| from typing import Any, AsyncIterator |
|
|
| from fastapi import Response |
| from fastapi.responses import StreamingResponse |
|
|
|
|
| async def prepend_async_item(first_item: Any, iterator: AsyncIterator[Any]): |
| """Yield a prefetched item before continuing the original iterator.""" |
| yield first_item |
| async for item in iterator: |
| yield item |
|
|
|
|
| async def read_first_async_item(iterator: AsyncIterator[Any]) -> Any: |
| """Python 3.9-compatible async equivalent of built-in anext().""" |
| return await iterator.__anext__() |
|
|
|
|
| async def build_streaming_response_or_error( |
| iterator: AsyncIterator[Any], |
| media_type: str = "text/event-stream", |
| ): |
| """ |
| Prefetch the first async item so router code can return an upstream error |
| response directly before FastAPI commits a 200 streaming response. |
| """ |
| try: |
| first_item = await read_first_async_item(iterator) |
| except StopAsyncIteration: |
| return Response(status_code=204) |
|
|
| if isinstance(first_item, Response): |
| return first_item |
|
|
| return StreamingResponse( |
| prepend_async_item(first_item, iterator), |
| media_type=media_type, |
| ) |
|
|
