Demo that signals don't get spread

so_signals.py

from __future__ import print_function
import signal
import subprocess
import os
import sys
import time

me = sys.argv[1] if len(sys.argv) >= 2 else 'controller'
signal.signal(signal.SIGUSR1, lambda *args: print(me + ': SIGUSR1 caught'))

assert os.path.exists(__file__)
if me == 'controller':
    cpid = os.fork()
    if cpid == 0:
        me = 'parent'
        print('PID of ' + me + ': ' + str(os.getpid()))
        subprocess.call(['python', __file__, 'child'])
    else:
        print('PID of ' + me + ': ' + str(os.getpid()))
        time.sleep(1)
        os.kill(cpid, signal.SIGUSR1)
elif me == 'child':
    print('PID of ' + me + ': ' + str(os.getpid()))
    time.sleep(999)