Rules · TUNA-daily

一般爬蟲的邏輯是：給定起始頁面，發起訪問，分析頁面包含的所有其他鏈接，然后將這些鏈接放入隊列，再逐次訪問這些隊列，直至邊界條件結束。為了針對列表頁+詳情頁這種模式，需要對鏈接抽取（link extractor）的邏輯進行限定。好在scrapy已經提供，關鍵是你知道這個接口，并靈活運用 ## 1 . url 特定截取 ~~~ rules = (Rule(SgmlLinkExtractor(allow=('category/20/index_\d+\.html'), restrict_xpaths=("//div[@class='left']"))), Rule(SgmlLinkExtractor(allow=('a/\d+/\d+\.html'), restrict_xpaths=("//div[@class='left']")), callback='parse_item'), ) ~~~ > 1. Rule是在定義抽取鏈接的規則，上面的兩條規則分別對應列表頁的各個分頁頁面和詳情頁，關鍵點在于通過**restrict_xpath**來限定只從頁面特定的部分來抽取接下來將要爬取的鏈接。 > 2. follow用途：第一：這是我爬取豆瓣新書的規則 rules = (Rule(LinkExtractor(allow=(r’^https://book.douban.com/subject/[0-9]*/’),),callback=’parse_item’,follow=False), )，在這條規則下，我只會爬取定義的start_urls中的和規則符合的鏈接。假設我把follow修改為True，那么爬蟲會start_urls爬取的頁面中在尋找符合規則的url，如此循環，直到把全站爬取完畢。第二：rule無論有無callback，都由同一個_parse_response函數處理，只不過他會判斷是否有follow和callback ## 2. CrawlSpider詳解 CrawlSpider基于Spider，但是可以說是為全站爬取而生。簡要說明 > CrawlSpider是爬取那些具有一定規則網站的常用的爬蟲，它基于Spider并有一些獨特屬性 > rules: 是Rule對象的集合，用于匹配目標網站并排除干擾 > parse_start_url: 用于爬取起始響應，必須要返回Item，Request中的一個。 > 因為rules是Rule對象的集合，所以這里也要介紹一下Rule。它有幾個參數：link_extractor、callback=None、cb_kwargs=None、follow=None、process_links=None、process_request=None > 其中的link_extractor既可以自己定義，也可以使用已有LinkExtractor類，主要參數為： > allow：滿足括號中“正則表達式”的值會被提取，如果為空，則全部匹配。 > deny：與這個正則表達式(或正則表達式列表)不匹配的URL一定不提取。 > allow_domains：會被提取的鏈接的domains。 > deny_domains：一定不會被提取鏈接的domains。 > restrict_xpaths：使用xpath表達式，和allow共同作用過濾鏈接。還有一個類似的restrict_css 下面是官方提供的例子，我將從源代碼的角度開始解讀一些常見問題： ~~~ 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 ~~~ 問題：CrawlSpider如何工作的？因為CrawlSpider繼承了Spider，所以具有Spider的所有函數。首先由start_requests對start_urls中的每一個url發起請求（make_requests_from_url)，這個請求會被parse接收。在Spider里面的parse需要我們定義，但CrawlSpider定義parse去解析響應（self._parse_response(response, self.parse_start_url, ~~~ cb_kwargs={}, follow=True)） _parse_response根據有無callback,follow和self.follow_links執行不同的操作 def _parse_response(self, response, callback, cb_kwargs, follow=True): ##如果傳入了callback，使用這個callback解析頁面并獲取解析得到的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 ## 其次判斷有無follow，用_requests_to_follow解析響應是否有符合要求的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又會獲取link_extractor（這個是我們傳入的LinkExtractor）解析頁面得到的link（link_extractor.extract_links(response)）,對url進行加工（process_links，需要自定義），對符合的link發起Request。使用.process_request(需要自定義）處理響應。問題：CrawlSpider如何獲取rules？ CrawlSpider類會在init方法中調用_compile_rules方法，然后在其中淺拷貝rules中的各個Rule獲取要用于回調(callback)，要進行處理的鏈接（process_links）和要進行的處理請求（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是怎么樣定義的呢？ ~~~ 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會傳給link_extractor。有callback的是由指定的函數處理，沒有callback的是由哪個函數處理的？由上面的講解可以發現_parse_response會處理有callback的（響應）respons。 cb_res = callback(response, **cb_kwargs) or () 而_requests_to_follow會將self._response_downloaded傳給callback用于對頁面中匹配的url發起請求（request）。 r = Request(url=link.url, callback=self._response_downloaded) 如何在CrawlSpider進行模擬登陸因為CrawlSpider和Spider一樣，都要使用start_requests發起請求，用從Andrew_liu大神借鑒的代碼說明如何模擬登陸： ##替換原來的start_requests，callback為 ~~~ 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' #下面這句話用于抓取請求網頁后返回網頁中的_xsrf字段的文字, 用于成功提交表單 xsrf = Selector(response).xpath('//input[@name="_xsrf"]/@value').extract()[0] print xsrf #FormRequeset.from_response是Scrapy提供的一個函數, 用于post表單 #登陸成功后, 會調用after_login回調函數 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會調用parse，就可以與CrawlSpider的parse進行銜接了 def after_login(self, response) : for url in self.start_urls : yield self.make_requests_from_url(url) ~~~ 最后貼上Scrapy.spiders.CrawlSpider的源代碼，以便檢查 ~~~ """ 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) ~~~