Link Extractors¶

Link Extractors 是用于从网页(scrapy.http.Response )中抽取会被follow的链接的对象。

Scrapy默认提供2种可用的 Link Extractor, 但你通过实现一个简单的接口创建自己定制的Link Extractor来满足需求｡ Scrapy 提供了 scrapy.contrib.linkextractors import LinkExtractor ，不过您也可以通过实现一个简单的接口来创建您自己的Link Extractor，满足需求。

每个LinkExtractor有唯一的公共方法是 extract_links ，其接收一个 Response 对象，并返回 scrapy.link.Link 对象｡ Link Extractors只实例化一次，其 extract_links 方法会根据不同的response被调用多次来提取链接｡

Link Extractors在 CrawlSpider 类(在Scrapy可用)中使用, 通过一套规则,但你也可以用它在你的Spider中,即使你不是从 CrawlSpider 继承的子类, 因为它的目的很简单: 提取链接｡

内置Link Extractor 参考¶

Scrapy 自带的Link Extractors类在 scrapy.contrib.linkextractors 模块提供｡

默认的link extractor 是 LinkExtractor ，其实就是 LxmlLinkExtractor:

from scrapy.contrib.linkextractors import LinkExtractor

在以前版本的Scrapy版本中提供了其他的link extractor，不过都已经被废弃了。

LxmlLinkExtractor¶

class scrapy.contrib.linkextractors.lxmlhtml.LxmlLinkExtractor(allow=(), deny=(), allow_domains=(), deny_domains=(), deny_extensions=None, restrict_xpaths=(), tags=('a', 'area'), attrs=('href', ), canonicalize=True, unique=True, process_value=None)¶

LxmlLinkExtractor is the recommended link extractor with handy filtering options. It is implemented using lxml’s robust HTMLParser.

参数:

allow (a regular expression (or list of)) – a single regular expression (or list of regular expressions) that the (absolute) urls must match in order to be extracted. If not given (or empty), it will match all links.
deny (a regular expression (or list of)) – a single regular expression (or list of regular expressions) that the (absolute) urls must match in order to be excluded (ie. not extracted). It has precedence over the allow parameter. If not given (or empty) it won’t exclude any links.
allow_domains (str or list) – a single value or a list of string containing domains which will be considered for extracting the links
deny_domains (str or list) – a single value or a list of strings containing domains which won’t be considered for extracting the links
deny_extensions (list) – a single value or list of strings containing extensions that should be ignored when extracting links. If not given, it will default to the IGNORED_EXTENSIONS list defined in the scrapy.linkextractor module.
restrict_xpaths (str or list) – is a XPath (or list of XPath’s) which defines regions inside the response where links should be extracted from. If given, only the text selected by those XPath will be scanned for links. See examples below.
tags (str or list) – a tag or a list of tags to consider when extracting links. Defaults to ('a', 'area').
attrs (list) – an attribute or list of attributes which should be considered when looking for links to extract (only for those tags specified in the tags parameter). Defaults to ('href',)
canonicalize (boolean) – canonicalize each extracted url (using scrapy.utils.url.canonicalize_url). Defaults to True.
unique (boolean) – whether duplicate filtering should be applied to extracted links.
process_value (callable) –
它接收来自扫描标签和属性提取每个值, 可以修改该值, 并返回一个新的, 或返回 None 完全忽略链接的功能｡如果没有给出, process_value 默认是 lambda x: x｡

例如,从这段代码中提取链接:
```
<a href="javascript:goToPage('../other/page.html'); return false">Link text</a>
```
你可以使用下面的这个 process_value 函数:
```
def process_value(value):
    m = re.search("javascript:goToPage\('(.*?)'", value)
    if m:
        return m.group(1)
```