Understanding subprocess module in python when running program from python script

Question

I usually run a program from my OpenSuse linux terminal by typing ./run file_name . This will bring up a series of options that I can choose from by typing a numeric value 0-9 and hitting return on my keyboard. Now I want to do this from a python script automatically. My example below is not working, but I can't understand where I'm failing and how to debug:

import subprocess
p = subprocess.Popen(["/path/to/program/run", file_name], stdin = subprocess.PIPE,stdout=subprocess.PIPE,shell=False)
print "Hello"
out, err = p.communicate(input='0\r\n')
print out
print err
for line in p.stdout.readlines():
    print line

The output of this program is just

>> Hello
>>

ie then it seems to freeze (I have no idea whats actually happening!) I would have expected to see what I see when I run ./run file_name and hit 0 and then return directly in my terminal, but I assure you this is not the case. What can I do to debug my code?

Edit 1: as suggested in comments

import subprocess

fileName = 'test_profile'
p = subprocess.Popen(["/path/to/program/run", fileName], stdin = subprocess.PIPE,stdout=subprocess.PIPE,shell=False)
print "Hello"

for line in iter(p.stdout.readline,""):
    print line

will indeed return the stdout of my program!

Answer 1

communicate waits for the completion of the program. For example:

import subprocess
p = subprocess.Popen(["cut", "-c2"], stdin=subprocess.PIPE, stdout=subprocess.PIPE,shell=False)
out, err = p.communicate(input='abc')
print("Result: '{}'".format(out.strip()))

# Result: 'b'

It sounds like you have a more interactive script, in which case you probably should try out pexpect

import pexpect

child = pexpect.spawn('cut -c2')
child.sendline('abc')
child.readline() # repeat what was typed
print(child.readline()) # prints 'b'