#-*- 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