Python单例模式的4种实现方法

#-*- 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