Python – 单例实现的多种方法

单例模式就是确保一个类只有一个实例.当你希望整个系统中,某个类只有一个实例时,单例模式就派上了用场.比如,某个服务器的配置信息存在在一个文件中,客户端通过AppConfig类来读取配置文件的信息.如果程序的运行的过程中,很多地方都会用到配置文件信息,则就需要创建很多的AppConfig实例,这样就导致内存中有很多AppConfig对象的实例,造成资源的浪费.其实这个时候AppConfig我们希望它只有一份,就可以使用单例模式.

单例模式是一种软件设计模型。在面向对象编程中,通过单例模型只能创建一个类实例,也就是一个类永远只有一个实例对象。在工作中,为了确保某一个类只会创建一个实例,就需要使用单例模型。实现单例的方法有:

1.通过装饰器的方式实现单例

def singleton_func(cls):
    instances = {}

    def _singleton(*args, **kwargs):
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return instances[cls]
    return _singleton

@singleton_func
class Phone(object):

    def Phone_id(self):
        return id(self)

p1 = Phone()
p2 = Phone()
print(p1.phone_id())
print(p2.phone_id())

2.使用实例化方式实现单例

class SingletonInstance(object):

    def __call__(self, *args, **kwargs):
        return self

SingletonInstance = SingletonInstance()
s1 = SingletonInstance()
s2 = SingletonInstance()
print(id(s1))
print(id(s2))

3.使用类装饰器实现单例

class SingletonDecorator(object):
    _instance = None

    def __init__(self, cls):
        self._cls = cls

    def __call__(self, *args, **kwargs):
        if self._instance is None:
            self._instance = self._cls(*args, **kwargs)
        return self._instance

@SingletonDecorator
class Phone(object):

    def phone_id(self):
        return id(self)

p1 = Phone()
p2 = Phone()
print(p1.phone_id())
print(p2.phone_id())

4.重写类的__new__方法实现单例

class SingletonClass(object):
    _instance = None

    def __new__(cls, *args, **kwargs):
        if cla._instance is None:
            cls._instance = super(SingletonClass, self).__new__(cls)
        return cls._instance

    _is_init = False

    def __init__(self):
        if self._is_init is False:
            print(‘-*-‘)
            self._is_init = True

s1 = SingletonClass()
s2 = SingletonClass()
print(id(s1))
print(id(s2))

②实现单例模式的方法有多种,之前再说元类的时候用call方法实现了一个单例模式,另外Python的模块就是一个天然的单例模式;

# 使用new关键字来实现一个单例模式
class Book:
    def __new__(cls, title):
        if nothasattr(cls, "_ins")
            cls._ins=super().__new__()
            print(‘in__new__‘)
            return cls._ins


    def __init__(self, title):
        print(‘in__init__‘)
        super().__init__()
        self.title = title
        
if __name__ == ‘__main__‘:
    b = Book(‘The Spider Book‘)
    b2 = Book(‘The Flask Book‘)
print(id(b))
print(id(b2))
print(b.title)
print(b2.title)

5.通过元类实现单例

class SingletonMeta(type, object):

    def __init__(self, *args, **kwargs):
        self._instance = None
        super(SingletonMeta, self).__init__(*arg, **kwargs)

    # _instance = None

    def __call__(self, *args, **kwargs):
        if self._instance is None
            self._instance = super(SingletonMeta, self).__call__(*args, **kwargs)
        return self._instance

class Phone(object, metaclass = SingletonMeta):

    def phone_id(self):
        return id(self)

p1 = Phone()
p2 = Phone()
print(p1.phone_id())
print(p2.phone_id())

②元类(metaclass)实现单例

class Singleton(type):
    def __init__(self, *args, **kwargs):
        print("in __init__")
        self.__instance = None
        super(Singleton, self).__init__(*args, **kwargs)

    def __call__(self, *args, **kwargs):
        print("in __call__")
        if self.__instance is None:
            self.__instance = super(Singleton, self).__call__(*args, **kwargs)
            return self.__instance

class Foo(metaclass=Singleton):
    pass

foo1 = Foo()
foo2 = Foo()
print(foo1 is foo2)

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