面向对象之单例模式

1. 单例模式

1.1 什么是单例模式

• 单例模式就是基于某种方法实例化多次得到的示例是同一个

class Foo():
    def __init__(self,name, age):
        self.name = name
        self.age = age
        
f1 = Foo('hades',13)
print(f1)
f2 = Foo('hades',13)
print(f2)
f3 = Foo('hades',13)
print(f3)

<__main__.Foo object at 0x0000023CB56AD518>
<__main__.Foo object at 0x0000023CB56AD550>
<__main__.Foo object at 0x0000023CB5688048>

上面演示的就不属于单例模式，虽然看起来实例化对象一摸一样，但从内存地址就能看出，其实都是单独的对象

1.2 为什么用单例模式

• 当实例化多次得到的对象存放的属性都是一样的时候，我们就没必要去占用更多的内存空间，实例化对象指向同一个内存

• 最终目的就是省内存

1.3 单例模式三种方式

1.3.1 使用类内部绑定方法特性，定义静态方法

NAME = 'hades'
AGE = 27

class People:
    __instance = None
    
    def __init__(self,name,age):
        self.name = name 
        self.age = age
        
    @classmethod
    def from_conf(cls):
        if cls.__instance:
            return cls.__instance
        
        cls.__instance = People.__init__(cls,NAME,AGE)
        return cls.__instance
        
p1 = People.from_conf()
p2 = People.from_conf()
print(p1==p2)

True

1.3.2 利用装饰器实现单例模式

NAME = 'hades'
AGE = 27

def deco(cls):
    cls.__instance = cls(NAME,AGE)
    
    def wrapper(*args,**kwargs):
        if len(args) == 0 and len(kwargs) == 0:
            return cls.__instance
        
        res = cls(*args,**kwargs)
        return res
    
    return wrapper

@deco
class People():
    
    def __init__(self,name,age):
        self.name = name
        self.age = age
        
p1 = People()
p2 = People()
p3 = People('bonnie',16)
print(p1 == p2)
print(p1 == p3)

True
False

1.3.3 利用元类实现单例模式

NAME = 'hades'
AGE = 27

class Mymeta(type):
    def __init__(self, class_name, class_bases, class_dict):
        super().__init__(class_name, class_bases, class_dict)
        
        self.__instance = self(NAME, AGE)
        
    def __call__(self, *args, **kwargs):
        
        if len(args) == 0 and len(kwargs) == 0:
            return self.__instance
        
        obj = self.__new__(self)
        self.__init__(obj, *args, **kwargs)
        
        return obj
        
class People(metaclass=Mymeta):
    def __init__(self,name,age):
        self.name = name
        self.age = age
        
p1 = People()
p2 = People()
p3 = People('bonnie',16)
print(p1 == p2)
print(p1 == p3)

True
False