def _active_downloads(self, slot):
        """ Return a number of requests in a Downloader for a given slot """
        if slot not in self.downloader.slots:
            return 0
        return len(self.downloader.slots[slot].active)
 Return a number of requests in a Downloader for a given slot 
stackoverflow/venv/lib/python3.6/site-packages/scrapy/pqueues.py