def download_request(self, request, spider):
        """Return a deferred for the HTTP download"""
        agent = ScrapyAgent(contextFactory=self._contextFactory, pool=self._pool,
            maxsize=getattr(spider, 'download_maxsize', self._default_maxsize),
            warnsize=getattr(spider, 'download_warnsize', self._default_warnsize),
            fail_on_dataloss=self._fail_on_dataloss)
        return agent.download_request(request)
Return a deferred for the HTTP download
stackoverflow/venv/lib/python3.6/site-packages/scrapy/core/downloader/handlers/http11.py