When I use the subprocess module to run a another process, I expect to be able to read the STDOUT and STDERR of the other process. This is not possible on Mac OS X. The attached test program has been tested on Linux and Windows and does work as expected. It pauses at the process.stdout.readline() line of code every time on Mac OS X.

Sorry, your program works for me on macOS 10.12.3 with the current python.org 2.7.13, the Apple-supplied system Python 2.7.10, and, with adding a decode to the readline, with 3.6.0.

The program failed for me on Mac OS 10.4.11 using Python 2.7.12 and Mac OS 10.6.8 using Python 2.7.13.

There certainly could be differences in behavior considering how old 10.6.8 and 10.4.11 are.  I'm not a subprocess expert but it seems to me that, if your program hangs doing a readline from process.stdout, chances are you are running into a pipe buffer deadlock as warned about in the subprocess module documentation:

"Warning - Use communicate() rather than .stdin.write, .stdout.read or .stderr.read to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process."

https://docs.python.org/dev/library/subprocess.html#popen-objects

Using communicate fixed the problem. Here is the program that works for me on Mac OS 10.4.11 with Python 2.7.12:

import subprocess
import time

print("Launch started")
program_name = "top"
list = [program_name]
process = subprocess.Popen(list)
while process.poll() == None:
    print("subprocess still running")
    print("Value = " + process.communicate())
    time.sleep(0.1)
    
print("Exit")



Thank you Ned.

The program does not work the way I wanted it to. I want the output of the top command to be seen only by the python program. The program I made causes the top command to print its output to the terminal. Using subprocess.Popen() with stdout=subprocess.PIPE does prevent the top command's output from being printed to the terminal. Now I need a way to actually work with that output. Would you know a way to do this?

I can't use the communicate() function because it blocks until the top command quits.

Please use Stack Overflow or python-list for usage questions (and please do not reopen this issue again)