python 设计模式-装饰器模式

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装饰器模式实现起来跟代理模式差多，其实可以是一样的，看你把它置于什么场景。代理模式侧重于隔离客户与被代理者，通过代理者控制访问被代理者的权限，而装饰器模式侧重于给被装饰的对象提供附加的功能，例如加上日志功能。python 语法层面就提供了装饰器，足以说明装饰器模式的重要性，这里我们就直接介绍python的装饰器吧。

先看看函数装饰器：

def work_wrapper(w):def func(*args, **kwargs):print('logging start ...')w(*args, **kwargs)print('logging end ...')return func@work_wrapper
def work():print('working')if __name__ == '__main__':work()

输出：

logging start ...
working
logging end ...

@符号是装饰器的标志，函数work上加一行@work_wrapper的意思是 work = work_wrapper(work)。装饰器可以叠加，我们修改下：

def work_wrapper_a(w):def func(*args, **kwargs):print('a logging start ...')w(*args, **kwargs)print('a logging end ...')return funcdef work_wrapper_b(w):def func(*args, **kwargs):print('b logging start ...')w(*args, **kwargs)print('b logging end ...')return func@work_wrapper_b
@work_wrapper_a
def work():print('working')if __name__ == '__main__':work()

输出

b logging start ...
a logging start ...
working
a logging end ...
b logging end ...

装饰器还可以带参数，如下：

def work_para_wrapper(work_times): def work_wrapper(w):def func(*args, **kwargs):print('logging start ...')[w(*args, **kwargs) for i in range(work_times)]print('logging end ...')return funcreturn work_wrapper@work_para_wrapper(3)
def work():print('working')if __name__ == '__main__':work()

其实跟不带参数的装饰器原理一样，work_para_wrapper(3)会返回一个不带参数装饰器。不但函数可以应用装饰器，类也可以，看下面例子：

def worker_wrapper(cls):class WorkerEnhanced(cls):def learn(self):print('learning new things, {}'.format(self._name))return WorkerEnhanced@worker_wrapper
class Worker:def __init__(self, name):self._name = namedef work(self):print('working')if __name__ == '__main__':w = Worker('Jack')print(type(w))w.learn()

输出

<class '__main__.worker_wrapper.<locals>.WorkerEnhanced'>
learning new things, Jack

另外，利用类的魔术方法 __call__，类也可以作为装饰器。

class WorkerEnhanced:def __init__(self, worker):self._worker = worker()def __call__(self, name):self._worker.Name = namereturn self def learn(self):print('{} is learning new things'.format(self._worker.Name))def work(self):self._worker.work()@WorkerEnhanced
class Worker:def __init__(self, name=None):self._name = name@propertydef Name(self):return self._name@Name.setterdef Name(self, val):self._name = valdef work(self):print('{} is working'.format(self._name))if __name__ == '__main__':w = Worker('Jack')print(type(w))w.work()w.learn()

输出：

<class '__main__.WorkerEnhanced'>
Jack is working
Jack is learning new things

这个例子没什么意义，只是为了说明下怎么使用类来装饰类。最后强调一下，不论是函数装饰器还是类装饰器，原理都是同一个 $\frac{@decorate}{to\_be\_decorated}$ ==> $to\_be\_decorated=decorate(to\_be\_decorated)$

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