Question

this is the idea. I’ll have ‘main’ python script that will start (using subprocess) app1 and app2. ‘main’ script will send input to app1 and output result to app2 and vice versa (and main script will need to remember what was sent so I can’t send pipe from app1 to app2).

This is main script.

import subprocess
import time

def main():
    prvi = subprocess.Popen(['python', 'random1.py'], stdin = subprocess.PIPE , stdout = subprocess.PIPE, stderr = subprocess.STDOUT)

    while 1:
        prvi.stdin.write('131231\n')
        time.sleep(1) # maybe it needs to wait
        print "procitano", prvi.stdout.read()

if __name__ == '__main__':
    main()

And this is ‘random1.py’ file.

import random

def main():
    while 1:
        inp = raw_input()
        print inp, random.random()
if __name__ == '__main__':
    main()

First I’ve tried with only one subprocess just to see if it’s working. And it’s not. It only outputs ‘procitano’ and waits there.
How can I read output from ‘prvi’ (without communicate(). When I use it, it exits my app and that’s something that I don’t want)?

Answer 1

Add prvi.stdin.flush() after prvi.stdin.write(...).

Explanation: To optimize communication between processes, the OS will buffer 4KB of data before it sends that whole buffer to the other process. If you send less data, you need to tell the OS “That’s it. Send it now” -> flush()

[EDIT] The next problem is that prvi.stdout.read() will never return since the child doesn’t exit.

You will need to develop a protocol between the processes, so each knows how many bytes of data to read when it gets something. A simple solution is to use a line based protocol (each “message” is terminated by a new line). To do that, replace read() with readline() and don’t forget to append \n to everything you send + flush()