Python Singleton

singleton.py

from typing import Any, Callable, ParamSpec, Type, TypeVar

T = TypeVar('T')
P = ParamSpec('P')

def singleton(cls: Type[T]) -> Callable[P, T]:
    """
    Example:
        >>> @singleton
        ... class C: ...
        >>> assert C() is C()
        >>> # import types; assert isinstance(C, types.FunctionTYpe)
        >>> import inspect; assert inspect.isfunction(C)  # bad
    """
    instances: dict[Type[T], T] = {}

    def getinstance(*args: P.args, **kwargs: P.kwargs):
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return instances[cls]

    return getinstance
  
 
class Singleton(type):
    """
    Example:
        >>> class S(metaclass=Singleton): ...
        >>> assert S() is S()
    """
    _instances: dict[type, Any] = {}

    def __call__(cls, *args: Any, **kwargs: Any):
        if cls not in cls._instances:
            cls._instances[cls] = super().__call__(*args, **kwargs)
        return cls._instances[cls]