How can a metaclass be used to implement the Singleton design pattern more robustly than overriding __new__ alone?

A Singleton metaclass overrides __call__(cls, *args, **kwargs) (the method invoked when you 'call' the class to create an instance) to check a per-class instance cache and return the existing instance instead of creating a new one. This centralizes the singleton logic in one reusable metaclass applicable to any class, rather than duplicating __new__ overrides in every singleton class.

class SingletonMeta(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super().__call__(*args, **kwargs)
        return cls._instances[cls]

class Config(metaclass=SingletonMeta):
    pass

print(Config() is Config())  # True