来源：http://blog.csdn.net/ghostfromheaven/article/details/7671853
#-*- encoding=utf-8 -*- print '----------------------方法1--------------------------' #方法1,实现__new__方法 #并在将一个类的实例绑定到类变量_instance上, #如果cls._instance为None说明该类还没有实例化过,实例化该类,并返回 #如果cls._instance不为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检测 print id(one) #29097904 print id(two) #29097904 print one == two #True print one is two #True print '----------------------方法2--------------------------' #方法2,共享属性;所谓单例就是所有引用(实例、对象)拥有相同的状态(属性)和行为(方法) #同一个类的所有实例天然拥有相同的行为(方法), #只需要保证同一个类的所有实例具有相同的状态(属性)即可 #所有实例共享属性的最简单最直接的方法就是__dict__属性指向(引用)同一个字典(dict) #可参看: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是两个不同的对象,id, ==, is对比结果可看出 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（同一个__dict__属性）,见: print id(one.__dict__) #30104000 print id(two.__dict__) #30104000 print '----------------------方法3--------------------------' #方法3:本质上是方法1的升级（或者说高级）版 #使用__metaclass__（元类）的高级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的升级（高级）版本, #使用装饰器(decorator), #这是一种更pythonic,更elegant的方法, #单例类本身根本不知道自己是单例的,因为他本身(自己的代码)并不是单例的 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