import subprocess
from time import sleep

# job is a Popen object, see the docs for other options that allow you
# to get stderr and stdout:

# https://docs.python.org/3.6/library/subprocess.html
job = subprocess.Popen(['/bin/sleep', '5'])

# the poll method checks the return code, and is 'None' if the program is
# still running
while job.poll() is None:
    print("still waiting to finish...")
    sleep(0.5)
else:
    print("All done, return status:", job.poll())