    def from_crawler(cls, crawler):
        splash_base_url = crawler.settings.get('SPLASH_URL',
                                               cls.default_splash_url)
        log_400 = crawler.settings.getbool('SPLASH_LOG_400', True)
        slot_policy = crawler.settings.get('SPLASH_SLOT_POLICY',
                                           cls.default_policy)
        if slot_policy not in SlotPolicy._known:
            raise NotConfigured("Incorrect slot policy: %r" % slot_policy)

        return cls(crawler, splash_base_url, slot_policy, log_400)