code: def assign_site_properties(self, slab, height=0.9):
          
          if 'surface_properties' in slab.site_properties.keys():
              return slab
          else:
              surf_sites = self.find_surface_sites_by_height(slab, height)
          surf_props = ['surface' if site in surf_sites
                        else 'subsurface' for site in slab.sites]
          return slab.copy(
              site_properties={'surface_properties': surf_props})
docs: Assigns site properties.