Selemium WebDriver:循环不工作

运行代码时

 def extract():
    driver = webdriver.Firefox()
    driver.get('http://example.com/')

    while True:
        elm = driver.find_element_by_link_text(">>").click()
        elm.click()

if __name__ == '__main__':
    extract()

页面加载,同时单击“下一步”按钮(链接)并加载下一页, 新加载的页面也有相同的链接按钮,但它没有单击 而且,

正在获取错误:

Traceback (most recent call last):
  File "C:\Users\Admin\sel\click_next.py", line 14, in <module>
    extract_top_news()
  File "C:\Users\Admin\sel\click_next.py", line 11, in extract_top_news
    elm.click()
AttributeError: 'NoneType' object has no attribute 'click'

这是试图单击的链接按钮

<li>
<a href='http://example.com/page.php?page=2'>&gt;&gt;</a>
</li>

我错过了什么!