code: def render_grid(self, bbox, grid_fields, layer, width=None, height=None):
          
          width = width or self.tilesize
          height = height or self.tilesize
          self._prepare_rendering(bbox, width=width, height=height)
  
          grid = mapnik.Grid(width, height)
          mapnik.render_layer(self._mapnik, grid, layer=layer, fields=grid_fields)
          grid = grid.encode()
          return json.dumps(grid)
docs: Render the specified grid with Mapnik