Get wrong output in python Singleton design pattern

I was making a Singleton design pattern and everything goes fine.

Look at the code below:

class Singleton():
    def __new__(cls):
        if not hasattr(cls, 'instance'):
            cls.instance = super().__new__(cls)
        return cls.instance

s1 = Singleton()
s2 = Singleton()

print(s1 is s2)
# True

But this is not Singleton because user can use delattr(Singleton,'instance') between creating s1 and s2 and with this simple function s1 is s2 returns False

So I decided to change instance to __instance (to stop user using delattr(Singleton,'instance')) but when I do that I get False when printing s1 is s2 (My new code (__instance))

class Singleton():
    def __new__(cls):
        if not hasattr(cls, '__instance'):
            cls.__instance = super().__new__(cls)
        return cls.__instance

s1 = Singleton()
s2 = Singleton()

print(s1 is s2)
# False

But where is the problem? Why when I change instance to __instance the result I get is False?

(I know other methods of creating Singleton I Just want to know what is different in instance and __instance when I'm in the class and __instance is not private for myself)