[QUESTION] How to send 204 response?

I tried to send 204 response in delete method

Example handler

@router.delete('/{order_id}/', tags=['smart order'], status_code=204)
async def cancel_smart_order(
        session: Session = Depends(get_session),
        order_id: UUID = Path(...)
):
    order = await session.get(order_id)

    if order.status != OrderStatus.open:
        raise HTTPException(409, f'Order have status: {order.status}')

    order.status = OrderStatus.canceled

    await session.commit_only(order)

But got error h11._util.LocalProtocolError: Too much data for declared Content-Length. Seems framefork convert None to null but set content length 0.

[dmontagu]

Instead of returning None, and instead of injecting the response, just return a newly created response. It would look like:

@api.get(
    "/test_request_results/{test_request_id}",
    response_model=schemas.TestRequestResults,
    responses={204: {"model": None}},
)
async def read_test_request_results(test_request_id: int):
    # ... snipped ...
    if not complete:
        return Response(status_code=HTTP_204_NO_CONTENT)

The reason you are getting content is because FastAPI uses a JSONResponse by default (instead of a Response), which converts the returned value None to "null". By returning a Response directly, you prevent FastAPI from using the JSONResponse and encoding the None.

[iacopo-papalini]

This is a bug: FastAPI is not conforming to the HTTP RFC: https://tools.ietf.org/html/rfc2616#section-10.2.5

The 204 response MUST NOT include a message-body, and thus is always terminated by the first empty line after the header fields.

FastAPI should made it difficult to produce an invalid HTTP response, not allowing to create a valid one with a workaround.