python基础之单例模式

单例模式：

　　什么是单例模式？

　　　　基于某种方法实例化多次得到实例是同一个

　　实现方法：

ip = '1.1.1.1'
port = 3306
# 假装来自配置文件

#方法一：定义类方法进行判断
class Mysql:
    __instance = None

    def __init__(self, ip, port):
        self.ip = ip
        self.port = port

    @classmethod
    def from_settings(cls):
        if not cls.__instance:
            cls.__instance = cls(ip, port)
        return cls.__instance

p1 = Mysql.from_settings()
p2 = Mysql.from_settings()
p3 = Mysql.from_settings()
p4 = Mysql('1.1.3.2',2203)
print(p1 is p2 is p3)  # Ture
print(p1)
print(p4)

#方法二：使用装饰器

def single(cls):
    _instance = cls(ip, port)

    def inner(*args, **kwargs):
        if len(args) + len(kwargs) == 0:
            return _instance
        return cls(*args, **kwargs)
    return inner

@single
class Mysql:
    __instance = None

    def __init__(self, ip, port):
        self.ip = ip
        self.port = port

p1 = Mysql()
p2 = Mysql()
p3 = Mysql()
p4 = Mysql('1.1.3.2',2203)
print(p1 is p2 is p3) # True
print(p1)
print(p4)

方法三：使用元类控制

class Mymeta(type):
    def __init__(self,class_name,class_bases,class_dic):
        super().__init__(class_name,class_bases,class_dic)
        self.__instance = self.__new__(self)
        self.__init__(self.__instance,ip,port)

    def __call__(self, *args, **kwargs):
        if len(args) + len(kwargs) == 0:
            return self.__instance
        obj = self.__new__(self)
        self.__init__(obj,*args,**kwargs)
        return obj

class Mysql(object,metaclass=Mymeta):
    __instance = None

    def __init__(self, ip, port):
        self.ip = ip
        self.port = port

p1 = Mysql()
p2 = Mysql()
p3 = Mysql()
p4 = Mysql('1.1.3.2',2203)
print(p1 is p2 is p3)  # True
print(p1)
print(p4)