How to kill a python child process created with subprocess.check_output() when the parent dies?

Yes, you can achieve this by two methods. Both of them require you to use Popen instead of check_output. The first is a simpler method, using try..finally, as follows:

from contextlib import contextmanager

@contextmanager
def run_and_terminate_process(*args, **kwargs):
try:
    p = subprocess.Popen(*args, **kwargs)
    yield p        
finally:
    p.terminate() # send sigterm, or ...
    p.kill()      # send sigkill

def main():
    with run_and_terminate_process(args) as running_proc:
        # Your code here, such as running_proc.stdout.readline()

This will catch sigint (keyboard interrupt) and sigterm, but not sigkill (if you kill your script with -9).

The other method is a bit more complex, and uses ctypes' prctl PR_SET_PDEATHSIG. The system will send a signal to the child once the parent exits for any reason (even sigkill).

import signal
import ctypes
libc = ctypes.CDLL("libc.so.6")
def set_pdeathsig(sig = signal.SIGTERM):
    def callable():
        return libc.prctl(1, sig)
    return callable
p = subprocess.Popen(args, preexec_fn = set_pdeathsig(signal.SIGTERM))

Your problem is with using subprocess.check_output - you are correct, you can't get the child PID using that interface. Use Popen instead:

proc = subprocess.Popen(["ls", "-l"], stdout=PIPE, stderr=PIPE)

# Here you can get the PID
global child_pid
child_pid = proc.pid

# Now we can wait for the child to complete
(output, error) = proc.communicate()

if error:
    print "error:", error

print "output:", output

To make sure you kill the child on exit:

import os
import signal
def kill_child():
    if child_pid is None:
        pass
    else:
        os.kill(child_pid, signal.SIGTERM)

import atexit
atexit.register(kill_child)