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屡次获得实例是同一个

　　实现方法：

ip = '1.1.1.1'
port = 3306
# 伪装来自配置文件

#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 objclass Mysql(object,metaclass=Mymeta):    __instance = None    def __init__(self, ip, port):        self.ip = ip        self.port = portp1 = Mysql()p2 = Mysql()p3 = Mysql()p4 = Mysql('1.1.3.2',2203)print(p1 is p2 is p3)  # Trueprint(p1)print(p4)