什么是单例模式：

单例模式即一个类有且仅有一个实例

class Mysql:
    __instance = None

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

    @classmethod
    def from_conf(cls):
        if cls.__instance is None:
            cls.__instance = cls("127.0.0.0", 8888)
        return cls.__instance


# 单例模式
obj1 = Mysql.from_conf()
obj2 = Mysql.from_conf()
obj3 = Mysql.from_conf()
print(obj1)
print(id(obj1))
print(obj2)
print(id(obj2))
print(obj3)
print(id(obj3))  # obj1、obj2、obj3的id一样，为一个实例

# 不单例
print("*"*50)
obj4 = Mysql("127.0.0.0", 9999)
print(obj4)
print(id(obj4))  # obj4的id改变，不是单例

<__main__.Mysql object at 0x10ea9cfa0>
4540977056
<__main__.Mysql object at 0x10ea9cfa0>
4540977056
<__main__.Mysql object at 0x10ea9cfa0>
4540977056
**************************************************
<__main__.Mysql object at 0x10ea9cee0>
4540976864

Process finished with exit code 0