python构建自定义回调函数详解

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()

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,**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):

  @callback
  def crawl(self,**kwargs):
    url = quote_chinese(_build_url(url.strip(),kwargs.get('params')))
    return requests.get(url)