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HomeBackend DevelopmentPython TutorialPython回调函数用法实例详解

本文实例讲述了Python回调函数用法。分享给大家供大家参考。具体分析如下:

一、百度百科上对回调函数的解释:

回调函数就是一个通过函数指针调用的函数。如果你把函数的指针(地址)作为参数传递给另一个函数,当这个指针被用为调用它所指向的函数时,我们就说这是回调函数。回调函数不是由该函数的实现方直接调用,而是在特定的事件或条件发生时由另外的一方调用的,用于对该事件或条件进行响应。

二、什么是回调:

软件模块之间总是存在着一定的接口,从调用方式上,可以把他们分为三类:同步调用、回调和异步调用。同步调用是一种阻塞式调用,调用方要等待对方执行完毕才返回,它是一种单向调用;回调是一种双向调用模式,也就是说,被调用方在接口被调用时也会调用对方的接口;异步调用是一种类似消息或事件的机制,不过它的调用方向刚好相反,接口的服务在收到某种讯息或发生某种事件时,会主动通知客户方(即调用客户方的接口)。回调和异步调用的关系非常紧密,通常我们使用回调来实现异步消息的注册,通过异步调用来实现消息的通知。同步调用是三者当中最简单的,而回调又常常是异步调用的基础,因此,下面我们着重讨论回调机制在不同软件架构中的实现。

三、一个小例子:

#call.py 
import called 
def callback(): 
  print "in callback" 
def main(): 
  #called.test() 
  called.test_call(callback) 
  print "in call.py" 
main() 
#called.py 
''''' 
def test(): 
  print "in called.py test()" 
''' 
def test_call(p_call): 
  print "in called.py test_call()" 
  p_call() 
joe@joe:~/test/python$ python call.py 
in called.py test_call() 
in callback 
in call.py 
joe@joe:~/test/python$

网上搜到的一个面向对象实现的例子:

当你要加入回调(Callback)功能的时候,代码往往会偏重于回调的实现而不是问题本身了。一个解决方法就是实现一个通用的基础类来解决回调的需求,然后再来实现你为某个事件(Event)所绑定(Binding)的方法(Method)。

代码如下:

class CallbackBase: 
  def __init__(self): 
 self.__callbackMap = {} 
 for k in (getattr(self, x) for x in dir(self)): 
   if hasattr(k, "bind_to_event"): 
 self.__callbackMap.setdefault(k.bind_to_event, []).append(k) 
   elif hasattr(k, "bind_to_event_list"): 
 for j in k.bind_to_event_list: 
   self.__callbackMap.setdefault(j, []).append(k) 
  ## staticmethod is only used to create a namespace 
  @staticmethod 
  def callback(event): 
 def f(g, ev = event): 
   g.bind_to_event = ev 
   return g 
 return f 
  @staticmethod 
  def callbacklist(eventlist): 
 def f(g, evl = eventlist): 
   g.bind_to_event_list = evl 
   return g 
 return f 
  def dispatch(self, event): 
 l = self.__callbackMap[event] 
 f = lambda *args, **kargs: \ 
   map(lambda x: x(*args, **kargs), l) 
 return f 
## Sample 
class MyClass(CallbackBase): 
  EVENT1 = 1 
  EVENT2 = 2 
  @CallbackBase.callback(EVENT1) 
  def handler1(self, param = None): 
 print "handler1 with param: %s" % str(param) 
 return None 
  @CallbackBase.callbacklist([EVENT1, EVENT2]) 
  def handler2(self, param = None): 
 print "handler2 with param: %s" % str(param) 
 return None 
  def run(self, event, param = None): 
 self.dispatch(event)(param) 
if __name__ == "__main__": 
  a = MyClass() 
  a.run(MyClass.EVENT1, 'mandarina') 
  a.run(MyClass.EVENT2, 'naranja') 

这里有一个类,它有两个事件(EVENT1和EVENT2)和两个处理函数(handler)。第一个处理函数handler1注册了EVENT1,而第二个处理函数handler2当EVENT1或者EVENT2发生的时候都会执行(即注册了全部的事件)。

运行函数(run)在MyClass的主循环中,它会将对应的事件派送(dispatch)出去。这(这里指dispatch函数)会返回一个函数,我们可以把所有需要传给这个函数的参数列表传给它。这个函数运行结束会返回一个列表(list),列表中是所有的返回值。

也许,使用Metaclass能够实现的更优雅一些吧。

希望本文所述对大家的Python程序设计有所帮助。

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