'Raise a NotFound if the parent region does not exist.
If the region_ref has a specified parent_region_id, check that
the parent exists, otherwise, raise a NotFound.'

def _check_parent_region(self, session, region_ref):

    parent_region_id = region_ref.get('parent_region_id')
    if (parent_region_id is not None):
        self._get_region(session, parent_region_id)
