dcc.Upload callback at the start?

Dash Python
1

Hi,

I am using the dcc.Upload component and would like to have a notification while the data is being loaded, that gets updated once the upload successful.
We can easily have a callback at the end of the data upload, but I could not find a way on how to trigger a callback when the data upload is starting (either by dropping files on the selected zone or clicking on the file browser)…

Is this possible? Am I missing something obvious?

Thanks

2

Hello @hcoohb,

For this, you can use a background callback, which gives the option of giving a progress indicator.

As a byproduct, you can also change the state of a loading spinner as well if you wanted.

3

hi,
this below can also do this.

4

@jinnyzor and @stu , Thanks for the quick reply.

But my problem is more that I do not see a trigger for the Upload start.
From the doc https://dash.plotly.com/dash-core-components/upload, defining the callback:

@app.callback(Output('output-image-upload', 'children'),
              Input('upload-image', 'contents'))
def update_output(list_of_contents, ):
    ...

means the Input is only triggered once the upload is already finished.
So how would I start a background callback or BlockingCallbackTransform at the start of the data upload?

Thanks

5

@hcoohb,

Can you provide more code? How big are the files that you are trying to transfer? That seems pretty big to want to see that something is currently being transfer…

6

Have you tried using loading_state to trigger?