参考

# 第一种方法 new 方法

class Singleton(object):

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

        if not hasattr(cls,'_instance'):

            cls._instance = super(Singleton,cls).__new__(cls,*args,**kw)

        return cls._instance

s1 = Singleton()

s2 = Singleton()

s1 == s2

# 第二种方法升级为元类 call 控制，实质跟方法一差不多

class SingletonMetaclass(type):

    def __call__(cls,*args,**kw):

        if not hasattr(cls,'_instance'):

            cls._instance = super(SingletonMetaclass,cls).__call__(*args,**kw)

#         cls.__init__(cls._instance,*args,**kw)

        return cls._instance

class Singleton(object,metaclass=SingletonMetaclass):

    pass

s1 = Singleton()

s2 = Singleton()

s1 == s2

# 第三种，使用装饰器

def singleton(cls,*args,**kw):

    instance = {}

    def get_instance():

        if cls not in instance:

            instance[cls] = cls.__new__(cls,*args,**kw)

        return instance[cls]

    return get_instance

@singleton

class Singleton(object):

    pass

s1 = Singleton()

s2 = Singleton()

s1 == s2

线程安全

# 线程安全的 写法

# 装饰器

import threading

def Singleton(cls,*args,**kw):

    instance = {}

    _instance_lock = threading.Lock()

    def get_instance():

        if cls not in instance:

            with _instance_lock:

                if cls not in instance:

                    instance[cls] = cls.__new__(cls,*args,**kw)

                    cls.__init__(instance[cls],*args,**kw)

        return instance[cls]

    return get_instance

@Singleton

class Demo(object):

    pass

d1 = Demo()

d2 = Demo()

d1 is d2

# 基类

class Singleton(object):

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

        if not hasattr(cls,'_instance'):

                cls._instance = super(Singleton,cls).__new__(cls,*args,**kw)

        return cls._instance

s1 = Singleton()

s2 = Singleton()

print(s1 == s2)

# 升级为元类

import threading

class SingletonMetaclass(type):

    def __call__(cls,*args,**kw):

        _instance_lock = threading.Lock()

        if not hasattr(cls,'_instance'):

            with _instance_lock:

                if not hasattr(cls,'_instance'):

                    cls._instance = cls.__new__(cls,*args,**kw)

                    cls.__init__(cls._instance,*args,**kw)

        return cls._instance

class Demo(object,metaclass=SingletonMetaclass):

    pass

d2 = Demo()

d3 = Demo()

d2 is d3

mysingleton.py

class Singleton(object):

    def foo(self):

        pass

singleton = Singleton()

将上面的代码保存在文件 mysingleton.py 中，要使用时，直接在其他文件中导入此文件中的对象，这个对象即是单例模式的对象

from mysingleton import singleton

方法四：Borg模式

利用“类变量对所有对象唯一”,即__share_state

class Foo:

   __share_state = {}

   def __init__(self):

       self.__dict__ = self.__share_state