How to give input multiple times to another program on python

Question

I want to make python file which opens two programs. This two programs have to get input from each other multiple times. I opened two programs and know how to give input to one

Answer 1

To exchange messages via pipes dynamically (not tested):

#!/usr/bin/env python3
from subprocess import Popen, PIPE

with Popen('p1', stdin=PIPE, stdout=PIPE, universal_newlines=True) as p1, \
     Popen('p2', stdin=PIPE, stdout=PIPE, universal_newlines=True) as p2:
    # give_input(n)
    print(n, file=p1.stdin, flush=True)
    print(n, file=p2.stdin, flush=True)
    for i in range(n):
        # t_1 = get_output(t_2) //give input t_2, and get output t_1
        print(p1.stdout.read(1), file=p2.stdin, flush=True)
        # t_2 = get_output(t_1) //give input t_1, and get output t_2
        print(p2.stdout.read(1), file=p1.stdin, flush=True)

It assumes that the child processes expect a line as a request and return a single character as a response.

To make it work:

1. p1, p2 should disable their internal stdout buffering (callsetvbuf(stdout, NULL, _IONBF, 0); at the start of the program) or call fflush() after each printf(). Related: Python C program subprocess hangs at “for line in iter”
2. Put the directory with p1, p2 into $PATH or pass a full path instead 'p1' into Popen().