た際に（例: [MongoDB の MotorClient](https://stackoverflow.com/questions/41584243/runtimeerror-task-attached-to-a-different-loop) 使用時）、`RuntimeError: Task attached to a different loop` に遭遇した場合は、イベントループを必要とするオブジェクトは非同期関数内でのみインスタンス化するようにしてください。例えば `@app.on_event("startup")` コールバック内で行います。

///...

        return decorator

    @deprecated(
        """
        on_event is deprecated, use lifespan event handlers instead.

        Read more about it in the
        [FastAPI docs for Lifespan Events](https://fastapi.tiangolo.com/advanced/events/).
        """
    )
    def on_event(
        self,
        event_type: Annotated[
            str,
            Doc(

            self.on_shutdown.append(func)

    @deprecated(
        """
        on_event is deprecated, use lifespan event handlers instead.

        Read more about it in the
        [FastAPI docs for Lifespan Events](https://fastapi.tiangolo.com/advanced/events/).
        """
    )
    def on_event(
        self,
        event_type: Annotated[
            str,
            Doc(

wriggling down from the sky!  Ugh, Serpent!'

  `But I'm NOT a serpent, I tell you!' said Alice.  `I'm a--I'm
a--'

  `Well!  WHAT are you?' said the Pigeon.  `I can see you're
trying to invent something!'

  `I--I'm a little girl,' said Alice, rather doubtfully, as she
remembered the number of changes she had gone through that day.

  `A likely story indeed!' said the Pigeon in a tone of the

wriggling down from the sky!  Ugh, Serpent!'

  `But I'm NOT a serpent, I tell you!' said Alice.  `I'm a--I'm
a--'

  `Well!  WHAT are you?' said the Pigeon.  `I can see you're
trying to invent something!'

  `I--I'm a little girl,' said Alice, rather doubtfully, as she
remembered the number of changes she had gone through that day.

  `A likely story indeed!' said the Pigeon in a tone of the

* ⬆️ Upgrade development dependencies. PR [#14854](https://github.com/fastapi/fastapi/pull/14854) by [@tiangolo](https://github.com/tiangolo).

## 0.128.3 (2026-02-06)

### Refactors

* ♻️ Re-implement `on_event` in FastAPI for compatibility with the next Starlette, while keeping backwards compatibility. PR [#14851](https://github.com/fastapi/fastapi/pull/14851) by [@tiangolo](https://github.com/tiangolo).

### Upgrades