code: def layers(self):
          
          layer = ['NONE'] * len(self.entities)
          for i, e in enumerate(self.entities):
              if hasattr(e, 'layer'):
                  layer[i] = str(e.layer)
          return layer
docs: If entities have a layer defined, return it.
  
          Returns
          ---------
          layers: (len(entities), ) list of str