scrapy request delay middleware

ScrapyDelayMiddleware.py

class RequestDelayMiddleware(object):
    def process_request(self, request, spider):
        # Add delay_by key in meta
        delay_s = request.meta.get('delay_by', None)
        if not delay_s:
            return

        deferred = Deferred()
        reactor.callLater(delay_s, deferred.callback, None)
        return deferred