Daemon Threads Explanation

Question

In the Python documentation it says:

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.

Does anyone have a clearer explanation of what that means or a practical example showing where you would want to set threads as daemonic?

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To clarify for me:

so the only time you wouldn't set threads as daemonic is if you wanted them to continue running after the main thread exits?

Answer 1

Chris already explained what daemon threads are, so let's talk about practical usage. Many thread pool implementations use daemon threads for task workers. Workers are threads which execute tasks from task queue.

Worker needs to keep waiting for tasks in task queue indefinitely as they don't know when new task will appear. Thread which assigns tasks (say main thread) only knows when tasks are over. Main thread waits on task queue to get empty and then exits. If workers are user threads i.e. non-daemon, program won't terminate. It will keep waiting for these indefinitely running workers, even though workers aren't doing anything useful. Mark workers daemon threads, and main thread will take care of killing them as soon as it's done handling tasks.

Answer 2

Quoting Chris: "... when your program quits, any daemon threads are killed automatically.". I think that sums it up. You should be careful when you use them as they abruptly terminate when main program executes to completion.

Answer 3

Other posters gave some examples for situations in which you'd use daemon threads. My recommendation, however, is never to use them.

It's not because they're not useful, but because there are some bad side effects you can experience if you use them. Daemon threads can still execute after the Python runtime starts tearing down things in the main thread, causing some pretty bizarre exceptions.

More info here:

https://joeshaw.org/python-daemon-threads-considered-harmful/

https://mail.python.org/pipermail/python-list/2005-February/343699.html

Strictly speaking you never need them, it just makes implementation easier in some cases.

Answer 4

When your second thread is non-Daemon, your application's primary main thread cannot quit because its exit criteria is being tied to the exit also of non-Daemon thread(s). Threads cannot be forcibly killed in python, therefore your app will have to really wait for the non-Daemon thread(s) to exit. If this behavior is not what you want, then set your second thread as daemon so that it won't hold back your application from exiting.

Answer 5

A simpler way to think about it, perhaps: when main returns, your process will not exit if there are non-daemon threads still running.

A bit of advice: Clean shutdown is easy to get wrong when threads and synchronization are involved - if you can avoid it, do so. Use daemon threads whenever possible.

Answer 6

Some threads do background tasks, like sending keepalive packets, or performing periodic garbage collection, or whatever. These are only useful when the main program is running, and it's okay to kill them off once the other, non-daemon, threads have exited.

Without daemon threads, you'd have to keep track of them, and tell them to exit, before your program can completely quit. By setting them as daemon threads, you can let them run and forget about them, and when your program quits, any daemon threads are killed automatically.