爬虫2（页面解析和数据提取）

处理HTML文件，常用Xpath，先将HTML文件转换成XML文档，然后用Xpath查找HTML节点或元素。

一、HTML与XML

二、XPath

1、XPath路径表达式

三、Lxml库

html = etree.HTML(text)  # 将字符串转换成HTML格式

# print(etree.tostring(html))  # 补全HTML

result = html.xpath('//li/a/text()')  # 获取li标签下的a标签的文本

r = html.xpath('//li[contains(@class,"item-")]')  # 获取属性为class且包含item-的li标签

案例：下载百度贴吧页面的图片

import requests
from lxml import etree

index_url = 'https://tieba.baidu.com/p/5475267611'

response = requests.get(index_url).text

selector = etree.HTML(response)

image_urls = selector.xpath('//img[@class="BDE_Image"]/@src')

offset = 0

for image_url in image_urls:
    
    image_content = requests.get(image_url).content
    
    with open('{}.jpg'.format(offset), 'wb') as f:
        f.write(image_content)
    
    offset = offset + 1