echo.py

while True:
  print raw_input("Echo?")

foo.py

import subprocess

process = subprocess.Popen(["python", "./echo.py"], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
for i in xrange(10):
    print i
    assert process.stdout.read(len("Echo?\n")) == "Echo?"
    message = "%d\n" % i
    process.stdin.write(message)
    assert process.stdout.read(len(message)) == message
process.terminate()