parent.py write to and read from the child.py

child.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-


while True:

    s = raw_input('Child: ')
    if not s:
        break

    print 'Child:', s

print 'End of child.'

parent.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-


import os
import subprocess


pty_master_fd, pty_slave_fd = os.openpty()
p = subprocess.Popen(
    ['python', 'child.py'],
    stdin=subprocess.PIPE,
    stdout=pty_slave_fd,
)
pty_master_f = os.fdopen(pty_master_fd)

while True:

    s = raw_input('Parent: ')

    p.stdin.write(s)
    p.stdin.write('\n')

    print '--- Output From Child ---'
    print pty_master_f.readline()
    print '--- End ---'
    print

    if not s:
        break

print 'End of parent.'

py_child_py.txt

$ py child.py
Child: 1
Child: 1
Child: 2
Child: 2
Child: 3
Child: 3
Child: ^CTraceback (most recent call last):
  File "child.py", line 7, in <module>
    s = raw_input('Child: ')
KeyboardInterrupt

py_parent_py.txt

$ py parent.py
Parent: 1
--- Output From Child ---
Child: Child: 1

--- End ---

Parent: 2
--- Output From Child ---
Child: Child: 2

--- End ---

Parent: 3
--- Output From Child ---
Child: Child: 3

--- End ---

Parent: ^CTraceback (most recent call last):
  File "child.py", line 7, in <module>
    s = raw_input('Child: ')
KeyboardInterrupt
Traceback (most recent call last):
  File "parent.py", line 20, in <module>
    s = raw_input('Parent: ')
KeyboardInterrupt

You need to add sys.stdout.flush() in the child.py, right after the print

Updated parent.py with the pty trick while child.py stays same.