code: def p_postfix_expr(self, p):
          
          if len(p) == 2:
              p[0] = p[1]
          else:
              p[0] = ast.UnaryOp(op=p[2], value=p[1], postfix=True)
docs: postfix_expr : left_hand_side_expr
                          | left_hand_side_expr PLUSPLUS
                          | left_hand_side_expr MINUSMINUS