Can't understand pattern-matching behavior in the last documentation example

Dash Python
1

I have the following snippet:


app.layout = html.Div([
    html.Div('Dash To-Do list'),
    dcc.Input(id="new-item"),
    html.Button("Add", id="add"),
    html.Button("Clear Done", id="clear-done"),
    html.Div(id="list-container"),
    html.Div(id="totals")
])

style_todo = {"display": "inline", "margin": "10px"}
style_done = {"textDecoration": "line-through", "color": "#888"}
style_done.update(style_todo)


@app.callback(
    [
        Output("list-container", "children"),
        Output("new-item", "value")
    ],
    [
        Input("add", "n_clicks"),
        Input("new-item", "n_submit"),
        Input("clear-done", "n_clicks")
    ],
    [
        State("new-item", "value"),
        State({"index": ALL}, "children"),
        State({"index": ALL, "type": "done"}, "value")
    ]
)
def edit_list(add, add2, clear, new_item, items, items_done):
    triggered = [t["prop_id"] for t in dash.callback_context.triggered]
    adding = len([1 for i in triggered if i in ("add.n_clicks", "new-item.n_submit")])
    clearing = len([1 for i in triggered if i == "clear-done.n_clicks"])
    new_spec = [
        (text, done) for text, done in zip(items, items_done)
        if not (clearing and done)
    ]
    if adding:
        new_spec.append((new_item, []))
    new_list = [
        html.Div([
            dcc.Checklist(
                id={"index": i, "type": "done"},
                options=[{"label": "", "value": "done"}],
                value=done,
                style={"display": "inline"},
                labelStyle={"display": "inline"}
            ),
            html.Div(text, id={"index": i}, style=style_done if done else style_todo)
        ], style={"clear": "both"})
        for i, (text, done) in enumerate(new_spec)
    ]
    return [new_list, "" if adding else new_item]


@app.callback(
    Output({"index": MATCH}, "style"),
    Input({"index": MATCH, "type": "done"}, "value")
)
def mark_done(done):
    return style_done if done else style_todo


@app.callback(
    Output("totals", "children"),
    Input({"index": ALL, "type": "done"}, "value"), # prevent_initial_call=True
)
def show_totals(done):
    count_all = len(done)
    count_done = len([d for d in done if d])
    result = "{} of {} items completed".format(count_done, count_all)
    if count_all:
        result += " - {}%".format(int(100 * count_done / count_all))
    return result

What i see when running the thing in debug mode is that by opening the website, i land in show_totals(done). Also worth noting that i do not land afterwards in ‘def edit_list’, only show_totals.

This is due to the initialization, but on a fresh state, i don’t have any input of type “done”. Why do i land in here?
I tried deleting all functions but not ‘def show_totals’ but i get the same behavior.

if i add ‘prevent_initial_call = True’ to the show totals decorator on the other hand i land in edit_list and dash.callback_context.triggered is empty. I feel lost.