baseimage-tutorial-framework/lib/tfw/components/history_monitor.py

from watchdog.observers import Observer
from watchdog.events import FileSystemEventHandler
from os.path import dirname


class CallbackEventHandler(FileSystemEventHandler):
    def __init__(self, *callbacks):
        super().__init__()
        self.callbacks = callbacks

    def on_modified(self, event):
        for callback in self.callbacks:
            callback()


class HistoryMonitor:
    def __init__(self, histfile):
        self.histfile = histfile
        self._history = []
        self.observer = Observer()
        self.observer.schedule(CallbackEventHandler(self._fetch_history), dirname(self.histfile))

    @property
    def history(self):
        return self._history

    def _fetch_history(self):
        with open(self.histfile, 'r') as ifile:
            self._history = ifile.readlines()

    def watch(self):
        self.observer.start()

    def stop(self):
        self.observer.stop()
        self.observer.join()