Python 3 - Singleton Access Pattern

singleton.py

import os
from typing import Any, Dict


class Singleton(type):
    _instances: Dict[Any, Any] = {}

    def __call__(cls, *args: Any, **kwargs: Any) -> Any:
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]
      
# test file

import pytest


pytestmark = pytest.mark.asyncio


class ClassA:
    def __init__(self) -> None:
        self.rand = os.urandom(10)


class ClassB(ClassA, metaclass=Singleton):
    pass


async def test_singleton() -> None:
    instance_a1 = ClassA()
    instance_a2 = ClassA()
    instance_b1 = ClassB()
    instance_b2 = ClassB()

    assert instance_a1 != instance_a2
    assert instance_a1 != instance_b1

    assert instance_b2 is instance_b1
    assert instance_b1.rand == instance_b2.rand