单列模式

#导包方式引入

#new方法实现

class Student():
    def __new__(self):
        if not hasattr(self,‘instance‘):
            self.instance =super(Student,self).__new__(self)
        return self.instance
a = Student()
b = Student()
print(id(a),id(b))


class Cat():
    def __new__(self):
        if not hasattr(self,‘instance‘):
            self.instane = super(Cat,self).__new__()
            return self.instane



class Person():
    def __new__(self):
        if not hasattr(self,‘instance‘):
            self.instance = super(Person,self).__new__()

        return self.instance


#装饰器

def singleton(cls,*arg,**kwargs):
    instance = {}
    def  _singleton():
        if cls not in instance:
            instance[cls] = cls(*arg,**kwargs)
        return instance[cls]
    return _singleton

@singleton
class Box():
    pass



s = Box()
m = Box()


print(id(s),id(m))