setDaemon() method of threading.Thread

Question

I am a newbie in python programming, what I understand is that a process can be a daemon, but a thread in a daemon mode, I couldn't understand the usecase of this, I would request the python gurus to help me in understanding this.

Answer 1

Here is some basic code using threading:

import Queue import threading  def basic_worker(queue):     while True:         item = queue.get()         # do_work(item)         print(item)         queue.task_done() def basic():     # http://docs.python.org/library/queue.html     queue = Queue.Queue()     for i in range(3):          t = threading.Thread(target=basic_worker,args=(queue,))          t.daemon = True          t.start()     for item in range(4):         queue.put(item)     queue.join()       # block until all tasks are done     print('got here')  basic() 

When you run it, you get

% test.py 0 1 2 3 got here 

Now comment out the line:

         t.daemon = True 

Run it again, and you'll see that the script prints the same result, but hangs. The main thread ends (note that got here was printed), but the second thread never finishes.

In contrast, when t.daemon is set to True, the thread t is terminated when the main thread ends.

Note that "daemon threads" has little to do with daemon processes.

Answer 2

It looks like people intend to use Queue to explain threading, but I think there should be a much simpler way, by using time.sleep(), to demo a daemon thread.

Create daemon thread by setting the daemon parameter (default as None):

from threading import Thread import time  def worker():     time.sleep(3)     print('daemon done')  thread = Thread(target=worker, daemon=True) thread.start()  print('main done') 

Output:

main done  Process finished with exit code 0 

Remove the daemon argument, like:

thread = Thread(target=worker) 

Re-run and see the output:

main done daemon done  Process finished with exit code 0 

Here we already see the difference of a daemon thread:

The entire Python program can exit if only daemon thread is left.

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isDaemon() and setDaemon() are old getter/setter API. Using constructor argument, as above, or daemon property is recommended.