Python之美--Decorator深入详解(一)
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)
def build(user, build_name): if not is_user_valid(user): redirect("/auth/login/") return False #do something here return create_building(build_na
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
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)
def salesgirl(method): def serve(*args): print "Salesgirl:Hello, what do you want?", method.__name__ method(*args) return serve @salesgirldef 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(
结果是：
Salesgirl:Hello, what do you want? try_this_shirt
I:38 inches is just enough
Salesgirl:Hello, what do you want? try_this_shirtI: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
Salesgirl:Hello, what do you want? try_this_shirtI:38 inches is just enoughMum: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$."
else:
print "Salesgirl: Well, how about trying another style?"
return result
return serve
@salesgirl
def try_this_shirt(size):
if size < 35:
print "I: %d inches is to small to me" %(size)
return False
else:
print "I:%d inches is just enough" %(size)
return True
result = try_this_shirt(38)
print "Mum:do you want to buy this?", result
def salesgirl(method): def serve(*args): print "Salesgirl:Hello, what do you want?", method.__name__ result = method(*args) if result: print "Salesgirl: This shirt is 50$." else: print "Salesgirl: Well, how about trying another style?" return result return serve @salesgirldef 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 Trueresult = try_this_shirt(38)print "Mum:do you want to buy this?", resul
结果是：
Salesgirl:Hello, what do you want? try_this_shirt
I:38 inches is just enough
Salesgirl: This shirt is 50$.
Mum:do you want to buy this? True
Salesgirl:Hello, what do you want? try_this_shirtI:38 inches is just enoughSalesgirl: This shirt is 50$.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$.As an old user, we promised to discount at %d%%" %(discount)
else:
print "Salesgirl: Well, how about trying another style?"
return result
return serve
return expense
@salesgirl(50)
def try_this_shirt(size):
if size < 35:
print "I: %d inches is to small to me" %(size)
return False
else:
print "I:%d inches is just enough" %(size)
return True
result = try_this_shirt(38)
print "Mum:do you want to buy this?", result
def salesgirl(discount): def expense(method): def serve(*args): print "Salesgirl:Hello, what do you want?", method.__name__ result = method(*args) if result: print "Salesgirl: This shirt is 50$.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 Trueresult = 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$.As an old user, we promised to discount at 50%
Mum:do you want to buy this? True
Salesgirl:Hello, what do you want? try_this_shirtI:38 inches is just enoughSalesgirl: This shirt is 50$.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。
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