[python]?view plaincopy

#-*-?encoding=utf-8?-*-??

print?'----------------------方法1--------------------------'??

#方法1,實現__new__方法??

#并在將一個類的實例綁定到類變量_instance上,??

#如果cls._instance為None說明該類還沒有實例化過,實例化該類,并返回??

#如果cls._instance不為None,直接返回cls._instance??

class?Singleton(object):??

????def?__new__(cls,?*args,?**kw):??

????????if?not?hasattr(cls,?'_instance'):??

????????????orig?=?super(Singleton,?cls)??

????????????cls._instance?=?orig.__new__(cls,?*args,?**kw)??

????????return?cls._instance??

??

class?MyClass(Singleton):??

????a?=?1??

??

one?=?MyClass()??

two?=?MyClass()??

??

two.a?=?3??

print?one.a??

#3??

#one和two完全相同,可以用id(),?==,?is檢測??

print?id(one)??

#29097904??

print?id(two)??

#29097904??

print?one?==?two??

#True??

print?one?is?two??

#True??

??

print?'----------------------方法2--------------------------'??

#方法2,共享屬性;所謂單例就是所有引用(實例、對象)擁有相同的狀態(屬性)和行為(方法)??

#同一個類的所有實例天然擁有相同的行為(方法),??

#只需要保證同一個類的所有實例具有相同的狀態(屬性)即可??

#所有實例共享屬性的最簡單最直接的方法就是__dict__屬性指向(引用)同一個字典(dict)??

#可參看:http://code.activestate.com/recipes/66531/??

class?Borg(object):??

????_state?=?{}??

????def?__new__(cls,?*args,?**kw):??

????????ob?=?super(Borg,?cls).__new__(cls,?*args,?**kw)??

????????ob.__dict__?=?cls._state??

????????return?ob??

??

class?MyClass2(Borg):??

????a?=?1??

??

one?=?MyClass2()??

two?=?MyClass2()??

??

#one和two是兩個不同的對象,id,?==,?is對比結果可看出??

two.a?=?3??

print?one.a??

#3??

print?id(one)??

#28873680??

print?id(two)??

#28873712??

print?one?==?two??

#False??

print?one?is?two??

#False??

#但是one和two具有相同的（同一個__dict__屬性）,見:??

print?id(one.__dict__)??

#30104000??

print?id(two.__dict__)??

#30104000??

??

print?'----------------------方法3--------------------------'??

#方法3:本質上是方法1的升級（或者說高級）版??

#使用__metaclass__（元類）的高級python用法??

class?Singleton2(type):??

????def?__init__(cls,?name,?bases,?dict):??

????????super(Singleton2,?cls).__init__(name,?bases,?dict)??

????????cls._instance?=?None??

????def?__call__(cls,?*args,?**kw):??

????????if?cls._instance?is?None:??

????????????cls._instance?=?super(Singleton2,?cls).__call__(*args,?**kw)??

????????return?cls._instance??

??

class?MyClass3(object):??

????__metaclass__?=?Singleton2??

??

one?=?MyClass3()??

two?=?MyClass3()??

??

two.a?=?3??

print?one.a??

#3??

print?id(one)??

#31495472??

print?id(two)??

#31495472??

print?one?==?two??

#True??

print?one?is?two??

#True??

??

print?'----------------------方法4--------------------------'??

#方法4:也是方法1的升級（高級）版本,??

#使用裝飾器(decorator),??

#這是一種更pythonic,更elegant的方法,??

#單例類本身根本不知道自己是單例的,因為他本身(自己的代碼)并不是單例的??

def?singleton(cls,?*args,?**kw):??

????instances?=?{}??

????def?_singleton():??

????????if?cls?not?in?instances:??

????????????instances[cls]?=?cls(*args,?**kw)??

????????return?instances[cls]??

????return?_singleton??

?

@singleton??

class?MyClass4(object):??

????a?=?1??

????def?__init__(self,?x=0):??

????????self.x?=?x??

??

one?=?MyClass4()??

two?=?MyClass4()??

??

two.a?=?3??

print?one.a??

#3??

print?id(one)??

#29660784??

print?id(two)??

#29660784??

print?one?==?two??

#True??

print?one?is?two??

#True??

one.x?=?1??

print?one.x??

#1??

print?two.x??

#1??

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