Why is Dash callback-function not triggered in python? (selected_columns)

Question

I want to trigger an event if the checkbox of a dash-table is clicked. When I start the webapp, I see below output

 * Serving Flask app 'app'
 * Debug mode: on
aaaa
[]
aaaa
[]

where "aaaa" indicating the callback is triggered. But when I click the checkbox after the initial load, nothing happens. I would expect that if I click the checkbox of a table, the callback-function dummy was triggered. Why is it not triggered?

import dash
import dash_bootstrap_components as dbc
from dash import html
from dash import dcc, dash_table
import plotly.express as px
from dash.dependencies import Input, Output
import pandas as pd

# data source: https://www.kaggle.com/chubak/iranian-students-from-1968-to-2017
# data owner: Chubak Bidpaa
df = pd.read_csv('https://raw.githubusercontent.com/Coding-with-Adam/Dash-by-Plotly/master/Bootstrap/Side-Bar/iranian_students.csv')

app = dash.Dash(__name__, external_stylesheets=[dbc.themes.BOOTSTRAP])

# styling the sidebar
SIDEBAR_STYLE = {
    "position": "fixed",
    "top": 0,
    "left": 0,
    "bottom": 0,
    "width": "16rem",
    "padding": "2rem 1rem",
    "background-color": "#f8f9fa",
}

# padding for the page content
CONTENT_STYLE = {
    "margin-left": "18rem",
    "margin-right": "2rem",
    "padding": "2rem 1rem",
}

sidebar = html.Div(
    [
        html.H2("", className="display-4"),
        html.Hr(),
        html.H2("Sidebar", className="display-4"),
        html.Hr(),
        html.P(
            "Number of students per education level", className="lead"
        ),
        dbc.Nav(
            [
                dbc.NavLink("Home", href="/", active="exact"),
                dbc.NavLink("Page 1", href="/page-1", active="exact"),
                dbc.NavLink("Page 2", href="/page-2", active="exact"),
            ],
            vertical=True,
            pills=True,
        ),
    ],
    style=SIDEBAR_STYLE,
)

navbar = dbc.NavbarSimple(
    children=[
        dbc.NavItem(dbc.NavLink("Page 1", href="#")),
        dbc.DropdownMenu(
            children=[
                dbc.DropdownMenuItem("More pages", header=True),
                dbc.DropdownMenuItem("Page 2", href="#"),
                dbc.DropdownMenuItem("Page 3", href="#"),
            ],
            nav=True,
            in_navbar=True,
            label="More",
        ),
    ],
    brand="NavbarSimple",
    brand_href="#",
    color="primary",
    dark=True,
)

fig1 = px.bar(df, barmode='group', x='Years',
                         y=['Girls Kindergarten', 'Boys Kindergarten'])
fig1.update_layout(plot_bgcolor='rgb(10,10,10)')
children1 = [html.H1('Kindergarten in Iran',
                        style={'textAlign':'center'}),
                dcc.Graph(id='bargraph',
                         figure=fig1),
                dash_table.DataTable(
        id='datatable-interactivity',
        columns=[
            {"name": i, "id": i, "deletable": True, "selectable": True} for i in df.columns
        ],
        data=df.to_dict('records'),
        editable=True,
        filter_action="native",
        sort_action="native",
        sort_mode="multi",
        column_selectable="single",
        row_selectable="multi",
        row_deletable=True,
        selected_columns=[],
        selected_rows=[],
        page_action="native",
        page_current= 0,
        page_size= 10,
    )]
content = html.Div(id="page-content", children=children1, style=CONTENT_STYLE)
app.layout = html.Div([
    dcc.Location(id="url"),
    sidebar,
    navbar,
    content
])


#fig1.update_layout(plot_bgcolor='rgb(10,10,10)')
"""
def update_styles(selected_columns):
    return [{
        'if': { 'column_id': i },
        'background_color': '#D2F3FF'
    } for i in selected_columns]
"""
@app.callback(
    Output('datatable-interactivity', 'style_data_conditional'),
    Input('datatable-interactivity', 'selected_columns')
)
def dummy(selected_columns):
    print("aaaa")
    print(selected_columns)

@app.callback(
    Output("page-content", "children"),
    [Input("url", "pathname")]
)
def render_page_content(pathname):
    if pathname == "/":
        return children1
    elif pathname == "/page-1":
        return children1
    elif pathname == "/page-2":
        return children1
    # If the user tries to reach a different page, return a 404 message
    return dbc.Jumbotron(
        [
            html.H1("404: Not found", className="text-danger"),
            html.Hr(),
            html.P(f"The pathname {pathname} was not recognised..."),
        ]
    )


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

Answer 1

The dummy function accepts a wrong input parameter. I have not worked with selected_columns, but selected_rows so far. A function input needed to be "chosen_rows" then for the selected_rows.