Z 2. CSS选择器基本语法

缺乏、安全感 2022-05-18 23:59 180阅读 0赞

*  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。
 *  表列出了CSS选择器的一些基本语法  
    ![css][]
 *  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')

●　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/][https_www.w3.org_TR_css3-selectors]。

[css]: /images/20220519/c6602b18567d4384be8475a029604e17.png
[https_www.w3.org_TR_css3-selectors]: https://www.w3.org/TR/css3-selectors/