单例模式

单例模式就是在多次实例化后得到的是同一个实例

实现单例的第一种方式:

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

    @classmethod
    def from_conf(cls):
        if not hasattr(cls,'_Mysql__instance'):
            cls.__instance =  cls(settings.IP,settings.PORT)
        return cls.__instance

或者:

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(settings.IP,settings.PORT)
        return cls.__instance

实现单例的第二种方式:

def foo(cls):
    __instance = cls(settings.IP,settings.PORT)
    def wrapper(*args,**kwargs):
        if args or kwargs:
            obj = cls(*args,**kwargs)
            return obj
        return __instance
    return wrapper

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

实现单例的第三种方法:

利用元类,来控制类的实例化的过程,在实例化过程中不传参数的时候都指向同一个实例

class Mymeta(type):
    def __init__(self,class_name,class_base,class_dic):
        self.__instance = self(settings.IP,settings.PORT)
    def __call__(self, *args, **kwargs):
        if args or kwargs:
            obj = self.__new__(self)
            self.__init__(obj,*args,**kwargs)
            return obj
        else:
            return self.__instance

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

实现单例的第四种方法:

利用导模块的方法实现单例,因为在模块被导入的时候就产生一次名称空间

# 在文件singleton.py中写一个类并且实例化

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

instance = Mysql('1.1.1.1',3306)

# 在别的文件中
from singleton import instance