Python装饰器decorator用法实例
本文实例讲述了Python装饰器decorator用法。分享给大家供大家参考。具体分析如下：
1. 闭包（closure）
闭包是Python所支持的一种特性，它让在非global scope定义的函数可以引用其外围空间中的变量，这些外围空间中被引用的变量叫做这个函数的环境变量。环境变量和这个非全局函数一起构成了闭包。
y = [1,2,3]
def inner():
print x
print y
return inner
x = 5 #这个x没有被引用
f = outer(2)
f()
print f.__closure__ #函数属性__closure__存储了函数的环境变量 def entrance(func):
= 5 #这个x没有被引用f = outer(2)f()print f.__closure__ #函数属性__closure__存储了函数的环境变量 def entrance(func):
x和y都是属于函数outer命名空间的，在inner中被引用，当outer函数退出后，outer的命名空间不存在了，但是inner依然维护了其定义时候对其外部变量x，y的连接。
程序输出：
2
[1, 2, 3]
(, )
装饰器是一个可调用对象（a callable），在Python中，函数是对象，当然也是可调用的，所以装饰器可以是一个函数，我们称其为函数装饰器。
这个可调用对象以一个函数作为参数，闭且返回另一个函数（来替换参数那个函数）。
比如：
def inner():
print "inside function :", func.__name__
func()
return inner
entrance是一个装饰器，它是一个函数，它可以接收一个函数func作为参数，返回了另一个函数inner。
那为什么叫装饰器了，在返回函数inner()的内部，调用了func()，而且还作了额外的操作，相当于“装饰”了函数func。
那如何使用装饰器？
pass
fun1 = entrance(fun1)
def fun2():
pass
fun2 = entrance(fun2)
fun1，fun2的名字都没有变，但是通过调用函数装饰器entrance()，它们已经指向了另一个函数inner()，“装饰了”自己。
@操作符
Python提供的@符号，实质上就是上面做的，对一个函数名进行从新赋值，是语法上的技巧。所以上面的代码等价于
def fun1():
pass
@entrance
def fun2():
pass
2. 装饰器的用途
从这个刻意构造的很简单的例子，可以看出装饰器的意义，如果一个函数需要一个功能，如果这个功能可以被使用在很多函数上，或是函数并不是自己实现，那可以写个装饰器来实现这些功能。
上面的装饰器entrance，装饰一个函数后，函数被调用时会打印出这个函数的名字。
但是有一个问题，这个装饰器从功能上看，是要应该可以用来装饰任何函数，但是如果我们用它来装饰了一个带参数的函数
def fun3(x):
pass
只要不调用fun3，这三行代码是不会让Python解释器报错的，因为我们已经知道，它等价于:
pass
fun3 = entrance(fun3)
我们定义了一个带参的函数fun3，然后把fun3指向了另一个函数inner()，当然不会有什么错。
但是，当我们使用fun3时，我们肯定会按照它定义时的样子去使用它，给它传入一个参数。
>>>fun3(1)
这里就会出错了，看看解释器怎么报错的
Traceback (most recent call last):
File “decorator.py”, line 23, in www.jb51.net
fun3(1)
TypeError: inner() takes no arguments (1 given)
当然我们已经很容易知到为什么会这样报错了，fun3已经不是指向它定义时那个函数了，它现在指向了”inner()”，而inner是没有参数的，当然会出错。
那怎么解决呢？
修改一下inner()的定义，让它可以就收任意个参数就可以了。
def inner(*args, **kvargs):
print "inside function : ", func.__name__
func(*args, **kvargs)
return inner
现在，给inner传任意个参数都不会出错了，也就是entrance可以被用来装饰任何一个函数了。
3. 写个装饰器logger
一个函数被调用时，在日志里记录其名称和被调用的实际参数
def inner(*args, **kvargs):
print func.__name__, 'called, arguments: ', args, kvargs
func(*args, **kvargs)
return inner
希望本文所述对大家的Python程序设计有所帮助。
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