The problem¶

Say you create a zombie process and instantiate a Process for it:

import os, time

def create_zombie():
    pid = os.fork()  # the zombie
    if pid == 0:
        os._exit(0)  # child exits immediately
    else:
        time.sleep(1000)  # parent does NOT call wait()

pid = create_zombie()
p = psutil.Process(pid)

Up until psutil 2.X, every time you tried to query it you'd get a NoSuchProcess exception:

>>> p.name()
  File "psutil/__init__.py", line 374, in _init
    raise NoSuchProcess(pid, None, msg)
psutil.NoSuchProcess: no process found with pid 123

This was misleading, because the PID technically still existed:

>>> psutil.pid_exists(p.pid)
True

Depending on the platform, some process information could still be retrieved:

>>> p.cmdline()
['python']

Worst of all, psutil.process_iter() didn't return zombies at all. That was a real problem, because identifying them is a legitimate use case: a zombie usually indicates a bug where a parent process spawns a child, kills it, but never calls wait() to reap it.

What changed¶

• A new ZombieProcess exception is raised whenever a process cannot be queried because it is a zombie.
• It replaces NoSuchProcess, which was incorrect and misleading.
• ZombieProcess inherits from NoSuchProcess, so existing code keeps working.
• psutil.process_iter() now correctly includes zombie processes, so you can reliably identify them:

import psutil

zombies = []
for p in psutil.process_iter():
    try:
        if p.status() == psutil.STATUS_ZOMBIE:
            zombies.append(p)
    except psutil.NoSuchProcess:
        pass