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.

Answer 1

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