code: def filter(self, zone='all'):
          
          if not self.is_connected():
              return None
  
          nodes = self.gce.list_nodes(zone)
          return Filter(nodes)
docs: Filter nodes by their attributes.
  
          :param  zone: A zone containing nodes.
          :type   zone: ``str``
  
          :return: A chainable filter object.
          :rtype:  ``object`` of :class:`Filter`