arg_list : ident_init_opt
             | arg_list COMMA ident_init_opt

def p_arg_list(p):
    """
    arg_list : ident_init_opt
             | arg_list COMMA ident_init_opt
    """
    if len(p) == 2:
        p[0] = node.expr_list([p[1]])
    elif len(p) == 4:
        p[0] = p[1]
        p[0].append(p[3])
    else:
        assert 0
    assert isinstance(p[0], node.expr_list)