example08.py

from functools import wraps
from threading import RLock


def singleton(cls):
    instances = {}
    lock = RLock()

    @wraps(cls)
    def wrapper(*args, **kwargs):
        if cls not in instances:
            with lock:
                if cls not in instances:
                    instances[cls] = cls(*args, **kwargs)
        return instances[cls]


@singleton
class President:
    pass


President = President.__wrapped__