为什么python报错但是能运行出一部分正确结果呢？

代码如下：爬取图片名和图片地址

import requests
from lxml import etree

if __name__ == '__main__':
    url='http://www.netbian.com/fengjing/'
    headers={'User-Agent':'Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.107 Mobile Safari/537.36'}
    res=requests.get(url,headers=headers)
    res.encoding='utf-8'
    html=res.text
    tree=etree.HTML(html)
    div=tree.xpath('///div[@class="list"]/ul/li')
    for li in div:
        img_alt = li.xpath('./a/img/@alt')[0]
        img_src = li.xpath('./a/img/@src')
        #img_alt.decode('gbk').encode('iso-8859-1')
        print(img_alt,img_src)

这个也是答案部分正确后面报错

原因是这一页第三张图片没读出alt和src(第三行只有一个.jpg）

我看第三个li挺正常啊，为什么我的代码读不出来

import requests
from lxml import etree

if __name__ == '__main__':
    url='http://www.netbian.com/fengjing/'
    headers={'User-Agent':'Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.107 Mobile Safari/537.36'}
    res=requests.get(url,headers=headers)
    res.encoding='utf-8'
    html=res.text
    tree=etree.HTML(html)
    div=tree.xpath('///div[@class="list"]/ul/li')
    for li in div:
        img_alt = li.xpath('./a/img/@alt')
        img_src = li.xpath('./a/img/@src')
        #img_alt.decode('gbk').encode('iso-8859-1')
        print(img_alt,img_src)

原网页class="list"的

• 不是每个
• 下都有alt和src，li.xpath('./a/img/@alt')为[]（空数组）时，img_alt = li.xpath('./a/img/@alt')[0]报错IndexError: list index out of range