Scrapy 1.3.3 . 有时发生的反应是无 .

Traceback（最近一次调用最后一次）：文件“/home/vetos/venvs/aliber/lib/python3.5/site-packages/twisted/internet/defer.py”，第1301行，在_inlineCallbacks中结果= g.send（结果）文件“/home/vetos/venvs/aliber/lib/python3.5/site-packages/scrapy/core/downloader/middleware.py”，第47行，在process_response断言响应中不是None，'在process_response中收到无'AssertionError：在process_response中收到无

我有一个中间件：

class CustomProcessProxyResponse(object):
EXCEPTIONS_TO_RETRY = (defer.TimeoutError, TimeoutError, DNSLookupError,
                       ConnectionRefusedError, ConnectionDone, ConnectError,
                       ConnectionLost, TCPTimedOutError, ResponseFailed,
                       IOError, TunnelError)
RETRY_HTTP_CODES = [500, 502, 503, 504, 408, 403, 401, 400, 404, 408]

def __init__(self, settings):
    pass

@classmethod
def from_crawler(cls, crawler):
    return cls(crawler.settings)

def process_response(self, request, response, spider):
    if response is None:
        reason = ('Response is None; Request {}'.format(request))
        return self._retry(request, reason, spider) or response
    if response.status in self.RETRY_HTTP_CODES:
        reason = response_status_message(response.status)
        return self._retry(request, reason, spider) or response
    return response

def process_exception(self, request, exception, spider):
    return self._retry(request, exception, spider)

def _retry(self, request, reason, spider):
    proxy = request.meta['proxy']
    logger.debug("Gave up fail %(request)s (failed %(proxy)s ): %(reason)s",
                 {'request': request, 'proxy': proxy, 'reason': reason},
                 extra={'spider': spider})
    ProxyManager().remove_from_databаse(proxy)
    retryreq = request.copy()
    retryreq.dont_filter = True
    retryreq.productId = request.productId
    return retryreq

settings.py：

DOWNLOADER_MIDDLEWARES = {
'aliexpress_affiliate_api.middlewares.CustomProxyMiddleware': 100,
'aliexpress_affiliate_api.middlewares.CustomProcessProxyResponse': 990,}


class CustomProxyMiddleware(object):
def process_request(self, request, spider):
    if spider.name == AliProductDetailExtender.name:
        proxy_manager = ProxyManager()
        proxy_server, country = proxy_manager.get_random_proxy_from_database()
        request.meta['proxy'] = proxy_server
        request.meta['proxy_country'] = country
        print(request.meta['proxy'])

执行错误拦截和重试请求但有时会发生此错误 . 当它是 None 时如何拦截答案？

python scrapy response