subprocess to print stderr and stdout in real-time

Say I have this program printer.py:

#!/usr/bin/env python3 import sys import time sys.stdout.write("STDOUT 1\n") time.sleep(1) sys.stderr.write("STDERR 2\n") time.sleep(1) sys.stdout.write("STDOUT 3\n") time.sleep(1) sys.stderr.write("STDERR 4\n") time.sleep(1) 

It prints to stdout and stderr to produce:

./printer.py STDOUT 1 STDERR 2 STDOUT 3 STDERR 4 

I would like to execute printer.py inside another python script, runner.py, and print in real time both stderr and stdout. The following version of runner.py does not work:

#!/usr/bin/env python3 import sys import subprocess def run_command(command): process = subprocess.Popen(command, shell=True, stdout=subprocess.PIPE) while True: output = process.stdout.readline().decode() if output == '' and process.poll() is not None: break if output: print(output.strip()) rc = process.poll() return rc rc = run_command('./printer.py') 

because it prints the stderr lines first in real-time and the stdout lines later all at once:

./runner.py STDERR 2 STDERR 4 STDOUT 1 STDOUT 3 

How can fix it to have the correct order 1, 2, 3, and 4 in real-time? The closer I could get is by using:

rc = run_command('./printer.py 1>&2') 

which is kind of ok, but I wonder whether I could make it do the proper thing and print to stdout and stderr in the same way as printer.py.

sys.stdout.flush() as suggested in comments makes no difference:

#!/usr/bin/env python3 import sys import subprocess def run_command(command): process = subprocess.Popen(command, shell=True, stdout=subprocess.PIPE) while True: output = process.stdout.readline().decode() if output == '' and process.poll() is not None: break if output: sys.stdout.write(output.strip() + '\n') sys.stdout.flush() rc = process.poll() return rc rc = run_command('./printer.py') 

./runner.py STDERR 2 STDERR 4 STDOUT 1 STDOUT 3 

The same for print(..., flush=True). Am I doing something wrong?