code: def alpha(self, opacity=None):
          
          if opacity is None:
              return self.GetProperty().GetOpacity()
  
          self.GetProperty().SetOpacity(opacity)
          bfp = self.GetBackfaceProperty()
          if bfp:
              if opacity < 1:
                  self._bfprop = bfp
                  self.SetBackfaceProperty(None)
              else:
                  self.SetBackfaceProperty(self._bfprop)
          return self
docs: Set/get actor's transparency. Same as `actor.opacity()`.