code: def computeNormals(self):
          
          poly = self.polydata(False)
          pnormals = poly.GetPointData().GetNormals()
          cnormals = poly.GetCellData().GetNormals()
          if pnormals and cnormals:
              return self
  
          pdnorm = vtk.vtkPolyDataNormals()
          pdnorm.SetInputData(poly)
          pdnorm.ComputePointNormalsOn()
          pdnorm.ComputeCellNormalsOn()
          pdnorm.FlipNormalsOff()
          pdnorm.ConsistencyOn()
          pdnorm.Update()
          return self.updateMesh(pdnorm.GetOutput())
docs: Compute cell and vertex normals for the actor's mesh.
  
          .. warning:: Mesh gets modified, can have a different nr. of vertices.