Try with this example:
import subprocess
proc = subprocess.Popen(['/bin/bash'], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
stdout = proc.communicate('ls -lash')
print stdout
You have to read more about stdin, stdout and stderr. This looks like good lecture: http://www.doughellmann.com/PyMOTW/subprocess/
EDIT:
Another example:
>>> process = subprocess.Popen(['/bin/bash'], shell=False, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
>>> process.stdin.write('echo it works!\n')
>>> process.stdout.readline()
'it works!\n'
>>> process.stdin.write('date\n')
>>> process.stdout.readline()
'wto, 13 mar 2012, 17:25:35 CET\n'
>>>