单例模式

class MySQL:
    __sign = None
    def __init__(self):
        self.host = "127.0.0.1"
        self.port = 3306

    @classmethod
    def signleton(cls):
        if not cls.__sign:
            obj = cls()
            cls.__sign = obj
        return cls.__sign

    def conn(self):
        pass

m1 =  MySQL.signleton()
m2 = MySQL.signleton()
print(m1 is m2,"1")

#但是这样必须调用类方法sighlenton来实例化。

"""
通过元类实现单例模式
"""


class MySQLmeta(type):
    __sign = None

    def __call__(self, *args, **kwargs):
        if not MySQLmeta.__sign:
            obj = object.__new__(self)
            self.__init__(obj, *args, **kwargs)
            MySQLmeta.__sign = obj
        return MySQLmeta.__sign


class MySQL(metaclass=MySQLmeta):
    def __init__(self):
        self.host = "127.0.0.1"
        self.port = 3306

    def conn(self):
        pass

m1 =  MySQL()
m2 = MySQL()
print(m1 is m2,"2")


class MySQLmeta(type):

    def __init__(self, class_name, class_bases, class_dict):
        super(MySQLmeta, self).__init__(class_name, class_bases, class_dict)
        self.__sign = None

    def __call__(self, *args, **kwargs):
        if not self.__sign:
            obj = object.__new__(self)
            self.__init__(obj, *args, **kwargs)
            self.__sign = obj
        return self.__sign


class MySQL(metaclass=MySQLmeta):
    def __init__(self):
        self.host = "127.0.0.1"
        self.port = 3306

    def conn(self):
        pass

m1 =  MySQL()
m2 = MySQL()
print(m1 is m2,"3")

"""
通过new方法实现单例模式
"""

class MySQL(metaclass=MySQLmeta):
    __sign = None

    def __new__(cls, *args, **kwargs):
        if not cls.__sign:
            obj = super().__new__(cls, *args, **kwargs)
            cls.__sign = obj
        return cls.__sign

    def __init__(self):
        self.host = "127.0.0.1"
        self.port = 3306

    def conn(self):
        pass

m1 =  MySQL()
m2 = MySQL()
print(m1 is m2,"4")

单例模式

原文：https://www.cnblogs.com/walle-zhao/p/10775964.html