code: def vertex_normals(self, values):
          
          if values is not None:
              values = np.asanyarray(values,
                                     order='C',
                                     dtype=np.float64)
              if values.shape == self.vertices.shape:
                  self._cache['vertex_normals'] = values
docs: Assign values to vertex normals
  
          Parameters
          -------------
          values : (len(self.vertices), 3) float
            Unit normal vectors for each vertex