process.stdout.readline() hangs. How to use it properly?

Question

I want to repeatedly send requests to process standard input and receive responses from standard output without calling subprocess mul

Answer 1

You need to use communicate to interact with your sub-process (https://docs.python.org/2/library/subprocess.html#subprocess.Popen.communicate). Here is an updated version of your main code:

import subprocess
p = subprocess.Popen(['python','subproc.py'],stdin=subprocess.PIPE,stdout=subprocess.PIPE)

while True:
    s = raw_input('Enter message:')
    response, _ = p.communicate(s)
    if response!= '':
        print "Process response:", response
    else:
        break

You also have to be careful that while you have a loop in your main code your subproc code only runs once. Only the first iteration will correctly receive a response the second communicate call will raise an exception as the stdin file of the subprocess will be closed by then.