代码改变世界

Python之美--Decorator深入详解(一)

2010-04-24 17:32  SeasonLee  阅读(35997)  评论(2编辑  收藏  举报

There should be one—--and preferably only one –--obvious way to do it.
                                                                                ----摘自The Zen of Python, by Time Peters



                                                         一些往事

  在正式进入Decorator话题之前,请允许我讲一个小故事。
  在最近的项目开发过程中,一位同事在读我的代码的时候,提出质疑,为什么同样的验证代码要重复出现在服务接口中呢?

  如某服务接口是游戏中的玩家想建造建筑:

def build(user, build_name):
if not is_user_valid(user):
redirect(
"/auth/login/")
return False
#do something here
return create_building(build_name)

  在build这个方法中,前3行是用来检测玩家是否已经验证过的,如果是非验证的玩家,就重定向到登陆页面;如果是验证过的玩家,就给他建造他想要的建筑。
  他指出这样的3行验证代码(虽然已经很短)将会出现任意一个玩家服务接口中——升级建筑、拆建筑、甚至是频繁的聊天,我说这只是Ctrl+C、Ctrl+V的时间啊,但是那时候我深想,如果现在需求有变动,要在原来的验证失败的情况下,写日志,那原来的3行代码就变成4行:
if not is_user_valid(user):
redirect(
"/auth/login/")
log_warning(
" %s is trying to enter without permission!" %(user["name"]))
return False

 

  更痛苦的是,你要在每个出现if not is_user_valid(user)的地方增加log_warning这一行,这些时间足够你泡一壶好茶、听完一首《Poker Face》、tweet好几次、甚至修复了一个bug……

  正如文章开头Time Peters所说的那样,总会有一个最好(合适)的方法来完成一件事。之后请教赖总,就祭出Decorator。以下是我基于Decorator改写的验证代码:

def authenticated(method):
def wrapper(user, *args):
if not is_user_valid(user):
redirect(
"/auth/login/")
log_warning(
" %s is trying to enter without permission!" %(user["name"]))
return False
return method(user, *args)
return wrapper
@authenticated
def build(user, build_name):
return create_building(build_name)

 

 

 


  使用Decorator的好处是玩家服务接口——升级建筑、拆建筑、聊天,需要进行验证的时候,只需要在方法前加上@authenticated就可,更重要的是因需求而对验证失败情况的处理时(如上面讲到的log),并不会影响原有代码的结构,因为你只要在authenticated方法中加入log_warning这一行就搞掂啦!


  毫无疑问Decorator对于维持代码结构起到神一样的作用,下面让我们进入Decorator之旅,PS:如果你已经是Decorator高手,请先别急着Ctrl+W,望不吝赐教,指出笔者在文中的问题和疏漏,不胜感激!

 

 

                                                                        初始Decorator

  Decorator,修饰符,是在Python2.4中增加的功能,也是pythoner实现元编程的最新方式,同时它也是最简单的元编程方式。为什么是“最简单”呢?是的,其实在Decorator之前就已经有classmethod()和staticmethod()内置函数,但他们的缺陷是会导致函数名的重复使用(可以看看David Mertz的Charming Python: Decorators make magic easy ),以下是摘自他本人的原文:

class C:
def foo(cls, y):
print "classmethod", cls, y
foo
= classmethod(foo)
  是的,classmethod做的只是函数转换,但是它却让foo这个名字另外出现了2次。记得有一句话是:人类因懒惰而进步。Decorator的诞生,让foo少出现2次。
class C:
@classmethod
def foo(cls, y):
print "classmethod", cls, y

  读者也许已经想到Decorator在python中是怎么处理的了(如果还没头绪的,强烈建议先去看看limodou写的Decorator学习笔记 )。下面我列出4种用法。

 

写法 使用Decorator 不使用Decorator
单个Decorator,不带参数 @dec
def method(args):
    pass
def method(args):
    pass
method = dec(method)
多个Decorator,不带参数 @dec_a
@dec_b
@dec_c
def method(args):
    pass
def method(args):
    pass
method = dec_a(dec_b(dec_c(method)))
单个Decorator,带参数 @dec(params)
def method(args):
    pass
def method(args):
    pass
method = dec(params)(method)
多个Decorator,带参数 @dec_a(params1)
@dec_b(params2)
@dec_c(params3)
def method(args):
    pass
def method(args):
    pass
method = dec_a(params1)(dec_b(params2)(dec_c(params)(method)))




