class Foo:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def show(self):
        print(self.name, self.age)

v = None

while True:
    if v:
        v.show()
    else:
        v = Foo('klvchen', 666)
        v.show()

运行结果:
无限循环 666

不需要重复生产对象

class Foo:

    __v = None

    @classmethod
    def get_instance(cls):
        if cls.__v:
            return cls.__v
        else:
            cls.__v = Foo()
            return cls.__v

obj1 = Foo.get_instance()
print(obj1)
obj2 = Foo.get_instance()
print(obj2)
obj3 = Foo.get_instance()
print(obj3)

运行结果：
<__main__.Foo object at 0x0000029958542518>
<__main__.Foo object at 0x0000029958542518>
<__main__.Foo object at 0x0000029958542518>

单例模式可以很好地保证当前内存中仅存在单个实例，避免内存浪费。