这是「进击的Coder」的第 689 篇技术分享作者：TheWeiJun来源：逆向与爬虫的故事“
阅读本文大概需要 13 分钟。
”时隔一个多月，scrapy 章节又迎来了重大更新，今天分享的主题是 RetryMiddleware 中间件。文中若有错误内容，欢迎各位读者多多指正。在阅读的同时不要忘记点赞+关注哦⛽️
目录一、问题思考
二、文档查寻
三、源码分析
四、源码重写
五、总结分享
趣味模块
娜娜是一名爬虫工程师，最近小娜在采集数据过程中遇到了难题。原因是因为任务积压代理超时了，所有的 request 全部无法下载了。娜娜很是苦恼，不知道如何解决这类型问题。后来小娜看了 TheWeiJun 发表的文章，存在的问题立马迎刃而解，接下来，让我们一起去看看他们是怎么做的吧。
一、问题思考
Question
①使用 scrapy 框架时，如果请求失败，如何保证该请求成功率？
Question
②scrapy 的重试机制是否了解，默认是几次？在什么样的情况下触发？
Question
③scrapy 重试机制，重试状态码有哪些，我们是否可以动态定义？
Question
④scrapy 在重试过程中，如何实时更换代理？如何清除失效的代理？
前言：那么带着这些问题，接下来我们对 Scrapy 源码进行分析探索吧，我相信这篇文章会让大家受益匪浅！
二、文档查寻
1、查看官网文档，搜索指定的模块 RetryMiddleware，搜索结果如下：
说明：观察搜索结果，我们发现官方文档中存在对 RetryMiddleware 模块的解释，接下来让我们点进去，一起去看看官方说明吧。
2、点击搜索结果，查看官方对当前模块的说明解释截图如下：
说明：观察上面的截图，我们发现上面提到的问题大家应该已经知道了部分答案吧。但是还是不够清晰，接下来，让我带大家进入源码分析环节一探究竟吧！
三、源码分析
RetryMiddleware 模块源码如下：
def get_retry_request(request: Request,*,spider: Spider,reason: Union[str, Exception] = unspecified,max_retry_times: Optional[int] = None,priority_adjust: Optional[int] = None,logger: Logger = retry_logger,stats_base_key: str = retry,):settings = spider.crawler.settingsstats = spider.crawler.statsretry_times = request.meta.get(retry_times, 0) + 1if max_retry_times is None:max_retry_times = request.meta.get(max_retry_times)if max_retry_times is None:max_retry_times = settings.getint(RETRY_TIMES)if retry_times <= max_retry_times:logger.debug("Retrying %(request)s (failed %(retry_times)d times): %(reason)s",{request: request, retry_times: retry_times, reason: reason},extra={spider: spider})new_request: Request = request.copy()new_request.meta[retry_times] = retry_timesnew_request.dont_filter = Trueif priority_adjust is None:priority_adjust = settings.getint(RETRY_PRIORITY_ADJUST)new_request.priority = request.priority + priority_adjustif callable(reason):reason = reason()if isinstance(reason, Exception):reason = global_object_name(reason.__class__)stats.inc_value(f{stats_base_key}/count)stats.inc_value(f{stats_base_key}/reason_count/{reason})return new_requestelse:stats.inc_value(f{stats_base_key}/max_reached)logger.error("Gave up retrying %(request)s (failed %(retry_times)d times): ""%(reason)s",{request: request, retry_times: retry_times, reason: reason},extra={spider: spider},)return Noneclass RetryMiddleware:# IOError is raised by the HttpCompression middleware when trying to# decompress an empty responseEXCEPTIONS_TO_RETRY = (defer.TimeoutError, TimeoutError, DNSLookupError, ConnectionRefusedError, ConnectionDone, ConnectError, ConnectionLost, TCPTimedOutError, ResponseFailed, IOError, TunnelError)def __init__(self, settings):if not settings.getbool(RETRY_ENABLED):raise NotConfiguredself.max_retry_times = settings.getint(RETRY_TIMES)self.retry_http_codes = set(int(x) for x in settings.getlist(RETRY_HTTP_CODES))self.priority_adjust = settings.getint(RETRY_PRIORITY_ADJUST)@classmethoddef from_crawler(cls, crawler):return cls(crawler.settings)def process_response(self, request, response, spider):if request.meta.get(dont_retry, False):return responseif response.status