为什么 scrapy middleware 使用 from_crawler 来初始化一个 middleware

[bytenoob]

例如：

class CookiesMiddleware(object):
    """This middleware enables working with sites that need cookies"""

    def __init__(self, debug=False):
        self.jars = defaultdict(CookieJar)
        self.debug = debug

    @classmethod
    def from_crawler(cls, crawler):
        if not crawler.settings.getbool('COOKIES_ENABLED'):
            raise NotConfigured
        return cls(crawler.settings.getbool('COOKIES_DEBUG'))

为什么不可以直接在init中使用 crawler 进行初始化呢，这样做有什么好处？

[rebornix]

你看下初始化 middleware 的代码 https://github.com/scrapy/scrapy/blob/6660175de9abddaf29be2b5f74e3f04a389618bf/scrapy/middleware.py#L33

mwcls = load_object(clspath)
if crawler and hasattr(mwcls, 'from_crawler'):
mw = mwcls.from_crawler(crawler)
elif hasattr(mwcls, 'from_settings'):
mw = mwcls.from_settings(settings)
else:
mw = mwcls()
middlewares.append(mw)