Get parent for given `tree` or `alt` if not found.

        Args:
            tree (obj): :class:`.Tree` instance, which is already stored in DB.
            alt (obj, default None): Alternative value returned when `tree` is
                not found.

        Returns:
            obj: :class:`.Tree` parent to given `tree`.

def get_parent(self, tree, alt=None):
        """
        Get parent for given `tree` or `alt` if not found.

        Args:
            tree (obj): :class:`.Tree` instance, which is already stored in DB.
            alt (obj, default None): Alternative value returned when `tree` is
                not found.

        Returns:
            obj: :class:`.Tree` parent to given `tree`.
        """
        parent = self.parent_db.get(tree.path)

        if not parent:
            return alt

        return list(parent)[0]