def _update_alpha(self, event=None):
        """Update display after a change in the alpha spinbox."""
        a = self.alpha.get()
        hexa = self.hexa.get()
        hexa = hexa[:7] + ("%2.2x" % a).upper()
        self.hexa.delete(0, 'end')
        self.hexa.insert(0, hexa)
        self.alphabar.set(a)
        self._update_preview()
Update display after a change in the alpha spinbox.