old_contents: #!/usr/bin/env python
  
  class Subject(object):
  
  
      def __init__(self):
          self.observers = []
  
      def subscribe(self, observer):
          self.observers.append(observer)
  
      def unsubscribe(self, observer):
          self.observers.remove(observer)
  
      def notify(self):
          for observer in self.observers:
              observer.update_data()
  
new_contents: #!/usr/bin/env python
  
  class Subject(object):
  
  
      def __init__(self):
          self._observers = []
  
      def subscribe(self, observer):
          self._observers.append(observer)
  
      def unsubscribe(self, observer):
          self._observers.remove(observer)
  
  
      def notify(self):
          for observer in self._observers:
              observer.update_data()
  
subject: change variable name observers in private name _observers