code: def set(self, alpha):
          
          if alpha > 255:
              alpha = 255
          elif alpha < 0:
              alpha = 0
          x = alpha / 255. * self.winfo_width()
          self.coords('cursor', x, 0, x, self.winfo_height())
          self._variable.set(alpha)
docs: Set cursor position on the color corresponding to the alpha value.
  
          :param alpha: new alpha value (between 0 and 255)
          :type alpha: int