Python: Using popen poll on background process

Question

I am running a long process (actually another python script) in the background. I need to know when it has finished. I have found that Popen.poll() always returns 0

Answer 1

You don't need to use the shell backgrounding & syntax, as subprocess will run the process in the background by itself

Just run the command normally, then wait until Popen.poll returns not None

import time
import subprocess

p = subprocess.Popen("sleep 30", shell=True)
# Better: p = subprocess.Popen(["sleep", "30"])

# Wait until process terminates
while p.poll() is None:
    time.sleep(0.5)

# It's done
print("Process ended, ret code:", p.returncode)