code: def layer_mapping(self, mapping):
          
          # For certain geoms, it is useful to be able to
          # ignore the default aesthetics and only use those
          # set in the layer
          if self.inherit_aes:
              aesthetics = defaults(self.mapping, mapping)
          else:
              aesthetics = self.mapping
  
          # drop aesthetic parameters or the calculated aesthetics
          calculated = set(get_calculated_aes(aesthetics))
          d = dict((ae, v) for ae, v in aesthetics.items()
                   if (ae not in self.geom.aes_params) and
                   (ae not in calculated))
          self._active_mapping = aes(**d)
          return self._active_mapping
docs: Return the mappings that are active in this layer
  
          Parameters
          ----------
          mapping : aes
              mappings in the ggplot call
  
          Notes
          -----
          Once computed the layer mappings are also stored
          in self._active_mapping