python selenuim如何判断下拉框是否加载出来,超过时间不再等待

s_flag = True
time_start = time.time()
while s_flag:
        doc = etree.HTML(unicode.encode(driver.page_source, encoding='utf-8'))
        from_list = doc.xpath("""//*[@id="ext-gen267"]/div""")
        if len(from_list) == 0:
            time_end = time.time()
            if int(time_end - time_start) > 15:
                # 超过 n 秒 没有 下拉框 ,不再等待
                s_flag = False
        else:
            # 有下拉框 ,退出 循环
            break

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