multithreading - Meaning of daemon property on Python Threads

Question

I'm a little confused about what setting a thread to be a daemon means.

The documentation says this:

A thread can be flagged as a “daemon thread”. The significance of this flag is that the entire Python program exits when only daemon threads are left. The initial value is inherited from the creating thread. The flag can be set through the daemon property.

I'm not sure what makes this different from a normal thread.

Is this saying that this program won't ever finish?

def threadfunc():
    while True:
        time.sleep(1)

threading.Thread(target=threadfunc).start()

Even though the main thread finishes it's execution. While will finish immediately?

def threadfunc():
    while True:
        time.sleep(1)

th = threading.Thread(target=threadfunc)
th.daemon = True
th.start()

I ask because I have a situation where in my main thread I'm calling sys.exit(), and the process just hangs and my other threads are running as I can see the log.

Does this have anything to do with sys.exit() being called with threads alive?

Can I interpret daemon=True as "this thread as a daemon of main thread"? — jtcloud

Jochen Ritzel Jochen Ritzel · Accepted Answer · 2010-12-01T23:19:19

Is this saying that this program won't ever finish?

Yes, that program won't finish, just try it out.

I ask because I have a situation where in my main thread I'm calling sys.exit(), and the process just hangs and my other threads are running as I can see the log. Does this have anything to do with sys.exit() being called with threads alive?

Yes, even exit won't stop other threads, it simply raises SystemExit in the main thread. So while the main thread will stop (just like it does on any other unhandled Exception), all other non-daemonic threads will continue to work.

multithreading - Meaning of daemon property on Python Threads

4 Answers