Basically this is it:

examples/python/qx.py

import subprocess

def qx(cmd):
   return subprocess.check_output(cmd, shell=True)

External code

In order to show how it works I've created a program (that happens to be in Python, but could be in any language) that prints to both STDOUT and STDERR:

examples/python/external.py

#!/usr/bin/env python

import sys

sys.stdout.write("Hello\n")
sys.stderr.write("Somthing to SDTERR\n")
sys.stdout.write("World\n")

If we run it we see the following on the screen:

examples/python/external.txt

Hello
Somthing to SDTERR
World

Capture using qx

examples/python/capture_stdout.py

import subprocess

def qx(cmd):
   return subprocess.check_output(cmd, shell=True)

out = qx('./external.py')

print("=={}==".format(out))

This will capture everything printed to STDOUT by the external code and assign it to the "out" variable.

Whatever printed to STDERR will be let through to STDERR.

This is the output:

examples/python/capture_stdout.txt

Somthing to SDTERR
==Hello
World
==

This is a blocking operation so your Python code will wait till the external program finishes to run.

qxx - capture both stdout and stderr

examples/python/qxx.py

import subprocess

def qxx(cmd):
    proc = subprocess.Popen(cmd,
               stdout = subprocess.PIPE,
               stderr = subprocess.STDOUT,
               shell = True,
    )
    stdout, stderr = proc.communicate()
    if proc.returncode != 0:
        raise Exception("Error executing {}".formart(cmd))
    return stdout

See also

examples/python/capture.py

import subprocess
import sys


def run(cmd):
    proc = subprocess.Popen(cmd,
        stdout = subprocess.PIPE,
        stderr = subprocess.PIPE,
    )
    stdout, stderr = proc.communicate()

    return proc.returncode, stdout, stderr

code, out, err = run([sys.executable, 'examples/python/run.py'])

print("out: '{}'".format(out))
print("err: '{}'".format(err))
print("exit: {}".format(code))

capture STDOUT, STDERR, and exit in python.