What is a python thread

1. Python threads are implemented using OS threads in all implementations I know (C Python, PyPy and Jython). For each Python thread, there is an underlying OS thread.

2. Some operating systems (Linux being one of them) show all different threads launched by the same executable in the list of all running processes. This is an implementation detail of the OS, not of Python. On some other operating systems, you may not see those threads when listing all the processes.

3. The process will terminate when the last non-daemon thread finishes. At that point, all the daemon threads will be terminated. So, those threads are part of your process, but are not preventing it from terminating (while a regular thread will prevent it). That is implemented in pure Python. A process terminates when the system _exit function is called (it will kill all threads), and when the main thread terminates (or sys.exit is called), the Python interpreter checks if there is another non-daemon thread running. If there is none, then it calls _exit, otherwise it waits for the non-daemon threads to finish.

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The daemon thread flag is implemented in pure Python by the threading module. When the module is loaded, a Thread object is created to represent the main thread, and it’s _exitfunc method is registered as an atexit hook.

The code of this function is:

class _MainThread(Thread):

    def _exitfunc(self):
        self._Thread__stop()
        t = _pickSomeNonDaemonThread()
        if t:
            if __debug__:
                self._note("%s: waiting for other threads", self)
        while t:
            t.join()
            t = _pickSomeNonDaemonThread()
        if __debug__:
            self._note("%s: exiting", self)
        self._Thread__delete()

This function will be called by the Python interpreter when sys.exit is called, or when the main thread terminates. When the function returns, the interpreter will call the system _exit function. And the function will terminate, when there are only daemon threads running (if any).

When the _exit function is called, the OS will terminate all of the process threads, and then terminate the process. The Python runtime will not call the _exit function until all the non-daemon thread are done.

All threads are part of the process.

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My interpretation/understanding was: main thread terminates when all
non-daemon threads are terminated.

So python daemon threads are not part of python program if “the entire
Python program exits when only daemon threads are left”?

Your understanding is incorrect. For the OS, a process is composed of many threads, all of which are equal (there is nothing special about the main thread for the OS, except that the C runtime add a call to _exit at the end of the main function). And the OS doesn’t know about daemon threads. This is purely a Python concept.

The Python interpreter uses native thread to implement Python thread, but has to remember the list of threads created. And using its atexit hook, it ensures that the _exit function returns to the OS only when the last non-daemon thread terminates. When using “the entire Python program”, the documentation refers to the whole process.

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The following program can help understand the difference between daemon thread and regular thread:

import sys
import time
import threading

class WorkerThread(threading.Thread):

    def run(self):
        while True:
            print 'Working hard'
            time.sleep(0.5)

def main(args):
    use_daemon = False
    for arg in args:
        if arg == '--use_daemon':
            use_daemon = True
    worker = WorkerThread()
    worker.setDaemon(use_daemon)
    worker.start()
    time.sleep(1)
    sys.exit(0)

if __name__ == '__main__':
    main(sys.argv[1:])

If you execute this program with the ‘–use_daemon’, you will see that the program will only print a small number of Working hard lines. Without this flag, the program will not terminate even when the main thread finishes, and the program will print Working hard lines until it is killed.