Scrapy HTTP状态代码未处理或不允许

from scrapy import signals
from scrapy.exceptions import IgnoreRequest

class HttpStatusCodeMiddleware:
    def __init__(self, allowed_status_codes=None):
        self.allowed_status_codes = allowed_status_codes or []

    @classmethod
    def from_crawler(cls, crawler):
        middleware = cls(crawler.settings.getlist('HTTP_STATUS_ALLOWED_CODES'))
        crawler.signals.connect(middleware.spider_opened, signal=signals.spider_opened)
        return middleware

    def spider_opened(self, spider):
        spider.logger.info('HttpStatusCodeMiddleware: Allowed status codes: %s',
                           self.allowed_status_codes)

    def process_response(self, request, response, spider):
        if response.status not in self.allowed_status_codes:
            spider.logger.warning('HttpStatusCodeMiddleware: Ignoring response with status code %d: %s',
                                  response.status, request.url)
            raise IgnoreRequest()
        return response

HTTP_STATUS_ALLOWED_CODES = [404, 500]
DOWNLOADER_MIDDLEWARES = {
    'myproject.middlewares.HttpStatusCodeMiddleware': 543,
}