scrapy框架中如何实现大文件的下载？

from twisted.web.client import Agent, getPage, ResponseDone, PotentialDataLoss
from twisted.internet import defer, reactor, protocol
from twisted.web._newclient import Response
from io import BytesIO

class _ResponseReader(protocol.Protocol):
    def __init__(self, finished, txresponse, file_name):
        self._finished = finished
        self._txresponse = txresponse
        self._bytes_received = 0
        self.f = open(file_name, mode='wb')
    def dataReceived(self, bodyBytes):
        self._bytes_received += len(bodyBytes)
        # 一点一点的下载
        self.f.write(bodyBytes)
        self.f.flush()
    def connectionLost(self, reason):
        if self._finished.called:
            return
        if reason.check(ResponseDone):
            # 下载完成
            self._finished.callback((self._txresponse, 'success'))
        elif reason.check(PotentialDataLoss):
            # 下载部分
            self._finished.callback((self._txresponse, 'partial'))
        else:
            # 下载异常
            self._finished.errback(reason)
        self.f.close()