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3.4 CSS选择器
CSS即层叠样式表，其选择器是一种用来确定HTML文档中某部分位置的语言。
CSS选择器的语法比XPath更简单一些，但功能不如XPath强大。实际上，当我们调用Selector对象的CSS方法时，在其内部会使用Python库cssselect将CSS选择器表达式翻译成XPath表达式，然后调用Selector对象的XPATH方法。
表3-2列出了CSS选择器的一些基本语法。
表3-2 CSS选择器
和学习XPath一样，通过一些例子展示CSS选择器的使用。
先创建一个HTML文档并构造一个HtmlResponse对象：
>>> from scrapy.selector import Selector
>>> from scrapy.http import HtmlResponse
>>> body = '''
... <html>
... <head>
... <base href='http://example.com/'/>
... <title>Example website</title>
... </head>
... <body>
... <div id='images-1'style="width: 1230px; ">
... <a href='image1.html'>Name: Image 1<br/><img src='image1.jpg'/></a>
... <a href='image2.html'>Name: Image 2<br/><img src='image2.jpg'/></a>
... <a href='image3.html'>Name: Image 3<br/><img src='image3.jpg'/></a>
... </div>
...
... <div id='images-2'class='small'>
... <a href='image4.html'>Name: Image 4<br/><img src='image4.jpg'/></a>
... <a href='image5.html'>Name: Image 5<br/><img src='image5.jpg'/></a>
... </div>
... </body>
... </html>
... '''
...
>>> response = HtmlResponse(url='http://www.example.com', body=body, encoding='utf8')
● E：选中E元素。
# 选中所有的img
>>> response.css('img')
[<Selector xpath='descendant-or-self::img' data='<img src="image1.jpg">'>,
<Selector xpath='descendant-or-self::img' data='<img src="image2.jpg">'>,
<Selector xpath='descendant-or-self::img' data='<img src="image3.jpg">'>,
<Selector xpath='descendant-or-self::img' data='<img src="image4.jpg">'>,
<Selector xpath='descendant-or-self::img' data='<img src="image5.jpg">'>]
● E1, E2：选中E1和E2元素。
# 选中所有base和title
>>> response.css('base, title')
[<Selector xpath='descendant-or-self::base | descendant-or-self::title' data='<base
href="http://example.com/">'>,
<Selector xpath='descendant-or-self::base | descendant-or-self::title' data='<title>Example
website</title>'>]
● E1 E2：选中E1后代元素中的E2元素。
# div后代中的img
>>> response.css('div img')
[<Selector xpath='descendant-or-self::div/descendant-or-self::*/img' data='<img src="image1.jpg">'>,
<Selector xpath='descendant-or-self::div/descendant-or-self::*/img' data='<img src="image2.jpg">'>,
<Selector xpath='descendant-or-self::div/descendant-or-self::*/img' data='<img src="image3.jpg">'>,
<Selector xpath='descendant-or-self::div/descendant-or-self::*/img' data='<img src="image4.jpg">'>,
<Selector xpath='descendant-or-self::div/descendant-or-self::*/img' data='<img src="image5.jpg">'>]
● E1>E2：选中E1子元素中的E2元素。
# body子元素中的div
>>> response.css('body>div')
[<Selector xpath='descendant-or-self::body/div' data='<div id="images-1" style="width: 1230px; '>,
<Selector xpath='descendant-or-self::body/div'data='<div id="images-2" class="small">\n '>]
● [ATTR]：选中包含ATTR属性的元素。
# 选中包含style属性的元素
>>> response.css('[style]')
[<Selector xpath='descendant-or-self::*[@style]' data='<div id="images-1" style="width: 1230px; '>]
● [ATTR=VALUE]：选中包含ATTR属性且值为VALUE的元素。
# 选中属性id值为images-1的元素
>>> response.css('[id=images-1]')
[<Selector xpath="descendant-or-self::*[@id = 'images-1']" data='<div id="images-1" style="width:1230px; '>]
● E:nth-child(n)：选中E元素，且该元素必须是其父元素的第n个子元素。
# 选中每个div的第一个a
>>> response.css('div>a:nth-child(1)')
[<Selector xpath="descendant-or-self::div/*[name() = 'a' and (position() = 1)]" data='<a
href="image1.html">Name: Image 1 <br>'>,
<Selector xpath="descendant-or-self::div/*[name() = 'a' and (position() = 1)]" data='<a
href="image4.html">Name: Image 4 <br>'>]
# 选中第二个div的第一个a
>>> response.css('div:nth-child(2)>a:nth-child(1)')
[<Selector xpath="descendant-or-self::*/*[name() = 'div' and (position() = 2)]/*[name() = 'a' and
(position() = 1)]" data='<a href="image4.html">Name: Image 4 <br>'>]
● E:first-child：选中E元素，该元素必须是其父元素的第一个子元素。
● E:last-child：选中E元素，该元素必须是其父元素的倒数第一个子元素。
# 选中第一个div的最后一个a
>>> response.css('div:first-child>a:last-child')
[<Selector xpath="descendant-or-self::*/*[name() = 'div' and (position() = 1)]/*[name() = 'a' and
(position() = last())]" data='<a href="image3.html">Name: Image 3 <br>'>]
● E::text：选中E元素的文本节点。
# 选中所有a的文本
>>> sel = response.css('a::text')
>>> sel
[<Selector xpath='descendant-or-self::a/text()' data='Name: Image 1 '>,
<Selector xpath='descendant-or-self::a/text()' data='Name: Image 2 '>,
<Selector xpath='descendant-or-self::a/text()' data='Name: Image 3 '>,
<Selector xpath='descendant-or-self::a/text()' data='Name: Image 4 '>,
<Selector xpath='descendant-or-self::a/text()' data='Name: Image 5 '>]
>>> sel.extract()
['Name: Image 1 ',
'Name: Image 2 ',
'Name: Image 3 ',
'Name: Image 4 ',
'Name: Image 5 ']
关于CSS选择器的使用先介绍到这里，更多详细内容可以参看CSS选择器文档：https://www.w3.org/TR/css3-selectors/。