Demo of Singleton in Python

singleton_1.py

from __future__ import annotations

from typing import reveal_type


class MetaSingleton(type):
    cached = None

    def __call__(cls, *args, **kwargs):
        if cls.cached is None:
            cls.cached = super().__call__(*args, **kwargs)
        return cls.cached


class Singleton(metaclass=MetaSingleton):
    def __init__(self):
        print("Instantiated")


ONE_TRUE_INSTANCE = Singleton()


def foo(a: int | type(ONE_TRUE_INSTANCE)) -> None:
    reveal_type(a)
    if isinstance(a, Singleton):
        print("a is of type Singleton")
        if a is ONE_TRUE_INSTANCE:
            print("it is the ONE_TRUE_INSTANCE")
    else:
        print("a is an int (probably)")


foo(15)

CertainSingleton = Singleton()
OtherSingleton = Singleton()

foo(CertainSingleton)
foo(OtherSingleton)