a. 爬虫中yield Request(...dont_filter=False) b. 类 from scrapy.dupefilter import BaseDupeFilterimport redisfrom scrapy.utils.request import request_fingerprintclass XzxDupefilter(BaseDupeFilter):def __init__(self,key):self.conn = Noneself.key = key@classmethoddef from_settings(cls, settings):key = settings.get('DUP_REDIS_KEY')return cls(key)def open(self):self.conn = redis.Redis(host='127.0.0.1',port=6379)def request_seen(self, request):fp = request_fingerprint(request)added = self.conn.sadd(self.key, fp)return added == 0c. settings中配置# 默认dupefilter# DUPEFILTER_CLASS = 'scrapy.dupefilter.RFPDupeFilter'DUPEFILTER_CLASS = 'xzx.dupfilter.XzxDupefilter'  # 可以自定义的 

　　　　　　　　补充：调度器中有一段代码来规定def enqueue_request(self, request):# dont_filter=True, =>  False  -> 添加到去重规则：False，True# dont_filter=False, => True  -> 添加到去重规则： False，Trueif not request.dont_filter and self.df.request_seen(request):return False# 添加到调度器dqok = self._dqpush(request)

a. 　　　　scrapy中下载中间件的作用？统一对所有请求批量对request对象进行下载前的预处理。b. 针对user-agent，默认中间件 内置的默认的执行， 获取的是stettings 中自己配置的user-agentclass UserAgentMiddleware(object):"""This middleware allows spiders to override the user_agent"""def __init__(self, user_agent='Scrapy'):self.user_agent = user_agent # USER_AGENT = 'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.99 Safari/537.36'@classmethoddef from_crawler(cls, crawler):o = cls(crawler.settings['USER_AGENT'])crawler.signals.connect(o.spider_opened, signal=signals.spider_opened)return odef spider_opened(self, spider):self.user_agent = getattr(spider, 'user_agent', self.user_agent)def process_request(self, request, spider):if self.user_agent:request.headers.setdefault(b'User-Agent', self.user_agent)c. 关于重定向   内置对的默认的class BaseRedirectMiddleware(object):enabled_setting = 'REDIRECT_ENABLED'def __init__(self, settings):if not settings.getbool(self.enabled_setting):raise NotConfiguredself.max_redirect_times = settings.getint('REDIRECT_MAX_TIMES')self.priority_adjust = settings.getint('REDIRECT_PRIORITY_ADJUST')@classmethoddef from_crawler(cls, crawler):return cls(crawler.settings)def _redirect(self, redirected, request, spider, reason):ttl = request.meta.setdefault('redirect_ttl', self.max_redirect_times)redirects = request.meta.get('redirect_times', 0) + 1if ttl and redirects <= self.max_redirect_times:redirected.meta['redirect_times'] = redirectsredirected.meta['redirect_ttl'] = ttl - 1redirected.meta['redirect_urls'] = request.meta.get('redirect_urls', []) + \[request.url]redirected.dont_filter = request.dont_filterredirected.priority = request.priority + self.priority_adjustlogger.debug("Redirecting (%(reason)s) to %(redirected)s from %(request)s",{'reason': reason, 'redirected': redirected, 'request': request},extra={'spider': spider})return redirectedelse:logger.debug("Discarding %(request)s: max redirections reached",{'request': request}, extra={'spider': spider})raise IgnoreRequest("max redirections reached")def _redirect_request_using_get(self, request, redirect_url):redirected = request.replace(url=redirect_url, method='GET', body='')redirected.headers.pop('Content-Type', None)redirected.headers.pop('Content-Length', None)return redirectedclass RedirectMiddleware(BaseRedirectMiddleware):"""Handle redirection of requests based on response statusand meta-refresh html tag."""def process_response(self, request, response, spider):if (request.meta.get('dont_redirect', False) orresponse.status in getattr(spider, 'handle_httpstatus_list', []) orresponse.status in request.meta.get('handle_httpstatus_list', []) orrequest.meta.get('handle_httpstatus_all', False)):return responseallowed_status = (301, 302, 303, 307, 308)if 'Location' not in response.headers or response.status not in allowed_status:return responselocation = safe_url_string(response.headers['location'])redirected_url = urljoin(request.url, location)if response.status in (301, 307, 308) or request.method == 'HEAD':redirected = request.replace(url=redirected_url)return self._redirect(redirected, request, spider, response.status)redirected = self._redirect_request_using_get(request, redirected_url)return self._redirect(redirected, request, spider, response.status)d. 关于cookie   是内置的默认的就执行　　　　　　　　　　　　　　用法 自己写的逻辑里 yield 加上meta={“cookieJar”:1}}：　　　　　　　　

def start_requests(self):    for url in self.start_urls:        yield Request(url=url,callback=self.parse,meta={"cookieJar":1})

