Scrapy基础——CrawlSpider详解

写在前面

在Scrapy基础——Spider中，我简要地说了一下Spider类。Spider基本上能做很多事情了，但是如果你想爬取知乎或者是简书全站的话，你可能需要一个更强大的武器。
CrawlSpider基于Spider，但是可以说是为全站爬取而生。

简要说明

CrawlSpider是爬取那些具有一定规则网站的常用的爬虫，它基于Spider并有一些独特属性

• rules: 是Rule对象的集合，用于匹配目标网站并排除干扰
• parse_start_url: 用于爬取起始响应，必须要返回Item，Request中的一个。

因为rules是Rule对象的集合，所以这里也要介绍一下Rule。它有几个参数：link_extractor、callback=None、cb_kwargs=None、follow=None、process_links=None、process_request=None
其中的link_extractor既可以自己定义，也可以使用已有LinkExtractor类，主要参数为：

• allow：满足括号中“正则表达式”的值会被提取，如果为空，则全部匹配。
• deny：与这个正则表达式(或正则表达式列表)不匹配的URL一定不提取。
• allow_domains：会被提取的链接的domains。
• deny_domains：一定不会被提取链接的domains。
• restrict_xpaths：使用xpath表达式，和allow共同作用过滤链接。还有一个类似的restrict_css

下面是官方提供的例子，我将从源代码的角度开始解读一些常见问题：

import scrapy
from scrapy.spiders import CrawlSpider, Rule
from scrapy.linkextractors import LinkExtractor

class MySpider(CrawlSpider):
    name = 'example.com'
    allowed_domains = ['example.com']
    start_urls = ['http://www.example.com']

    rules = (
        # Extract links matching 'category.php' (but not matching 'subsection.php')
        # and follow links from them (since no callback means follow=True by default).
        Rule(LinkExtractor(allow=('category\.php', ), deny=('subsection\.php', ))),

        # Extract links matching 'item.php' and parse them with the spider's method parse_item
        Rule(LinkExtractor(allow=('item\.php', )), callback='parse_item'),
    )

    def parse_item(self, response):
        self.logger.info('Hi, this is an item page! %s', response.url)
        item = scrapy.Item()
        item['id'] = response.xpath('//td[@id="item_id"]/text()').re(r'ID: (\d+)')
        item['name'] = response.xpath('//td[@id="item_name"]/text()').extract()
        item['description'] = response.xpath('//td[@id="item_description"]/text()').extract()
        return item

问题：CrawlSpider如何工作的？

因为CrawlSpider继承了Spider，所以具有Spider的所有函数。
首先由start_requests对start_urls中的每一个url发起请求（make_requests_from_url)，这个请求会被parse接收。在Spider里面的parse需要我们定义，但CrawlSpider定义parse去解析响应（self._parse_response(response, self.parse_start_url, cb_kwargs={}, follow=True)）
_parse_response根据有无callback,follow和self.follow_links执行不同的操作

    def _parse_response(self, response, callback, cb_kwargs, follow=True):
    ##如果传入了callback，使用这个callback解析页面并获取解析得到的reques或item
        if callback:
            cb_res = callback(response, **cb_kwargs) or ()
            cb_res = self.process_results(response, cb_res)
            for requests_or_item in iterate_spider_output(cb_res):
                yield requests_or_item
    ## 其次判断有无follow，用_requests_to_follow解析响应是否有符合要求的link。
        if follow and self._follow_links:
            for request_or_item in self._requests_to_follow(response):
                yield request_or_item

其中_requests_to_follow又会获取link_extractor（这个是我们传入的LinkExtractor）解析页面得到的link（link_extractor.extract_links(response)）,对url进行加工（process_links，需要自定义），对符合的link发起Request。使用.process_request(需要自定义）处理响应。

问题：CrawlSpider如何获取rules？

CrawlSpider类会在__init__方法中调用_compile_rules方法，然后在其中浅拷贝rules中的各个Rule获取要用于回调(callback)，要进行处理的链接（process_links）和要进行的处理请求（process_request)

    def _compile_rules(self):
        def get_method(method):
            if callable(method):
                return method
            elif isinstance(method, six.string_types):
                return getattr(self, method, None)

        self._rules = [copy.copy(r) for r in self.rules]
        for rule in self._rules:
            rule.callback = get_method(rule.callback)
            rule.process_links = get_method(rule.process_links)
            rule.process_request = get_method(rule.process_request)

那么Rule是怎么样定义的呢？

    class Rule(object):

        def __init__(self, link_extractor, callback=None, cb_kwargs=None, follow=None, process_links=None, process_request=identity):
            self.link_extractor = link_extractor
            self.callback = callback
            self.cb_kwargs = cb_kwargs or {}
            self.process_links = process_links
            self.process_request = process_request
            if follow is None:
                self.follow = False if callback else True
            else:
                self.follow = follow

