在python中怎么实现单例模式

在python中怎么实现单例模式？下面给大家带来七种不同的方法：

一：staticmethod

代码如下：

class Singleton(object):    instance = None    def __init__(self):        raise SyntaxError('can not instance, please use get_instance')    def get_instance():        if Singleton.instance is None:            Singleton.instance = object.__new__(Singleton)        return Singleton.instancea = Singleton.get_instance()b = Singleton.get_instance()print('a id=', id(a))print('b id=', id(b))

该方法的要点是在__init__抛出异常，禁止通过类来实例化，只能通过静态get_instance函数来获取实例；因为不能通过类来实例化，所以静态get_instance函数中可以通过父类object.__new__来实例化。

二：classmethod

和方法一类似，代码：

class Singleton(object):    instance = None    def __init__(self):        raise SyntaxError('can not instance, please use get_instance')    def get_instance(cls):        if Singleton.instance is None:            Singleton.instance = object.__new__(Singleton)        return Singleton.instancea = Singleton.get_instance()b = Singleton.get_instance()print('a id=', id(a))print('b id=', id(b))

该方法的要点是在__init__抛出异常，禁止通过类来实例化，只能通过静态get_instance函数来获取实例；因为不能通过类来实例化，所以静态get_instance函数中可以通过父类object.__new__来实例化。

三：类属性方法

和方法一类似， 代码：

class Singleton(object):    instance = None    def __init__(self):        raise SyntaxError('can not instance, please use get_instance')    def get_instance():        if Singleton.instance is None:            Singleton.instance = object.__new__(Singleton)        return Singleton.instancea = Singleton.get_instance()b = Singleton.get_instance()print(id(a))print(id(b))

该方法的要点是在__init__抛出异常，禁止通过类来实例化，只能通过静态get_instance函数来获取实例；因为不能通过类来实例化，所以静态get_instance函数中可以通过父类object.__new__来实例化。

四：__new__

常见的方法， 代码如下：

class Singleton(object):    instance = None    def __new__(cls, *args, **kw):        if not cls.instance:            # cls.instance = object.__new__(cls, *args)            cls.instance = super(Singleton, cls).__new__(cls, *args, **kw)        return cls.instancea = Singleton()b = Singleton()print(id(a))print(id(b))

五：装饰器

代码如下：

def Singleton(cls):    instances = {}    def getinstance():        if cls not in instances:            instances[cls] = cls()        return instances[cls]    return getinstanceclass MyClass:    passa = MyClass()b = MyClass()c = MyClass()print(id(a))print(id(b))print(id(c))

六：元类

python2版：

class Singleton(type):    def __init__(cls, name, bases, dct):        super(Singleton, cls).__init__(name, bases, dct)        cls.instance = None    def __call__(cls, *args):        if cls.instance is None:            cls.instance = super(Singleton, cls).__call__(*args)        return cls.instanceclass MyClass(object):    __metaclass__ = Singletona = MyClass()b = MyClass()c = MyClass()print(id(a))print(id(b))print(id(c))print(a is b)print(a is c)

或者：

class Singleton(type):    def __new__(cls, name, bases, attrs):        attrs["_instance"] = None        return super(Singleton, cls).__new__(cls, name, bases, attrs)    def __call__(cls, *args, **kwargs):        if cls._instance is None:            cls._instance = super(Singleton, cls).__call__(*args, **kwargs)        return cls._instanceclass Foo(object):    __metaclass__ = Singletonx = Foo()y = Foo()print(id(x))print(id(y))

python3版：

class Singleton(type):    def __new__(cls, name, bases, attrs):        attrs['instance'] = None        return super(Singleton, cls).__new__(cls, name, bases, attrs)    def __call__(cls, *args, **kwargs):        if cls.instance is None:            cls.instance = super(Singleton, cls).__call__(*args, **kwargs)        return cls.instanceclass Foo(metaclass=Singleton):    passx = Foo()y = Foo()print(id(x))print(id(y))

七：名字覆盖

代码如下：

class Singleton(object):    def foo(self):        print('foo')    def __call__(self):        return selfSingleton = Singleton()Singleton.foo()a = Singleton()b = Singleton()print(id(a))print(id(b))

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