expr_list : expr_list COMMA expr
                     | expr

def p_expr_list(self, p):
        '''expr_list : expr_list COMMA expr
                     | expr'''
        if len(p) == 4:
            p[1].append(p[3])
            p[0] = p[1]
        elif len(p) == 2:
            p[0] = [p[1]]