> ## Documentation Index
> Fetch the complete documentation index at: https://abxbus.archivebox.io/llms.txt
> Use this file to discover all available pages before exploring further.

# AutoReturnEventMiddleware

> Emit auto result events for non-None handler returns.

`AutoReturnEventMiddleware` emits `{OriginalEventType}ResultEvent` for successful non-`None` handler return values.

## Constructor params

None.

## Setup with EventBus

<Tabs>
  <Tab title="Python">
    ```python theme={null}
    from abxbus import EventBus
    from abxbus.middlewares import AutoReturnEventMiddleware

    bus = EventBus(
        name='AppBus',
        middlewares=[AutoReturnEventMiddleware()],
    )
    ```
  </Tab>

  <Tab title="Rust">
    ```rust theme={null}
    // Not implemented in the Rust crate yet.
    // Use explicit typed handler wrappers around bus.on(...) for this behavior for now.
    ```
  </Tab>
</Tabs>

## Behavior

* Runs on completed handler results.
* Emits auto result events only when:
  * handler returned a non-`None` value
  * handler did not error
  * return value is not itself a `BaseEvent`
* Auto event payload uses `data=<handler_return_value>`.
* Skips source events ending in `ErrorEvent` or `ResultEvent`.