Dynamic callback during running time

Dash Python
1

I am in situation where I need to assign the callback during the program’s running (for example in a callback).
I tried to assign the callback inside another callback, but it doesn’t work.
For example:
I start the view with this configuration:

dcc.Checklist(
            id='testing_checklist',
            options=OptionsFormatWrapper(['option1']).formatted_values,
            labelStyle={'display': 'block'}
 ),
 html.Div(
            id='testing_expand'
)

And I defined a callback for the checklist object (the callback creates a dropdown object in case the checklist is marked and assign a callback for the dropdown object):

@app.callback(Output('testing_expand', 'children'),
              [Input('testing_checklist', 'value')])
def test_callback(selected):
    if selected is None:
        raise dash.exceptions.PreventUpdate()
    global testing_lst

    returned = []
    for select in selected:
        obj = dcc.Dropdown(id=select, options=OptionsFormatWrapper(['option1']).formatted_values)
        if select not in testing_lst:
            app.callback(
                Output(select, 'style'),
                [Input(select, 'value')],
                [State(select, 'id')]
            )(temp)
            testing_lst.append(select)
        returned.append(obj)

    return returned

The problem is when I perform change in the dropdown value the callback isn’t been called as expected.

2

At this time this is not possible with Dash. Can you describe what you’re trying to achieve? Perhaps there’s another solution.

3

@sjtrny
I have checklist and when item in this checklist is marked a dropdown should be appear. The problem is that the items in the checklist are not known before I execute the program (only during the running the user select this items).

4

Below I have an example where I have a checklist, when the checklist is marked a dropdown appears. Since you know what is in the checklist, you also then know what will appear in the dropdown. I don’t think you need dynamic callbacks to solve this problem.

Just create your dropdown ahead of time and show/hide it dynamically.

app = dash.Dash(__name__)

app.layout = html.Div([
    dcc.Checklist(
        id="city",
        options=[
            {'label': 'New York City', 'value': 'NYC'},
            {'label': 'San Francisco', 'value': 'SF'}
        ],
        value=[]
    ),

    dcc.Dropdown(
        id='nyc',
        options=[
            {'label': 'Empire State Building', 'value': 'esb'},
            {'label': 'Central Park', 'value': 'central_park'},
        ],
        value=None,
        style={'display': 'none'}
    ),

    html.Div(id='result')

])

# Hide the Dropdown
@app.callback(Output('nyc', 'style'), [Input('city', 'value')])
def toggle_dropdown(checklist):
    if "NYC" in checklist:
        return {'display': 'block'}
    else:
        return {'display': 'none'}

# Change something else based on checklist and dropdown
@app.callback(Output('result', 'children'), [Input('city', 'value'), Input('nyc', 'value')])
def toggle_dropdown(checklist, nyc):
    return f"You selected {checklist} and {nyc}"

if __name__ == '__main__':
    app.run_server(debug=True)