title: Notify parent instance about property change
body: <p>I have these two classes:</p>
  
  <pre><code>class Status(object):
      def __init__(self):
          self._message = ''
  
      @property
      def message(self):
          return self._message
  
      @message.setter
      def message(self, value):
          self._message = value
  
  
  class Buddy(object):
      def __init__(self, name):
          self.name = name
          self.status = Status()
  
      def status_updated(self):
          # this should be called when self.status.message is changed
  </code></pre>
  
  <p>and I use them like this:</p>
  
  <pre><code>buddy = Buddy('John')
  buddy.status.message = 'Hello world!'  # this should call Buddy.status_updated
  </code></pre>
  
  <p>I want <code>Buddy.status_updated</code> to be called when I modify the <code>message</code> property of <code>Status</code>. How to achieve this?</p>