make-zombies.py

#!/usr/bin/env python
"""Make zombie processes that are garbage collected."""
import gc
import os
import time
from subprocess import Popen

def check_zombies(expected_nzombies):
    #NOTE: don't use Popen() here
    output = os.popen(r"ps aux | grep Z | grep -v grep").read()
    print(output)
    got_nzombies = len(output.splitlines()) - 1
    assert expected_nzombies == got_nzombies

check_zombies(0) # no zombies

# make zombies
n = 3
for _ in range(n):
    Popen("sleep 1", shell=True) # no references are kept

check_zombies(0) # no zombies

time.sleep(2) # wait for children to terminate and become zombies

check_zombies(n) # zombies.

unreachable_count = gc.collect() # collect garbage
assert unreachable_count == 0

check_zombies(n) # zombies!

print("if you see it then subprocess doesn't reap children automatically.")