How to use Starlettes Streaming response with FastAPI

[KoduIsGreat]

Description

How can I use Starlettes streaming response with synchronous and async generators in fastapi, Its mentioned on the front page of the docs, but no example (that I can find is provided, other than websocket)

Is it possible to this?

[LKay]

It is possible. You need to return an instance of starlette.responses.StreamingResponse in your route, providing a stream or a generator in its constructor. Below you can check some of my routes streaming a S3 stored file, I hope that helps.

@router.get(orders_export_download_uri,
            responses={
                404: responses.empty_response(404)
            })
async def get_download(job_id: UUID,
                       hash: str,
                       *,
                       config: ConfigTree = Depends(get_config)) -> Response:
    order_export_job = await order_export_dao.get(job_id)

    if not order_export_job or order_export_job.hash != hash or not order_export_job.filename:
        raise NotFoundError()

    async def s3_stream(chunk_size: int = 4096):
        try:
            async with AWSClientProvider.s3() as s3:
                s3_ob = await s3.get_object(
                    Bucket=config.get_string("aws.s3.exports.bucket"),
                    Key="{}_{}.csv".format(
                        order_export_job.job_id,
                        order_export_job.hash,
                    )
                )

                async with s3_ob["Body"] as stream:
                    chunk = await stream.read(chunk_size)
                    while chunk:
                        yield chunk
                        chunk = await stream.read(chunk_size)
        except ClientError as e:
            if e.response['Error']['Code'] == 'NoSuchKey':
                raise NotFoundError()
            else:
                raise e

    return StreamingResponse(
        s3_stream(),
        media_type="text/csv",
        headers={
            "Content-Disposition": "attachment;filename=export.csv"
        }
    )

[tiangolo]

There are now docs for StreamingResponse: https://fastapi.tiangolo.com/advanced/custom-response/#streamingresponse 🎉

[github-actions]

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