Streaming responses

Quart supports responses that are meant to be streamed to the client, rather than received in one block. If you are interested in streaming the request data see Consuming the request body or for duplex streaming see Using websockets.

To stream a response the view-function should return an asynchronous generator that yields bytes. This generator can be returned with a status code and headers as normal. For example to stream the time every second,

@app.route('/')
async def stream_time():
    async def async_generator():
        time = datetime.isoformat()
        yield time.encode()
    return async_generator(), 200, {'X-Something': 'value'}

With context

If you want to make use of the request context whilst streaming you will need to use the quart.helpers.stream_with_context() decorator,

@app.route('/')
async def stream_time():
    @stream_with_context
    async def async_generator():
        time = datetime.isoformat()
        yield time.encode()
    return async_generator(), 200, {'X-Something': 'value'}

Timeout

Quart by default will timeout long responses to protect against possible denial of service attacks, see Denial Of Service mitigations. This may be undesired for streaming responses, e.g. an indefinite stream. The timeout can be disabled globally, however this could make other routes DOS vulnerable, therefore the recommendation is to set the timeout attribute on a specific response to None,

from quart import make_response

@app.route('/sse')
async def stream_time():
    ...
    response = await make_response(async_generator())
    response.timeout = None  # No timeout for this route
    return response

Testing

The test client get() and associated methods will collate the entire streamed response. If you want to test that the route actually streams the response, or to test routes that stream until the client disconnects you will need to use the request() method,

async def test_stream() -> None:
    test_client = app.test_client()
    async with test_client.request(..) as connection:
        data = await connection.receive()
        assert data ...
        assert connection.status_code == 200
        ...
        await connection.disconnect()  # For infinite streams

See also

Server Sent Events