python 自定义回调函数

回调函数用起来比较爽。特别是在js中，满世界全是回调，那么在python中，怎么来优雅地实现自己的回调函数呢

下面贴一个我写的例子

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()

在这里实现的是当self.crawl指定callback方法时，self.crawl会在接收到start_url参数执行完成后回调self.index_url。

而实现方式其实就是通过kwargs获取到callback的对象/字符串，然后通过getattr(self, callback)来反射得到回调的方法，然后调用即可。

接着可以进一步抽象，增加回调方法的复用，于是BaseHandler可以改写成装饰器方式

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=$func)时,通过装饰器callback即可实现回调,在self.crawl方法中,只需要专注于得到$func的参数即可"""
    @callback
    def crawl(self, url, **kwargs):
        url = quote_chinese(_build_url(url.strip(), kwargs.get('params')))
        return requests.get(url)