參考了其他的博客：http://ghostfromheaven.iteye.com/blog/1562618

#-*- encoding=utf-8 -*-  
print ‘----------------------方法1--------------------------‘  
#方法1,實現__new__方法  
#並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,並返回  
#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)並不是單例的  
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

PYTHON設計模式學習（3）：Singleton pattern