.py

from threading import Lock, Thread

class Worker:
    def __init__(self, once):
        self.__thread = None
        self.__running_lock = Lock()
        self.__running = False
        self.__once = once

    def __loop(self):
        while True:
            with self.__running_lock:
                if not self.__running:
                    break
            self.__once()

    def start(self):
        with self.__running_lock:
            if self.__running:
                return
            self.__running = True
        self.__thread = Thread(target=self.__loop)
        self.__thread.start()

    def stop(self):
        with self.__running_lock:
            if not self.__running:
                return
            self.__running = False
        self.__thread.join()
        self.__thread = None

    def __enter__(self):
        pass

    def __exit__(self, exc_type, exc_value, traceback):
        self.stop()