class Singleton(type):
    """
    Singleton Metaclass.

    Enforces any object using this metaclass to only create a single instance.

    - External Usage documentation: U{https://github.com/tylerlaberge/PyPatterns/wiki/Creational-Pattern-Usage}
    - External Singleton Pattern documentation: U{https://en.wikipedia.org/wiki/Singleton_pattern}
    """
    __instance = None

    def __call__(cls, *args, **kwargs):
        """
        Override the __call__ method to make sure only one instance is created.
        """
        if cls.__instance is None:
            cls.__instance = type.__call__(cls, *args, **kwargs)

        return cls.__instance