Is there a simple, elegant...

Yes, there are several simple and elegant ways to define singletons in Python, each with its own advantages and considerations. Here are some of the most commonly used methods:

1. Using a Metaclass

A metaclass can control the instantiation of a class. By overriding the __call__ method in the metaclass, you can ensure that only one instance of a class is created.

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

class Singleton(metaclass=SingletonMeta):
    pass

This method is highly readable and reusable, and it's easy to test because you can mock the metaclass during testing[3][4].

2. Using a Decorator

A decorator can be used to wrap a class and control its instantiation. This approach is less intrusive and can be easily added or removed from a class.

def singleton(cls):
    instances = {}
    def get_instance(*args, **kwargs):
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return instances[cls]
    return get_instance

@singleton
class Singleton:
    pass

This method is intuitive and additive, allowing for clear modifications without altering the class structure itself[4].

3. Using a Base Class

Implementing singleton behavior in a base class from which other singletons can inherit is another approach. This method uses the __new__ method to control instance creation.

class SingletonBase:
    _instance = None
    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super().__new__(cls, *args, **kwargs)
        re...