像是一般的装饰器，如下：

def wrapper(func):
    def inner(arg):
        # do something
        return func(arg)
    return inner

@wrapper
def func(arg):
    print 'func'

我都理解为将func作为参数传递给wrapper函数，之后改变func指向的函数地址。然而，下面这个我就不是很懂了？

#!/usr/bin/env python
#coding:utf-8
  
def Before(request,kargs):
    print 'before'
      
def After(request,kargs):
    print 'after'
  
  
def Filter(before_func,after_func):
    def outer(main_func):
        def wrapper(request,kargs):
              
            before_result = before_func(request,kargs)
            if(before_result != None):
                return before_result;
              
            main_result = main_func(request,kargs)
            if(main_result != None):
                return main_result;
              
            after_result = after_func(request,kargs)
            if(after_result != None):
                return after_result;
              
        return wrapper
    return outer
      
@Filter(Before, After)
def Index(request,kargs):
    print 'index'