python面向对象之单例模式

一.什么叫单例模式

基于某种方法通过类实例化的得到的对象,或者说内存地址指向同一个.

总共有4种方式.最后一个方式就是当作包导入使用。

方式一

方式一
settings.py文件中内容:
ip = '1.1.1.1'
port = 3306

版本一
import settings

class Mysql:
    __instance = None
    
    def __init__(self, ip, port):
        self.ip = ip
        self.port = port

    @classmethod
    def from_conf(cls):
        if not cls.__instance: #cls = Mysql ==>如果Mysql.__instance为假,执行下面代码
            cls.__instance = cls(settings.ip, settings.port)# ===>调用Mysql(settings.ip, settings.port)进行实例化
        return cls.__instance

obj1 = Mysql.from_conf()
obj2 = Mysql.from_conf()
obj3 = Mysql('1.1.1.10', 3302)
print(obj1)
print(obj2)
print(obj3)

版本二
class Mysql:
    def __init__(self, ip, port):
        self.ip = ip
        self.port = port

    @classmethod
    def from_conf(cls):
        if not hasattr(cls,'_%s__instance'%cls.__name__):#通过反射方法hasattr来判断
            cls.__instance = cls(settings.ip, settings.port)# ===>调用Mysql(settings.ip, settings.port)进行实例化
        return cls.__instance


obj1 = Mysql.from_conf()
obj2 = Mysql.from_conf()
obj3 = Mysql('1.1.1.10', 3302)
print(obj1)
print(obj2)
print(obj3)

方式二

def singleton(cls):
    _instance = cls(settings.ip, settings.port)  # 先导入配置文件settings中的内容
    def wrapper(*args, **kwargs):  # *args,**kwargs用来接受参数
        if args or kwargs:  # 如果有参数
            obj = cls(*args, **kwargs)  # 实例化对象obj
            return obj
        return _instance  # 这说明wrapper没有参数,就返回默认的_instance
    return wrapper

@singleton  # Mysql=singleton(Mysql)==>Mysql = wrapper
class Mysql:
    def __init__(self, ip, port):
        self.ip = ip
        self.port = port

obj1 = Mysql()  # 这里的Mysql其实是wrapper函数,加括号调用wrapper
obj2 = Mysql()
obj3 = Mysql('1.1.1.10', 3302)  # obj3 = wrapper('1.1.1.10',3302)
print(obj1)
print(obj2)
print(obj3)

方式三

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

    def __call__(self, *args, **kwargs):
        if args or kwargs:
            obj = self.__new__(self) #创建一个空对象
            self.__init__(obj,*args, **kwargs) #初始化空对象
            return obj #返回初始化好的对象

        return self.__instance#返回一个创建好的Msql对象


class Mysql(metaclass=Mymeta): #Mysql = Mymeta(...)
    def __init__(self, ip, port):
        self.ip = ip
        self.port = port


obj1 = Mysql()
obj2 = Mysql()
obj3 = Mysql('1.1.1.2', 3307)
print(obj1)
print(obj2)
print(obj3)