python 实现单例模式的5种方法

一、classmethod装饰器

# 全局变量ip = ’192.168.13.98’port = ’3306’class MySQL: __instance = None def __init__(self, ip, port): self.ip = ip self.port = port @classmethod def instance(cls, *args, **kwargs): if args or kwargs: cls.__instance = cls(*args, **kwargs) return cls.__instance obj1 = MySQL.instance(ip, port)obj2 = MySQL.instance()obj3 = MySQL.instance()print(obj1)print(obj2, obj2.__dict__)print(obj3, obj3.__dict__)

输出结果

<main.MySQL object at 0x058D6F30><main.MySQL object at 0x058D6F30> {’ip’: ’192.168.13.98’, ’port’: ’3306’}<main.MySQL object at 0x058D6F30> {’ip’: ’192.168.13.98’, ’port’: ’3306’}

二、类的装饰器

def singlegon(cls): _instance = cls(ip, port) def wrapper(*args, **kwargs): if args or kwargs: return cls(*args, **kwargs) return _instance return wrapper @singlegonclass MySQL1: def __init__(self, ip, port): self.ip = ip self.port = port obj1 = MySQL1()obj2 = MySQL1()obj3 = MySQL1(’1.1.1.3’, 8080)print(obj1)print(obj2, obj2.__dict__)print(obj3, obj3.__dict__)

运行结果

<main.MySQL1 object at 0x04C102B0><main.MySQL1 object at 0x04C102B0> {’ip’: ’192.168.13.98’, ’port’: ’3306’}<main.MySQL1 object at 0x04C10310> {’ip’: ’1.1.1.3’, ’port’: 8080}

三、元类

class Mymetaclass(type): def __init__(self, class_name, class_bases, class_dic): super().__init__(class_name, class_bases, class_dic) self.__instance = self(ip, port) def __call__(self, *args, **kwargs): if args or kwargs: obj = self.__new__(self) self.__init__(obj, *args, **kwargs) self.__instance = obj return self.__instance class MySQL2(metaclass=Mymetaclass): def __init__(self, ip, port): self.ip = ip self.port = port obj1 = MySQL2()obj2 = MySQL2()obj3 = MySQL2(’1.1.1.3’, 80)print(obj1)print(obj2, obj2.__dict__)print(obj3, obj3.__dict__)

运行结果

<main.MySQL2 object at 0x04D003B0><main.MySQL2 object at 0x04D003B0> {’ip’: ’192.168.13.98’, ’port’: ’3306’}<main.MySQL2 object at 0x04D003D0> {’ip’: ’1.1.1.3’, ’port’: 80}

四、模块导入

# instance.py class MySQL: def __init__(self, ip, port): self.ip = ip self.port = port ip = ’192.168.13.98’port = 3306instance = MySQL(ip, port) # 测试代码import os, sys sys.path.append(os.path.dirname(os.path.dirname(__file__)))from test import instance obj1 = instance.instanceobj2 = instance.instanceobj3 = instance.MySQL(’1.1.1.3’, 80)print(obj1)print(obj2, obj2.__dict__)print(obj3, obj3.__dict__)

运行结果

<day30.instance.MySQL object at 0x052B0AB0><day30.instance.MySQL object at 0x052B0AB0> {’ip’: ’192.168.13.98’, ’port’: 3306}<day30.instance.MySQL object at 0x052B03F0> {’ip’: ’1.1.1.3’, ’port’: 80}

五、重写__new__()

class MySQL3(object): __instance = None __first_init = True def __init__(self, ip, port): if self.__first_init: self.ip = ip self.port = port self.__first_init = False def __new__(cls, *args, **kwargs): if not cls.__instance: cls.__instance = object.__new__(cls) return cls.__instance obj1 = MySQL3(ip, port)obj2 = MySQL3(ip, port)obj3 = MySQL3(’1.1.1.3’, 80)print(obj1)print(obj2, obj2.__dict__)print(obj3, obj3.__dict__)

运行结果

<main.MySQL3 object at 0x059603F0><main.MySQL3 object at 0x059603F0> {’ip’: ’192.168.13.98’, ’port’: ’3306’, ’_MySQL3__first_init’: False}<main.MySQL3 object at 0x059603F0> {’ip’: ’192.168.13.98’, ’port’: ’3306’, ’_MySQL3__first_init’: False}

注：前四种可以实现单例模式，但都不是绝对单例模式，可以创建新的对象，但是第五种方式是绝对单例模式，全局只能真正创建一次对象

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