Doubt about nested callbacks

Dash Python
1

Hi! Im starting with nested callbacks and I have a problem:

In this example:

@app.callback(
    Output('minor_cat_dd', 'options'),
    Input('major_cat_dd', 'value'))
def update_dd(major_cat_dd):
    # Filter options (list of dicts)
    return minor_options

@app.callback(
    Output('minor_cat_dd', 'value'),
    Input('minor_cat_dd', 'options'))
def update_dd(minor_cat_options):
    # Pick a default value
    return chosen_value

When changing the first dropdown, then what the second callback does is changing to a default value

Another example from dash documentation: Part 3. Basic Callbacks | Dash for Python Documentation | Plotly

# -*- coding: utf-8 -*-
from dash import Dash, dcc, html, Input, Output

external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']

app = Dash(__name__, external_stylesheets=external_stylesheets)

all_options = {
    'America': ['New York City', 'San Francisco', 'Cincinnati'],
    'Canada': [u'Montréal', 'Toronto', 'Ottawa']
}
app.layout = html.Div([
    dcc.RadioItems(
        list(all_options.keys()),
        'America',
        id='countries-radio',
    ),

    html.Hr(),

    dcc.RadioItems(id='cities-radio'),

    html.Hr(),

    html.Div(id='display-selected-values')
])


@app.callback(
    Output('cities-radio', 'options'),
    Input('countries-radio', 'value'))
def set_cities_options(selected_country):
    return [{'label': i, 'value': i} for i in all_options[selected_country]]


@app.callback(
    Output('cities-radio', 'value'),
    Input('cities-radio', 'options'))
def set_cities_value(available_options):
    return available_options[0]['value']


@app.callback(
    Output('display-selected-values', 'children'),
    Input('countries-radio', 'value'),
    Input('cities-radio', 'value'))
def set_display_children(selected_country, selected_city):
    return u'{} is a city in {}'.format(
        selected_city, selected_country,
    )


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

My doubt is, in the second callback, the one that is updating the default value, if you change the value of the second option, instead of just picking one value why the callback is not being triggered?

@app.callback(
    Output('minor_cat_dd', 'value'),
    Input('minor_cat_dd', 'options'))
def update_dd(minor_cat_options):
    # Pick a default value
    return chosen_value

I mean, what this callback does is updatint the callback to a default value. If you are picking one value, It shout be triggered and choose the default callbak

2

hi @Miguelxda
:wave: welcome to the community.

I’m not sure I fully get what you’re trying to do. However, to answer your question… A callback is triggered by the Input. In the second callback below, the only Input to trigger the callback is the options property. That means, whenever the options property changes (in this case, only through the first callback), the callback will be triggered.

@app.callback(
    Output('cities-radio', 'value'),
    Input('cities-radio', 'options'))
def set_cities_value(available_options):
    return available_options[0]['value']
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