old_contents: class Bookmark(object):
      def __init__(self, url, title, identifier, date):
          self.url = url
          self.title = title
          self.identifier = identifier
          self.date = date
  
  
new_contents: class Bookmark(object):
      _url = None
      _title = None
      _identifier = None
      _date = None
  
      def __init__(self, url, title, identifier, date):
          self._url = url
          self._title = title
          self._identifier = identifier
          self._date = date
  
      @property
      def url(self):
          return self._url
  
      @url.setter
      def url(self, url):
          self._url = url
  
      @property
      def title(self):
          return self._title
  
      @title.setter
      def title(self, title):
          self._title = title
  
      @property
      def identifier(self):
          return self._identifier
  
      @identifier.setter
      def identifier(self, identifier):
          self._identifier = identifier
  
      @property
      def date(self):
          return self._date
  
      @date.setter
      def date(self, date):
          self._date = date
  
subject: Use properties