"Fire and forget" a process from a Python script

Question

How do I start a process (another Python script, for example) from a Python script so the "child" process is completely detached from the "parent", so the parent can a) continue on its merry way without waiting for child to finish and b) can be terminated without terminating the child process?

Parent:

import os

print "Parent started"
os.system("./child.py")
print "Parent finished"

Child:

import time

print "Child started"
time.sleep(10)
print "Child finished"

Running parent.py prints:

Parent started
Child started
Child finished
Parent finished

What I want it to print:

Parent started
Child started
Parent finished
(seconds later)
Child finished

Answer 1

Since you mentioned os.system, I think it's worth to mention that you should have used os.spawn* with mode P_NOWAIT to achieve the "forget" part.

But subprocess module provides replacements for os.system, os,spawn*,etc so you should use that instead like so

import subprocess
p = subprocess.Popen("./child.py")
print "pid = ", p.pid

See Replacing os.spawn with subprocess.Popen

As I explained in the comments both processes parent.py and child.py are still on the same process group and therefore the terminal will forward signals (like Ctrl-C) to all process in the foreground process group so both will get killed when you Ctrl-C. So if you don't want that you can force child.py to be in a new process group with the following:

#!/usr/bin/env python
import subprocess
import time
import os
p = subprocess.Popen("./child.py", preexec_fn=os.setsid)
print "pid = ", p.pid
time.sleep(30) # Ctrl-C at this point will not kill child.py
print "parent exit"