Python单例模式的4种实现方法

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 #-*- 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