def datum_to_value(self, instance, datum):
        """Convert a given MAAS-side datum to a Python-side value.

        :param instance: The `Object` instance on which this field is
            currently operating. This method should treat it as read-only, for
            example to perform validation with regards to other fields.
        :param datum: The MAAS-side datum to validate and convert into a
            Python-side value.
        :return: A set of `cls` from the given datum.
        """
        if datum is None:
            return []
        if not isinstance(datum, Sequence):
            raise TypeError(
                "datum must be a sequence, not %s" % type(datum).__name__)
        # Get the class from the bound origin.
        bound = getattr(instance._origin, "FilesystemGroupDevices")
        return bound((
            get_device_object(instance._origin, item)
            for item in datum
            ))