scrapy之Selectors

练习url：https://doc.scrapy.org/en/latest/_static/selectors-sample1.html

一获取文本值

　　xpath

In [18]: response.selector.xpath('//title/text()').extract_first(default='')
Out[18]: 'Example website'

　　css

In [19]: response.selector.css('title::text').extract_first(default='')
Out[19]: 'Example website'

　　注：可以省略写成:response.xpath()

二获取属性值

　　xpath

In [23]: response.selector.xpath('//base/@href').extract_first()
Out[23]: 'http://example.com/'

　　css　

In [24]: response.selector.css('base::attr(href)').extract_first()
Out[24]: 'http://example.com/'

　　注: 可以省略写成：response.css

三 xpath,css嵌套使用

　　因为css，xpath返回的是 SelectorList 实例，所有可以嵌套便捷的使用。

　　ps：获取属性，xpath，@已经实现，并不需要 /text()

In [21]: response.selector.css('img').xpath('@src').extract()
Out[21]:
['image1_thumb.jpg',
 'image2_thumb.jpg',
 'image3_thumb.jpg',
 'image4_thumb.jpg',
 'image5_thumb.jpg']

四 .re()

　　.re()

　　.re_first()

　　ps :返回的是unicode构成的列表，所以，不能嵌套使用 .re()

In [1]: response.selector.css('div > p:nth-of-type(2)::text').extract()
Out[1]: ['333xxx']

In [2]: response.selector.css('div > p:nth-of-type(2)::text').extract_first()
Out[2]: '333xxx'

In [3]: response.selector.css('div > p:nth-of-type(2)::text').re_first('w+')
Out[3]: '333xxx'

In [4]: response.selector.css('div > p:nth-of-type(2)::text').re_first('[A-Za-z]+')
Out[4]: 'xxx'

In [5]: response.selector.css('div > p:nth-of-type(2)::text').re('[A-Za-z]+')
Out[5]: ['xxx']

五关于Xpath的相对路径查找的注意

　　查找div标签下p标签

<html lang="zh-CN">
<head>
</head>
<body>
    <p>11</p>
    <div>
        <p>222</p>
        <p>333</p>
    </div>
</body>
</html>

　　错误做法：

In [4]: divs = response.selector.xpath('//div')

In [5]: for p in divs.xpath('//p'):
   ...:     print(p.extract())
   ...:
<p>11</p>
<p>222</p>
<p>333</p>

　　正确做法 1：

In [6]: divs = response.selector.css('div')

In [7]: for p in divs.xpath('.//p'):
   ...:     print(p.extract())
   ...:
   ...:
<p>222</p>
<p>333</p>

　　正确做法 2：

In [8]: divs = response.selector.css('div')

In [9]: for p in divs.xpath('p'):
   ...:     print(p.extract())
   ...:
   ...:
   ...:
<p>222</p>
<p>333</p>