in self.retry_http_codes:reason = response_status_message(response.status)return self._retry(request, reason, spider) or responsereturn responsedef process_exception(self, request, exception, spider):if (isinstance(exception, self.EXCEPTIONS_TO_RETRY)and not request.meta.get(dont_retry, False)):return self._retry(request, exception, spider)def _retry(self, request, reason, spider):max_retry_times = request.meta.get(max_retry_times, self.max_retry_times)priority_adjust = request.meta.get(priority_adjust, self.priority_adjust)return get_retry_request(request,reason=reason,spider=spider,max_retry_times=max_retry_times,priority_adjust=priority_adjust,)环节说明：代码一共也就 94 行，但是却能实现多个功能。在好奇心的驱使下，我们还是对源码进行一一讲解分析吧。
from_crawler 函数
# 类方法,创建当前class的实例对象，参数：当前spider settings对象@classmethoddef from_crawler(cls, crawler): return cls(crawler.settings)__init__ 函数
"""这里涉及到了settings.py配置文件中定义的一些参数。RETRY_ENABLED: 用于开启中间件，默认为TrueRETRY_TIMES: 重试次数, 默认为2RETRY_HTTP_CODES: 遇到哪些返回状态码需要重试, 一个列表，默认为[500, 503, 504, 400, 408]RETRY_PRIORITY_ADJUST：调整相对于原始请求的重试请求优先级，默认为-1"""def __init__(self, settings):if not settings.getbool(RETRY_ENABLED):raise NotConfiguredself.max_retry_times = settings.getint(RETRY_TIMES)self.retry_http_codes = set(int(x) for x in settings.getlist(RETRY_HTTP_CODES))self.priority_adjust = settings.getint(RETRY_PRIORITY_ADJUST)process_response 函数
process_exception 函数
EXCEPTIONS_TO_RETRY = (defer.TimeoutError, TimeoutError, DNSLookupError, ConnectionRefusedError, ConnectionDone, ConnectError, ConnectionLost, TCPTimedOutError, ResponseFailed, IOError, TunnelError) def process_response(self, request, response, spider): # 处理request请求，确定是否需要请求重试，重试触发机制，前面提到的问题.if request.meta.get(dont_retry, False):return response # 检查response状态码是否在重试机制list中,如果存在就要调用_retry方法进行重试if response.status in self.retry_http_codes:reason = response_status_message(response.status)return self._retry(request, reason, spider) or response # 不存在会返回response，但会被spider parse方法是过滤掉，只处理200状态码return responsedef process_exception(self, request, exception, spider): # 如果产生了EXCEPTIONS_TO_RETRY列表中的异常错误并且重试机制为开启状态,则会调用_retry方法进行重试。if (isinstance(exception, self.EXCEPTIONS_TO_RETRY)and not request.meta.get(dont_retry, False)):return self._retry(request, exception, spider)_retry 函数
get_retry_request 函数
# 该方法获取最大重试次数，和请求重试优先级，然后调用get_retry_request方法def _retry(self, request, reason, spider):max_retry_times = request.meta.get(max_retry_times, self.max_retry_times)priority_adjust = request.meta.get(priority_adjust, self.priority_adjust)return get_retry_request(request,reason=reason,spider=spider,max_retry_times=max_retry_times,priority_adjust=priority_adjust,)"""读取当前重试次数和最大重试次数进行比较，如果小于等于最大重试次数：利用copy方法在原来的request上复制一个新request，并更新其retry_times，并将dont_filter设为True来防止因url重复而被过滤。