Python daemon threads and the "with" statement

Question

If I have the following code in a daemon thread and the main thread does not do invoke a join on the daemon. Will the file close safely since it is used inside "with" once the main thread exits or no? Anyway to make it safe? Thanks :D

while True:
    with open('file.txt', 'r') as f:
        cfg = f.readlines()
time.sleep(60)

Answer 1

From the docs:

Note: Daemon threads are abruptly stopped at shutdown. Their resources (such as open files, database transactions, etc.) may not be released properly. If you want your threads to stop gracefully, make them non-daemonic and use a suitable signalling mechanism such as an Event.

This suggests, but does not outright state, that daemon threads are terminated without a chance for __exit__ methods and finally blocks to run. We can run an experiment to verify that this is the case:

import contextlib
import threading
import time

@contextlib.contextmanager
def cm():
    try:
        yield
    finally:
        print 'in __exit__'

def f():
    with cm():
        print 'in with block'
        event.set()
        time.sleep(10)

event = threading.Event()

t = threading.Thread(target=f)
t.daemon = True
t.start()

event.wait()

where we start a daemon thread and leave it sleeping in a with block when the main thread exits. When we run the experiment, we get an output of

in with block

but no in __exit__, so the __exit__ method never gets a chance to run.

---

If you want cleanup, don't use a daemon thread. Use a regular thread, and tell it to shut down at the end of the main thread through the regular inter-thread communication channels.