you put some sys.stdout.flush() around? — Dacav
– Dacav, Commented May 15, 2015 at 7:00

ljk321 · Accepted Answer · 2015-05-16 01:11:47Z

4

Just as Mark and Dacav suggested, buffering is the problem. Here are some possible solutions:

Using python -u to run the script

python -u will unbuffer stdout and stderr. This is the easiest solution if it's acceptable to you.

Using sys.stdout.flush

sys.stdout.flush will flush the stdout from buffer.

delay = 0.5
while True:
    print("Loading."),
    sys.stdout.flush()
    time.sleep(delay)
    print("\b."),
    sys.stdout.flush()
    time.sleep(delay)
    print("\b."),
    sys.stdout.flush()
    time.sleep(delay)
    print("\r                        \r"),
    sys.stdout.flush()
return

edited May 16, 2015 at 1:11

answered May 15, 2015 at 13:04

ljk321

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