def METHOD_NAME(sender, instance, action, pk_set, **kwargs):
    """Update unit labels."""
    if (
        action not in ("post_add", "post_remove", "post_clear")
        or (action != "post_clear" and not pk_set)
        or not instance.is_source
    ):
        return
    if not instance.is_batch_update:
        instance.translation.component.invalidate_cache()