Get the cached value.

        Returns
        -------
        value : object
            The cached value.

        Raises
        ------
        Expired
            Raised when `dt` is greater than self.expires.

def unwrap(self, dt):
        """
        Get the cached value.

        Returns
        -------
        value : object
            The cached value.

        Raises
        ------
        Expired
            Raised when `dt` is greater than self.expires.
        """
        expires = self._expires
        if expires is AlwaysExpired or expires < dt:
            raise Expired(self._expires)
        return self._value