[Python-Dev] Is it a Python bug that the main thread of a process created in a daemon thread is a daemon itself?

Elizabeth Shashkova elizabeth.shashkova at gmail.com
Thu Jun 25 19:23:44 CEST 2015

Hello everybody!

When I call fork() inside a daemon thread, the main thread in the child
process has the "daemon" property set to True. This is very confusing,
since the program keeps running while the only thread is a daemon.
According to the docs, if all the threads are daemons the program should
exit. Here is an example:

import os
import threading

def child():
     assert not threading.current_thread().daemon  # This shouldn't fail

def parent():
    new_pid = os.fork()
    if new_pid == 0:
        os.waitpid(new_pid, 0)

t = threading.Thread(target=parent)

Is it a bug in the CPython implementation?
Also let's assume the second example. I have another non-daemon thread
in the child process and want to detect this situation. Does anybody
know a way to find such fake daemon threads that are really main

Best regards,
Elizaveta Shashkova.
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