master.py

import subprocess
import sys
import time

proc = subprocess.Popen(['/Users/marten/.rubies/ruby-2.1.1/bin/ruby', '/Users/marten/tmp/python-processes/slave.rb'],
                        stdin=subprocess.PIPE, stdout=sys.stdout)

for i in range(5):
  print "Python preparing to send newline"
  proc.stdin.write("\n")
  time.sleep(1)

slave.rb

puts "Ruby started"

while line = gets
  puts "Ruby[#{Process.pid}]: Got a line!"
  puts "Ruby[#{Process.pid}]: /tmp/foo.png"
end