singleton.py

class Singleton(type):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        key = (cls, args, str(kwargs))

        if key not in cls._instances:
            cls._instances[key] = super(Singleton, cls).__call__(*args, **kwargs)

        return cls._instances[key]

class Sample(object, metaclass=Singleton):
  pass