async_proc.py

from subprocess import Popen, PIPE, STDOUT                                                          
import time                                                                                         
                                                                                                    
def process_line(line):                                                                             
    print 'New line: "%s"' % line.strip('\n')                                                                                           
                                                                                                    
proc = Popen(['bash', 'print_lines.sh'], stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)       
while True:                                                                                         
    line = proc.stdout.readline()                                                                   
    if line != '':                                                                                  
        process_line(line)                                                                          
    else:                                                                                           
        break                                                                                       

print_lines.sh

echo "Line 1"                                                                                       
sleep 1                                                                                             
echo "Line 2" 1>&2;                                                                                                                     
sleep 1                                                                                             
echo "Line 3"