Scarpy源码分析9

2021SC@SDUSC

跟了这么远，我们现在回到 ExecutionEngine 中的 _next_request_from_scheduler 方法，为了方便，我把这段代码再复制下放到这里来

# scrapy/core/engine.py

class ExecutionEngine(object):
    # ...

    def _next_request_from_scheduler(self, spider):
        slot = self.slot
        request = slot.scheduler.next_request()
        if not request:
            return
        d = self._download(request, spider)
        d.addBoth(self._handle_downloader_output, request, spider)
        d.addErrback(lambda f: logger.info('Error while handling downloader output',
                                           exc_info=failure_to_exc_info(f),
                                           extra={'spider': spider}))
        d.addBoth(lambda _: slot.remove_request(request))
        d.addErrback(lambda f: logger.info('Error while removing request from slot',
                                           exc_info=failure_to_exc_info(f),
                                           extra={'spider': spider}))
        d.addBoth(lambda _: slot.nextcall.schedule())
        d.addErrback(lambda f: logger.info('Error while scheduling new request',
                                           exc_info=failure_to_exc_info(f),
                                           extra={'spider': spider}))

再跟下 _handle_downloader_output

class ExecutionEngine(object):
    # ...

    def _handle_downloader_output(self, response, request, spider):
        assert isinstance(response, (Request, Response, Failure)), response
        # downloader middleware can return requests (for example, redirects)
        if isinstance(response, Request):
            self.crawl(response, spider)
            return
        # response is a Response or Failure
        d = self.scraper.enqueue_scrape(response, request, spider)
        d.addErrback(lambda f: logger.error('Error while enqueuing downloader output',
                                            exc_info=failure_to_exc_info(f),
                                            extra={'spider': spider}))

• 如果 response 是请求的话，执行 crawl。因为响应会经过下载器中间件，所以有可能返回的是请求
• 将响应以及请求加到 scraper 的处理队列中，这部分操作和 Spider Middleware 和 Item Pipelines 有关联。感兴趣的也可以像跟 downloader 那样慢慢跟下去，多看几遍，应该能大致理解，这里也不展开了