def turn(self):
        """Turn the ring for a single position.
        For example, [a, b, c, d] becomes [b, c, d, a]."""
        first = self._data.pop(0)
        self._data.append(first)
def turn ( self ) : first = self . _data . pop ( 0 ) self . _data . append ( first )
Turn the ring for a single position.
        For example, [a, b, c, d] becomes [b, c, d, a].