#!/usr/bin/env python

 # -*- coding:utf-8 -*-

 __author__ = 'Andy'

 """

 大话设计模式

 设计模式——单例模式(Singleton Pattern):保证类仅有一个实例，并提供一个访问它的全局访问点.

 (python中模块化的操作,可以代替单例模式,生成一个类的实例作为全局变量,其他地方只引用这个实例就可以实现单例)

 """

 #因为__new__在__init__前被执行,利用__new__实现单例

 #创建过的单例带有属性__instance

 class Singleton1(object):

     def __new__(cls, *args, **kwargs):

         if not hasattr(cls,'__instance'):

             cls.__instance = super(Singleton1, cls).__new__(cls, *args, **kwargs)

             return cls.__instance

 #利用__dict__来引用同一个字典

 class Singleton2(object):

     _state = {}

     def __new__(cls, *args, **kwargs):

         ob = super(Singleton2, cls).__new__(cls, *args, **kwargs)

         ob.__dict__ = cls._state

         return ob

 #利用元类在创建方法时用__metaclass__来创建

 class Singleton3(type):

     def __new__(cls, name, bases, dct):

         if not hasattr(cls, '__instance'):

             cls.__instance = super(Singleton3, cls).__new__(cls, name, bases, dct)

             return cls.__instance

 #利用装饰器

 def singleton4(cls, *args, **kwargs):

     instances = {}

     def _single():

         if cls not in instances:

             instances[cls] = cls(*args, **kwargs)

         return instances[cls]

     return _single

 class Myclass1(Singleton1):

     a = 1

 class Myclass2(Singleton2):

     a = 2

 class Myclass3(object):

     __metaclass__ = Singleton3

     a = 3

 @singleton4

 class Myclass4(object):

     a = 4

 if __name__ == "__main__":

     s1 = Myclass1()

     s2 = Myclass1()

     print s1.a,id(s1.a),id(s2.a) # 1 140630246306072 140630246306072

     s3 = Myclass2()

     s4 = Myclass2()

     print s3.a,id(s3.a),id(s4.a) # 2 140628399791616 140628399791616

     s5 = Myclass3()

     s6 = Myclass3()

     print s5.a,id(s5.a),id(s6.a) # 3 140530304439208 140530304439208

     s7 = Myclass4()

     s8 = Myclass4()

     print s7.a,id(s7.a),id(s8.a) # 4 140684486002032 140684486002032

作者：Andy
出处：http://www.cnblogs.com/onepiece-andy/
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