App callback without an output

Dash Python
22

I would like to bump up the topic of callbacks without outputs again because I discovered this to be a common use-case if you use Dash to communicate with other processes.

One example that comes directly into my mind is if you communicate with a Redis server and want to write some values depending on the user input in the Dash app. In a current case I need to control a process that captures the data. The parameters for the capturing like sample time, buffer size and measurement can be controlled by the user via Dash client. So I find myself creating dummy divs for each parameter for being able to transfer them to the Redis server. I know this is the recommended workaround however I think it is not a nice solution and creates unnecessary complexity in the frontend.

I would very much prefer a solution like this:

@app.callback(Output(None), [Input("samplerate_slider", "value"])
def update_samplerate(value: int) -> None:
    redis_client.set("samplerate", value)

Another use-cases I can think of that currently needs dummy divs and could be simplified by this approach:

So maybe we can revive the discussion about callbacks without outputs again :slight_smile: Iā€™m also very willing to offer my help when it comes to implementation.

2 Likes
23
24

@trav Thanks for the link but I can not see how this addresses the issue at hand :confused:

1 Like
25

Necessity of creating a dummy div seems weird. What if I want to do something on timer and donā€™t need to return anything immediately?

1 Like
26

Typically, you would poll updates using an interval component.

27

You still need an Output e.g. a dummy div for this to work which is not a nice solution if you want to pass information to the Backend (e.g. Redis server).

28

Yes, but you can just return a blank string. Itā€™s not super elegant, but I donā€™t see that it does much harm.

29

I agree on that. It is also cumbersome if you have multiple controls that only do backend operations. You need to create a separate dummy Div for each of these controls. My point is that it would be more convenient to create callbacks without beeing forced to add Outputs.

1 Like
30

I made a wrapper for that purpose,

But it just does the same thing under the hood.

1 Like
31

Nice work. I didnā€™t know about your extension and I will definetely give it a try :+1: I would still like to know why Outputs are enforced by Dash. Is it some limitation of React?

32

Thanks! No, it is a ā€˜limitationā€™ (though I would rather call it a design choice) of Dash itself.

33

Yes it looks this way. I would like to understand the reason for it though. It does not make much sense to force such workarounds from my point of view.

34

Well, a key design point of dash is keeping the server stateless. With that ind mind, what you are doing is practically a hack, and I can thus understand why the plotly team didnā€™t make any efforts to make it pretty.

35

I get your point. I use Dash to write GUIs for test stations and machines. So it is mandatory for me that it can handle states. I keep asking myself if Dash is the right choice for this kind of task.

36

I have previously used Dash for this kind of application, and i found it to be a good compromise between flexibility and ease of use. The reason Dash was designed with a stateless server in mind is to enable scaling to many (thousands) of users. You donā€™t really need that (I assume?), so it that sense it doesnā€™t matter that you violate this design principle.

37

Thanks, thatā€™s reassuring :slight_smile: So far I didnā€™t get in trouble with my hacks, either. And yes the use-case is only 1 or a few users at a time.