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