单例模式

以下方法 中只有方法4 最好， 其它都有些问题

# -*- encoding=utf-8 -*-print '----------------------方法1--------------------------'# 方法1 通过重写__new__方法from threading import Lockclass SingleClass(object):    lock = Lock()    def __init__(self, value):        self.value = value    def __new__(cls, *args, **kw):        if not hasattr(cls, '_instance'):            cls.lock.required()            if not hasattr(cls, '_instance'):                cls._instance = object.__new__(cls, *args)            cls.lock.release()        return cls._instanceone = SingleClass(2)two = SingleClass(3)print one is two  # Trueprint one.value, two.value  # 3, 3one.value = 4print one.value, two.value  # 4, 4print '----------------------方法2--------------------------'# 方法2 共享属性(所有实例共享同一个状态字典(dict))  保证不同实例有相同的状态;class Borg(object):    _state = {}    def __new__(cls, *args, **kw):        ob = super(Borg, cls).__new__(cls, *args)        ob.__dict__ = cls._state        return obclass SingleClass(Borg):    def __init__(self, value):        self.value = valueone = SingleClass(2)two = SingleClass(3)print one is two  # Falseprint two.__dict__ is one.__dict__  # Trueprint one.value, two.value  # 3 3one.value = 4print one.value, two.value  # 4 4print '----------------------方法3--------------------------'# 方法3: 重写__metaclass__（元类)的call方法class Singleton(type):    def __call__(cls, *args, **kwargs):        if not hasattr(cls, '_instance'):            cls._instance = super(Singleton, cls).__call__(*args, **kwargs)        return cls._instanceclass SingleClass(object):    __metaclass__ = Singleton    def __init__(self, value):        self.value = valueone = SingleClass(2)two = SingleClass(3)print one is two  # Falseprint one.value, two.value  # 2 2one.value = 4print one.value, two.value  # 4 4print '----------------------方法4--------------------------'# 方法4: 使用装饰器(decorator)def singleton(cls):    instances = {}    def _singleton(*args, **kwargs):        if cls not in instances:            instances[cls] = cls(*args, **kwargs)        return instances[cls]    return _singleton@singletonclass SingleClass(object):    def __init__(self, value):        self.value = valueone = SingleClass(2)two = SingleClass(3)print one is two  # Trueprint one.value, two.value  # 2 2one.value = 4print one.value, two.value  # 4 4print '----------------------方法5--------------------------'# 类简单工厂模式class Factory(object):    class __SingleClass(object):        def __init__(self, value):            self.value = value    @classmethod    def get_one(cls, *args, **kwargs):        if not hasattr(cls, '_Factory__instance'):            cls.__instance = cls.__SingleClass( *args, **kwargs)        return cls.__instanceone = Factory.get_one(2)two = Factory.get_one(3)print one is two  # Trueprint one.value, two.value  # 2 2one.value = 4print one.value, two.value  # 4 4