child.py

#!/usr/bin/env python3
"""Print a random number of lines in response to each input line.

Terminate each response with 'END'.
"""
import sys
import random


def log(msg):
    return print(msg, file=sys.stderr)


for line in sys.stdin:
    log('input: %r' % line)
    response = line * random.randint(1, 5) + 'END'
    log('response: %r' % response)
    # NOTE: flush stdout buffer to make the response available to the parent
    print(response, flush=True)

parent.py

#!/usr/bin/env python3
"""Run interactive subprocess.

Input depends on the previous output from the child process.

The child process has to flush its stdout buffer at the end of each
response.  Otherwise the code blocks. Threads, non-blocking IO won't
help here: the next input can't be sent until the previous response is
flushed by the child. The buffering in the child has to change
https://stackoverflow.com/questions/20503671/python-c-program-subprocess-hangs-at-for-line-in-iter

"""
import sys
from subprocess import Popen, PIPE

with Popen([sys.executable, 'child.py'], stdin=PIPE, stdout=PIPE,
           bufsize=1, universal_newlines=True) as p:
    s = 'a'
    while len(s) < 10: # an arbitrary condition to stop
        print(s, file=p.stdin)                         # send input
        s = ''.join(iter(p.stdout.readline, 'END\n'))  # get output