如果超出最大重试次数：记录重试失败请求量，并放弃该请求记录到logger日志中,logger级别为：error"""def get_retry_request(request: Request,*,spider: Spider,reason: Union[str, Exception] = unspecified,max_retry_times: Optional[int] = None,priority_adjust: Optional[int] = None,logger: Logger = retry_logger,stats_base_key: str = retry,):settings = spider.crawler.settingsstats = spider.crawler.statsretry_times = request.meta.get(retry_times, 0) + 1if max_retry_times is None:max_retry_times = request.meta.get(max_retry_times)if max_retry_times is None:max_retry_times = settings.getint(RETRY_TIMES)if retry_times <= max_retry_times:logger.debug("Retrying %(request)s (failed %(retry_times)d times): %(reason)s",{request: request, retry_times: retry_times, reason: reason},extra={spider: spider})new_request: Request = request.copy()new_request.meta[retry_times] = retry_timesnew_request.dont_filter = Trueif priority_adjust is None:priority_adjust = settings.getint(RETRY_PRIORITY_ADJUST)new_request.priority = request.priority + priority_adjustif callable(reason):reason = reason()if isinstance(reason, Exception):reason = global_object_name(reason.__class__)stats.inc_value(f{stats_base_key}/count)stats.inc_value(f{stats_base_key}/reason_count/{reason})return new_requestelse:stats.inc_value(f{stats_base_key}/max_reached)logger.error("Gave up retrying %(request)s (failed %(retry_times)d times): ""%(reason)s",{request: request, retry_times: retry_times, reason: reason},extra={spider: spider},)return None环节总结：整个源码分析流程到这里就结束了，接下来我们一起进入源码重写环节来解决下娜娜遇到的问题吧，我相信大家会豁然开朗的。
四、源码重写
重写 RetryMiddleware 源码后完整代码如下：
class RetryMiddleware:EXCEPTIONS_TO_RETRY = (defer.TimeoutError, TimeoutError, DNSLookupError, ConnectionRefusedError, ConnectionDone, ConnectError, ConnectionLost, TCPTimedOutError, ResponseFailed, IOError, TunnelError)def __init__(self, settings):if not settings.getbool(RETRY_ENABLED):raise NotConfiguredself.max_retry_times = settings.getint(RETRY_TIMES)self.retry_http_codes = set(int(x) for x in settings.getlist(RETRY_HTTP_CODES))self.priority_adjust = settings.getint(RETRY_PRIORITY_ADJUST)@classmethoddef from_crawler(cls, crawler):return cls(crawler.settings)def process_response(self, request, response, spider):if request.meta.get(dont_retry, False):return response if response.status in self.retry_http_codes: # 可以自定义重试状态码reason = response_status_message(response.status)response.last_content = request.metareturn self._retry(request, reason, spider) or responsereturn responsedef process_exception(self, request, exception, spider):if (isinstance(exception, self.EXCEPTIONS_TO_RETRY)and not request.meta.get(dont_retry, False)):return self._retry(request, exception, spider)def _retry(self, request, reason, spider):max_retry_times = request.meta.get(max_retry_times, self.max_retry_times)priority_adjust = request.meta.get(priority_adjust, self.priority_adjust)request.meta[proxy] = "xxx:xxxx"request.headers[Proxy-Authorization] = "proxyauth"return get_retry_request(request,reason=reason,spider=spider,max_retry_times=max_retry_times,priority_adjust=priority_adjust,)重写总结：我们只需要在 _retry 函数中实时更换代理即可，如果涉及到代理池需要剔除失败代理的问题，同样在 _retry 函数中删除代理池中指定代理即可。我们还可以自定义重试机制状态码，大家可自行添加即可！
五、总结分享
通过本次案例分析，上面的几个问题我们都已经得到了答案。今天分享到这里就结束了，欢迎大家关注下期文章，我们不见不散⛽️。最后希望大家多多转发、点赞、在看支持一波
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