Tim Peters added the comment:
I'll take a crack at these ancient comments ;-)
"""
daemon
The process’s daemon flag, a Boolean value. This must be set before start()
is called.
The initial value is inherited from the creating process. [1]
When a process exits, it attempts to terminate all of its daemonic child
processes. [2]
[1] this sentence is weird: since daemonic processes are not allowed to have
children, isn't this flag always False in a new Process instance ?
"""
Yes. I agree it's confusing; looks like it was pasted from the `threading`
docs a long time ago, in which context it had a non-degenerate meaning.
I've personally found this `multiprocessing` restriction (daemonic processes
can't have children) to be nothing but a pain in the ass, especially since
`Pool` worker processes are (although this doesn't appear to be documented)
created as daemonic processes. (Note that
`concurrent.futures.ProcessPoolExecutor` doesn't have this restriction.)
"""
[2] typo, it meant "all of its NON-daemonic child processes" instead, didn't it
?
"""
No, this was correct. When a process exits, it waits to join its non-daemonic
children, but brutally terminates its daemonic children midstream. For
example, run this:
import multiprocessing as mp
from time import sleep
def e(tag):
for i in range(10):
print(tag, i)
sleep(1)
if __name__ == '__main__':
p = mp.Process(target=e, args=("daemon",), daemon=True)
q = mp.Process(target=e, args=("normal",))
p.start()
q.start()
sleep(5)
You'll see that, after 5 seconds, the main process exits and terminates the
daemonic process immediately, but waits for the non-daemonic process to finish:
daemon 0
normal 0
daemon 1
normal 1
daemon 2
normal 2
daemon 3
normal 3
daemon 4
normal 4
normal 5
normal 6
normal 7
normal 8
normal 9
----------
nosy: +tim.peters
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<http://bugs.python.org/issue5906>
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