            class CookiesMiddleware(object):"""This middleware enables working with sites that need cookies"""def __init__(self, debug=False):self.jars = defaultdict(CookieJar)self.debug = debug@classmethoddef from_crawler(cls, crawler):if not crawler.settings.getbool('COOKIES_ENABLED'):raise NotConfiguredreturn cls(crawler.settings.getbool('COOKIES_DEBUG'))def process_request(self, request, spider):if request.meta.get('dont_merge_cookies', False):return# cookiejarkey = 1cookiejarkey = request.meta.get("cookiejar")jar = self.jars[cookiejarkey] # CookieJar对象-> 空容器cookies = self._get_request_cookies(jar, request)for cookie in cookies:jar.set_cookie_if_ok(cookie, request)# set Cookie headerrequest.headers.pop('Cookie', None)jar.add_cookie_header(request)self._debug_cookie(request, spider)def process_response(self, request, response, spider):if request.meta.get('dont_merge_cookies', False):return response# extract cookies from Set-Cookie and drop invalid/expired cookiescookiejarkey = request.meta.get("cookiejar")jar = self.jars[cookiejarkey]jar.extract_cookies(response, request)self._debug_set_cookie(response, spider)return responsedef _debug_cookie(self, request, spider):if self.debug:cl = [to_native_str(c, errors='replace')for c in request.headers.getlist('Cookie')]if cl:cookies = "\n".join("Cookie: {}\n".format(c) for c in cl)msg = "Sending cookies to: {}\n{}".format(request, cookies)logger.debug(msg, extra={'spider': spider})def _debug_set_cookie(self, response, spider):if self.debug:cl = [to_native_str(c, errors='replace')for c in response.headers.getlist('Set-Cookie')]if cl:cookies = "\n".join("Set-Cookie: {}\n".format(c) for c in cl)msg = "Received cookies from: {}\n{}".format(response, cookies)logger.debug(msg, extra={'spider': spider})def _format_cookie(self, cookie):# build cookie stringcookie_str = '%s=%s' % (cookie['name'], cookie['value'])if cookie.get('path', None):cookie_str += '; Path=%s' % cookie['path']if cookie.get('domain', None):cookie_str += '; Domain=%s' % cookie['domain']return cookie_strdef _get_request_cookies(self, jar, request):if isinstance(request.cookies, dict):cookie_list = [{'name': k, 'value': v} for k, v in \six.iteritems(request.cookies)]else:cookie_list = request.cookiescookies = [self._format_cookie(x) for x in cookie_list]headers = {'Set-Cookie': cookies}response = Response(request.url, headers=headers)return jar.make_cookies(response, request)默认中间件：DOWNLOADER_MIDDLEWARES_BASE = {# Engine side'scrapy.downloadermiddlewares.robotstxt.RobotsTxtMiddleware': 100,'scrapy.downloadermiddlewares.httpauth.HttpAuthMiddleware': 300,'scrapy.downloadermiddlewares.downloadtimeout.DownloadTimeoutMiddleware': 350,'scrapy.downloadermiddlewares.defaultheaders.DefaultHeadersMiddleware': 400,'scrapy.downloadermiddlewares.useragent.UserAgentMiddleware': 500,'scrapy.downloadermiddlewares.retry.RetryMiddleware': 550,'scrapy.downloadermiddlewares.ajaxcrawl.AjaxCrawlMiddleware': 560,'scrapy.downloadermiddlewares.redirect.MetaRefreshMiddleware': 580,'scrapy.downloadermiddlewares.httpcompression.HttpCompressionMiddleware': 590,'scrapy.downloadermiddlewares.redirect.RedirectMiddleware': 600,'scrapy.downloadermiddlewares.cookies.CookiesMiddleware': 700,'scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware': 750,'scrapy.downloadermiddlewares.stats.DownloaderStats': 850,'scrapy.downloadermiddlewares.httpcache.HttpCacheMiddleware': 900,# Downloader side}

编写中间件class XzxSpiderMiddleware(object):# Not all methods need to be defined. If a method is not defined,# scrapy acts as if the spider middleware does not modify the# passed objects.@classmethoddef from_crawler(cls, crawler):# This method is used by Scrapy to create your spiders.s = cls()return sdef process_spider_input(self, response, spider):# Called for each response that goes through the spider# middleware and into the spider.# Should return None or raise an exception.return Nonedef process_spider_output(self, response, result, spider):# Called with the results returned from the Spider, after# it has processed the response.# Must return an iterable of Request, dict or Item objects.for i in result:yield idef process_spider_exception(self, response, exception, spider):# Called when a spider or process_spider_input() method# (from other spider middleware) raises an exception.# Should return either None or an iterable of Response, dict# or Item objects.passdef process_start_requests(self, start_requests, spider):# Called with the start requests of the spider, and works# similarly to the process_spider_output() method, except# that it doesn’t have a response associated.# Must return only requests (not items).for r in start_requests:yield r配置文件：SPIDER_MIDDLEWARES = {'xzx.middlewares.XzxSpiderMiddleware': 543,}内置爬虫中间件 settings 中的配置 ：深度 ：DEPTH_LIMIT = 8优先级DEPTH_PRIORITY = 1， 请求的优先级：0 -1  -2  -3 。。。。DEPTH_PRIORITY = -1，请求的优先级：0 1 2 3 。。。。SPIDER_MIDDLEWARES_BASE = {# Engine side'scrapy.spidermiddlewares.httperror.HttpErrorMiddleware': 50,'scrapy.spidermiddlewares.offsite.OffsiteMiddleware': 500,'scrapy.spidermiddlewares.referer.RefererMiddleware': 700,'scrapy.spidermiddlewares.urllength.UrlLengthMiddleware': 800,'scrapy.spidermiddlewares.depth.DepthMiddleware': 900,# Spider side}