                                                          
单个 Decorator,不带参数

  设想一个情景,你平时去买衣服的时候,跟售货员是怎么对话的呢?售货员会先向你问好,然后你会试穿某件你喜爱的衣服。

def salesgirl(method):
def serve(*args):
print "Salesgirl:Hello, what do you want?", method.__name__
method(
*args)
return serve

@salesgirl
def try_this_shirt(size):
if size < 35:
print "I: %d inches is to small to me" %(size)
else:
print "I:%d inches is just enough" %(size)
try_this_shirt(
38)

 


  结果是:

Salesgirl:Hello, what do you want? try_this_shirt
I:38 inches is just enough

 

 

  这只是一个太简单的例子,以至一些“细节”没有处理好,你试穿完了好歹也告诉salesgirl到底要不要买啊。。。这样try_this_shirt方法需要改成带返回值 (假设是bool类型,True就是要买,False就是不想买),那么salesgirl中的serve也应该带返回值,并且返回值就是 method(*args)。


修改后的salesgirl  

 

def salesgirl(method):
def serve(*args):
print "Salesgirl:Hello, what do you want?", method.__name__
return method(*args)
return serve

@salesgirl
def try_this_shirt(size):
if size < 35:
print "I: %d inches is to small to me" %(size)
return False
else:
print "I:%d inches is just enough" %(size)
return True
result
= try_this_shirt(38)
print "Mum:do you want to buy this?", result

  结果是:

Salesgirl:Hello, what do you want? try_this_shirt
I:38 inches is just enough
Mum:do you want to buy this? True

 

 

 

  现在我们的salesgirl还不算合格,她只会给客人打招呼,但是客人要是买衣服了,也不会给他报价;客人不买的话,也应该推荐其他款式!


  会报价的salesgirl:

def salesgirl(method):
def serve(*args):
print "Salesgirl:Hello, what do you want?", method.__name__
result
= method(*args)
if result:
print "Salesgirl: This shirt is 50$."
else:
print "Salesgirl: Well, how about trying another style?"
return result
return serve

@salesgirl
def try_this_shirt(size):
if size < 35:
print "I: %d inches is to small to me" %(size)
return False
else:
print "I:%d inches is just enough" %(size)
return True
result
= try_this_shirt(38)
print "Mum:do you want to buy this?", result

 


  结果是:

Salesgirl:Hello, what do you want? try_this_shirt
I:38 inches is just enough
Salesgirl: This shirt is 50$.
Mum:do you want to buy this? True

 

  这样的salesgirl总算是合格了,但离出色还很远,称职的salesgirl是应该对熟客让利,老用户总得有点好处吧?


 

                                                            单个 Decorator,带参数
  会报价并且带折扣的salesgirl:

def salesgirl(discount):
def expense(method):
def serve(*args):
print "Salesgirl:Hello, what do you want?", method.__name__
result
= method(*args)
if result:
print "Salesgirl: This shirt is 50$.As an old user, we promised to discount at %d%%" %(discount)
else:
print "Salesgirl: Well, how about trying another style?"
return result
return serve
return expense

@salesgirl(
50)
def try_this_shirt(size):
if size < 35:
print "I: %d inches is to small to me" %(size)
return False
else:
print "I:%d inches is just enough" %(size)
return True
result
= try_this_shirt(38)
print "Mum:do you want to buy this?", result

 

 
  结果是:

Salesgirl:Hello, what do you want? try_this_shirt
I:38 inches is just enough
Salesgirl: This shirt is 50$.As an old user, we promised to discount at 50%
Mum:do you want to buy this? True

 

 

  这里定义的salesgirl是会给客户50%的折扣,因为salesgirl描述符是带参数,而参数就是折扣。如果你是第一次看到这个 salesgirl,会被她里面嵌套的2个方法而感到意外,没关系,当你习惯了Decorator之后,一切都变得很亲切啦。


                                                                  未完,待续

  上面这些也只是一个开胃汤,在下一篇正餐中,我将带来更多Decorator的高级用法,敬请留意。