code: def bounds_at_zoom(self, zoom=None):
          
          return () if self.area_at_zoom(zoom).is_empty else Bounds(
              *self.area_at_zoom(zoom).bounds)
docs: Return process bounds for zoom level.
  
          Parameters
          ----------
          zoom : integer or list
  
          Returns
          -------
          process bounds : tuple
              left, bottom, right, top