code: def vertex_normals(self):
          
          # make sure we have faces_sparse
          assert hasattr(self.faces_sparse, 'dot')
          vertex_normals = geometry.mean_vertex_normals(
              vertex_count=len(self.vertices),
              faces=self.faces,
              face_normals=self.face_normals,
              sparse=self.faces_sparse)
          return vertex_normals
docs: The vertex normals of the mesh. If the normals were loaded
          we check to make sure we have the same number of vertex
          normals and vertices before returning them. If there are
          no vertex normals defined or a shape mismatch we  calculate
          the vertex normals from the mean normals of the faces the
          vertex is used in.
  
          Returns
          ----------
          vertex_normals : (n,3) float
            Represents the surface normal at each vertex.
            Where n == len(self.vertices)