what the function of "flag" in example2 how_to_determine_if_a_thread_has_started?

[wwang-chcn]

I don't understand the function of flag in example2 how_to_determine_if_a_thread_has_started (12.2). The scripts works fine when I remove the flag part likes below.

import threading
import time
class PeriodicTimer:
    def __init__(self, interval):
        self._interval = interval
        self._cv = threading.Condition()
    
    def start(self):
        t = threading.Thread(target=self.run)
        t.daemon = True
        t.start()
    
    def run(self):
        print('running start')
        while True:
            time.sleep(self._interval)
            with self._cv:
                self._cv.notify_all()
    
    def wait_fot_tick(self):
        with self._cv:
            self._cv.wait()


ptimer = PeriodicTimer(5)
ptimer.start()

def countdown(nticks):
    print('countdown start')
    while nticks > 0:
        ptimer.wait_fot_tick()
        print('T-minus', nticks)
        nticks -= 1


def countup(last):
    n = 0
    print('counting start')
    while n < last:
        ptimer.wait_fot_tick()
        print('Counting', n)
        n += 1


threading.Thread(target=countdown, args=(10,)).start()
threading.Thread(target=countup, args=(5,)).start()