@classmethod
def unwrap(cls, key):
    """
        Unpack a :class:`dict` key, whether optional or not.

        Args:
            key:
                Plain or :class:`.Optional` key.

        Returns:
            Tuple of ``(schema, default)``, where ``default`` is :attr:`.Optional.MISSING` if
            :data:`key` is not an :class:`.Optional` instance.
        """
    return (key.schema, key.default) if isinstance(key, cls) else (key, cls
        .MISSING)
