Java's Observer and Observable translated to Python

observer.py

# -*- coding: utf-8 -*-

from __future__ import unicode_literals
import threading


class Observer(object):

    def update(self, o, *args, **kwargs):
        """
        This method is called whenever the observed object is changed

        Arguments:
          o - observable
          args and kwargs - any arguments supplied by the observed object
        """
        pass


class Observable(object):

    def __init__(self):
        self._changed = False
        self._lock = threading.Lock()
        self._obs = set()

    def add_observer(self, o):
        with self._lock:
            self._obs.add(o)

    def delete_observer(self, o):
        with self._lock:
            self._obs.remove(o)

    def notify_observers(self, *args, **kwargs):
        with self._lock:
            if not self._changed:
                return
            self.clear_changed()
            snapshot = self._obs.copy()
        for obs in snapshot:
            obs.update(*args, **kwargs)

    def delete_observers(self):
        with self._lock:
            self._obs.clear()

    def set_changed(self):
        with self._lock:
            self._changed = True

    def clear_changed(self):
        with self._lock:
            self._changed = False

    def has_changed(self):
        with self._lock:
            return self._changed

    def count_observers(self):
        with self._lock:
            return len(self._obs)