code: def normals(self, cells=False):
          
          if cells:
              vtknormals = self.polydata(True).GetCellData().GetNormals()
          else:
              vtknormals = self.polydata(True).GetPointData().GetNormals()
          return vtk_to_numpy(vtknormals)
docs: Retrieve vertex normals as a numpy array.
  
          :params bool cells: if `True` return cell normals.