因此LinkExtractor会传给link_extractor。

有callback的是由指定的函数处理，没有callback的是由哪个函数处理的？

由上面的讲解可以发现_parse_response会处理有callback的（响应）respons。
cb_res = callback(response, **cb_kwargs) or ()
而_requests_to_follow会将self._response_downloaded传给callback用于对页面中匹配的url发起请求（request）。
r = Request(url=link.url, callback=self._response_downloaded)

如何在CrawlSpider进行模拟登陆

因为CrawlSpider和Spider一样，都要使用start_requests发起请求，用从Andrew_liu大神借鉴的代码说明如何模拟登陆：

##替换原来的start_requests，callback为
def start_requests(self):
    return [Request("http://www.zhihu.com/#signin", meta = {'cookiejar' : 1}, callback = self.post_login)]
def post_login(self, response):
    print 'Preparing login'
    #下面这句话用于抓取请求网页后返回网页中的_xsrf字段的文字, 用于成功提交表单
    xsrf = Selector(response).xpath('//input[@name="_xsrf"]/@value').extract()[0]
    print xsrf
    #FormRequeset.from_response是Scrapy提供的一个函数, 用于post表单
    #登陆成功后, 会调用after_login回调函数
    return [FormRequest.from_response(response,   #"http://www.zhihu.com/login",
                        meta = {'cookiejar' : response.meta['cookiejar']},
                        headers = self.headers,
                        formdata = {
                        '_xsrf': xsrf,
                        'email': '1527927373@qq.com',
                        'password': '321324jia'
                        },
                        callback = self.after_login,
                        dont_filter = True
                        )]
#make_requests_from_url会调用parse，就可以与CrawlSpider的parse进行衔接了
def after_login(self, response) :
    for url in self.start_urls :
        yield self.make_requests_from_url(url)

理论说明如上，有不足或不懂的地方欢迎在留言区和我说明。
其次，我会写一段爬取简书全站用户的爬虫来说明如何具体使用CrawlSpider

最后贴上Scrapy.spiders.CrawlSpider的源代码，以便检查

"""
This modules implements the CrawlSpider which is the recommended spider to use
for scraping typical web sites that requires crawling pages.

See documentation in docs/topics/spiders.rst
"""

import copy
import six

from scrapy.http import Request, HtmlResponse
from scrapy.utils.spider import iterate_spider_output
from scrapy.spiders import Spider


def identity(x):
    return x


class Rule(object):

    def __init__(self, link_extractor, callback=None, cb_kwargs=None, follow=None, process_links=None, process_request=identity):
        self.link_extractor = link_extractor
        self.callback = callback
        self.cb_kwargs = cb_kwargs or {}
        self.process_links = process_links
        self.process_request = process_request
        if follow is None:
            self.follow = False if callback else True
        else:
            self.follow = follow


class CrawlSpider(Spider):

    rules = ()

    def __init__(self, *a, **kw):
        super(CrawlSpider, self).__init__(*a, **kw)
        self._compile_rules()

    def parse(self, response):
        return self._parse_response(response, self.parse_start_url, cb_kwargs={}, follow=True)

    def parse_start_url(self, response):
        return []

    def process_results(self, response, results):
        return results

    def _requests_to_follow(self, response):
        if not isinstance(response, HtmlResponse):
            return
        seen = set()
        for n, rule in enumerate(self._rules):
            links = [lnk for lnk in rule.link_extractor.extract_links(response)
                     if lnk not in seen]
            if links and rule.process_links:
                links = rule.process_links(links)
            for link in links:
                seen.add(link)
                r = Request(url=link.url, callback=self._response_downloaded)
                r.meta.update(rule=n, link_text=link.text)
                yield rule.process_request(r)

    def _response_downloaded(self, response):
        rule = self._rules[response.meta['rule']]
        return self._parse_response(response, rule.callback, rule.cb_kwargs, rule.follow)

    def _parse_response(self, response, callback, cb_kwargs, follow=True):
        if callback:
            cb_res = callback(response, **cb_kwargs) or ()
            cb_res = self.process_results(response, cb_res)
            for requests_or_item in iterate_spider_output(cb_res):
                yield requests_or_item

        if follow and self._follow_links:
            for request_or_item in self._requests_to_follow(response):
                yield request_or_item

    def _compile_rules(self):
        def get_method(method):
            if callable(method):
                return method
            elif isinstance(method, six.string_types):
                return getattr(self, method, None)

        self._rules = [copy.copy(r) for r in self.rules]
        for rule in self._rules:
            rule.callback = get_method(rule.callback)
            rule.process_links = get_method(rule.process_links)
            rule.process_request = get_method(rule.process_request)

    @classmethod
    def from_crawler(cls, crawler, *args, **kwargs):
        spider = super(CrawlSpider, cls).from_crawler(crawler, *args, **kwargs)
        spider._follow_links = crawler.settings.getbool(
            'CRAWLSPIDER_FOLLOW_LINKS', True)
        return spider

    def set_crawler(self, crawler):
        super(CrawlSpider, self).set_crawler(crawler)
        self._follow_links = crawler.settings.getbool('CRAWLSPIDER_FOLLOW_LINKS', True)