python 自定義回調函數

回調函數用起來比較爽。特別是在js中，滿世界全是回調，那么在python中，怎么來優雅地實現自己的回調函數呢

下面貼一個我寫的例子

class BaseHandler(object):

    def crawl(self, url, **kwargs):
        if kwargs.get('callback'):
            callback = kwargs['callback']
            if isinstance(callback, basestring) and hasattr(self, callback):
                func = getattr(self, callback)
            elif hasattr(callback, 'im_self') and callback.im_self is self:
                func = callback

        resp = requests.get(url)
        return func(resp)


class Anjuke(BaseHandler):
    def on_start(self):
        self.crawl(start_url, callback=self.index_url)

    def index_url(self, response):
        print response.text

a = Anjuke()
print a.on_start()

在這里實現的是當self.crawl指定callback方法時，self.crawl會在接收到start_url參數執行完成后回調self.index_url。

而實現方式其實就是通過kwargs獲取到callback的對象/字符串，然后通過getattr(self, callback)來反射得到回調的方法，然后調用即可。

接着可以進一步抽象，增加回調方法的復用，於是BaseHandler可以改寫成裝飾器方式

def callback(f):
    def wrapper(obj, *args, **kwargs):
        resp = f(obj, *args, **kwargs)
        if kwargs.get('callback'):
            mcallback = kwargs['callback']
            if isinstance(callback, basestring) and hasattr(obj, mcallback):
                func = getattr(obj, mcallback)
            elif hasattr(mcallback, 'im_self') and mcallback.im_self is obj:
                func = mcallback
                kwargs['callback'] = func.__name__
            else:
                raise NotImplementedError("self.%s() not implemented!" % callback)

        return func(resp)
    return wrapper


class BaseHandler(object):
　　"""在self.crawl(url,callback=$func)時,通過裝飾器callback即可實現回調,在self.crawl方法中,只需要專注於得到$func的參數即可"""
    @callback
    def crawl(self, url, **kwargs):
        url = quote_chinese(_build_url(url.strip(), kwargs.get('params')))
        return requests.get(url)