code: def mptt_before_insert(mapper, connection, instance):
      
      table = _get_tree_table(mapper)
      db_pk = instance.get_pk_column()
      table_pk = getattr(table.c, db_pk.name)
  
      if instance.parent_id is None:
          instance.left = 1
          instance.right = 2
          instance.level = instance.get_default_level()
          tree_id = connection.scalar(
              select(
                  [
                      func.max(table.c.tree_id) + 1
                  ]
              )
          ) or 1
          instance.tree_id = tree_id
      else:
          (parent_pos_left,
           parent_pos_right,
           parent_tree_id,
           parent_level) = connection.execute(
              select(
                  [
                      table.c.lft,
                      table.c.rgt,
                      table.c.tree_id,
                      table.c.level
                  ]
              ).where(
                  table_pk == instance.parent_id
              )
          ).fetchone()
  
          # Update key of right side
          connection.execute(
              table.update(
                  and_(table.c.rgt >= parent_pos_right,
                       table.c.tree_id == parent_tree_id)
              ).values(
                  lft=case(
                      [
                          (
                              table.c.lft > parent_pos_right,
                              table.c.lft + 2
                          )
                      ],
                      else_=table.c.lft
                  ),
                  rgt=case(
                      [
                          (
                              table.c.rgt >= parent_pos_right,
                              table.c.rgt + 2
                          )
                      ],
                      else_=table.c.rgt
                  )
              )
          )
  
          instance.level = parent_level + 1
          instance.tree_id = parent_tree_id
          instance.left = parent_pos_right
          instance.right = parent_pos_right + 1
docs: Based on example
      https://bitbucket.org/zzzeek/sqlalchemy/src/73095b353124/examples/nested_sets/nested_sets.py?at=master