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'Bug' in Callback Context

Hey there,

I am facing a problem with the code snippet in the FAQ which refers to callback contexts (how to check which input has changed, see below).
If I execute this snippet in my own python project and visit localhost to see it there is already the ‘btn-3’ marked under ‘Most recent clicked’ even if I did not clicked it. It has also still the count 0.

I tried some other implementation which include the callback_context functionality and it always pretends that something was clicked even if it is not the case.

import json

import dash
import dash_html_components as html
from dash.dependencies import Input, Output

app = dash.Dash(__name__)

app.layout = html.Div([
    html.Button('Button 1', id='btn-1'),
    html.Button('Button 2', id='btn-2'),
    html.Button('Button 3', id='btn-3'),
    html.Div(id='container')
])


@app.callback(Output('container', 'children'),
              [Input('btn-1', 'n_clicks'),
               Input('btn-2', 'n_clicks'),
               Input('btn-3', 'n_clicks')])
def display(btn1, btn2, btn3):
    ctx = dash.callback_context

    if not ctx.triggered:
        button_id = 'No clicks yet'
    else:
        button_id = ctx.triggered[0]['prop_id'].split('.')[0]

    ctx_msg = json.dumps({
        'states': ctx.states,
        'triggered': ctx.triggered,
        'inputs': ctx.inputs
    }, indent=2)

    return html.Div([
        html.Table([
            html.Tr([html.Th('Button 1'),
                     html.Th('Button 2'),
                     html.Th('Button 3'),
                     html.Th('Most Recent Click')]),
            html.Tr([html.Td(btn1 or 0),
                     html.Td(btn2 or 0),
                     html.Td(btn3 or 0),
                     html.Td(button_id)])
        ]),
        html.Pre(ctx_msg)
    ])


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

Yes, for some reason I do not fully understand, the ctx.triggered is not empty, however the value of its element is None, you can use this as in the code below to have a more intuitive behaviour.

import json

import dash
import dash_html_components as html
from dash.dependencies import Input, Output

app = dash.Dash(__name__)

app.layout = html.Div([
    html.Button('Button 1', id='btn-1'),
    html.Button('Button 2', id='btn-2'),
    html.Button('Button 3', id='btn-3'),
    html.Div(id='container')
])


@app.callback(Output('container', 'children'),
              [Input('btn-1', 'n_clicks'),
               Input('btn-2', 'n_clicks'),
               Input('btn-3', 'n_clicks')])
def display(btn1, btn2, btn3):
    ctx = dash.callback_context

    if not ctx.triggered or not ctx.triggered[0]['value']:
        button_id = 'No clicks yet'
    else:
        button_id = ctx.triggered[0]['prop_id'].split('.')[0]

    ctx_msg = json.dumps({
        'states': ctx.states,
        'triggered': ctx.triggered,
        'inputs': ctx.inputs
    }, indent=2)

    return html.Div([
        html.Table([
            html.Tr([html.Th('Button 1'),
                     html.Th('Button 2'),
                     html.Th('Button 3'),
                     html.Th('Most Recent Click')]),
            html.Tr([html.Td(btn1 or 0),
                     html.Td(btn2 or 0),
                     html.Td(btn3 or 0),
                     html.Td(button_id)])
        ]),
        html.Pre(ctx_msg)
    ])


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

1 Like

Hi @MicSto and @Emmanuelle I could not find the documentation for the callback contexts.
Could someone provide me with the link?
Are there any updates regarding this bug?

Thank you :slight_smile: