[BUG] @callbacks not being registered when @app.callback and @callback with duplicate outputs are used

Empieza ejecutando la reproducción proporcionada de Dash 3.2.0 y compara el registro de callbacks al usar @callback y @app.callback con salidas duplicadas y prevent_initial_call=True. Rastrea la ruta de registro que produce el comportamiento informado y, después, verifica que el caso de salidas duplicadas genere DuplicateCallback mientras los callbacks no relacionados sigan funcionando.

I'm aware that this is an edge case, but it's something we believe should be documented (and eventually fixed) in case someone runs into a similar issue.

The general recommendation would be to choose between @app.callback and @callback to use consistently across the app, even though combining them sometimes works, like in the following case:

@callback(
    Output("div1", "children", allow_duplicate=True),
    Input("b1", "n_clicks"),
    prevent_initial_call=True
)
...

@app.callback(
    Output("div1", "children"),
    Input("b2", "n_clicks"),
    prevent_initial_call=True
)

Description of the issue

If we have something like this in the code:

@app.callback(
    Output("div1", "children"),
    Input("b1", "n_clicks"),
    prevent_initial_call=True
)
...

@callback(
    Output("div1", "children"),
    Input("b2", "n_clicks"),
    prevent_initial_call=True
)

any callback defined with @callback will not work. Every callback defined with @app.callback will still work.

The code for the video app is at the end of this message.

https://github.com/user-attachments/assets/39899de0-8fdd-472e-8f7a-20789105774c

Defining duplicate callbacks like this is wrong according to the documentation. However, instead of this "all @callbacks not working" behaviour, we should see a "duplicate output warning" (see next section for more details).

Variations of the issue:

• The order in which the @app.callback and @callback callbacks are defined for the duplicate output doesn't affect the occurrence of the issue (it happens either way).
• If prevent_initial_call=True is not specified for (at least) @app.callback (the one with the duplicate output), the expected error will be raised:

dash.exceptions.DuplicateCallback: The callback `div1.children` provided with `dash.callback` was already assigned with `app.callback`.

• However, if prevent_initial_call=True is specified for @app.callback and not for @callback (the ones for the duplicate outputs), the issue will still occur, i.e. no error will be raised.

The last two points mean that:

• This will raise the expected error/exception:

@app.callback(
    Output("div1", "children"),
    Input("b1", "n_clicks"),
)
...
@callback(
    Output("div1", "children"),
    Input("b2", "n_clicks"),
    prevent_initial_call=True
)

• This will cause the issue - no error will be raised and other callbacks defined with @callback will not work.

@app.callback(
    Output("div1", "children"),
    Input("b1", "n_clicks"),
    prevent_initial_call=True
)
...
@callback(
    Output("div1", "children"),
    Input("b2", "n_clicks"),
)

If both callbacks are defined with @callback or @app.callback, the (expected) error appears in the debug menu. The app is loaded but no callback (not even the ones defined with @app.callback work:

https://github.com/user-attachments/assets/7d98a477-ae3a-4fd6-aa28-593341f5d450

If the callbacks are defined as the docs specify, with prevent_initial_call=True and allow_duplicate=True, like this, every callback will work (no matter if they are defined with @callback or @app.callback`):

@app.callback(
    Output("div1", "children", allow_duplicate=True),
    Input("b1", "n_clicks"),
    prevent_initial_call=True
)
...
@callback(
    Output("div1", "children"),
    Input("b2", "n_clicks"),
    prevent_initial_call=True
)

Expected behavior

Any case where callbacks with duplicate outputs are defined incorrectly should raise the below exception or an error in the debug menu:

dash.exceptions.DuplicateCallback: The callback `div1.children` provided with `dash.callback` was already assigned with `app.callback`.

So these two cases (and potentially more) should raise the exception:

# CASE 1: prevent_initial_call in app.callback and callback
@app.callback(
    Output("div1", "children"),
    Input("b1", "n_clicks"),
    prevent_initial_call=True
)
...

@callback(
    Output("div1", "children"),
    Input("b2", "n_clicks"),
    prevent_initial_call=True
)

# CASE 2: only prevent_initial_call in app.callback 
@app.callback(
    Output("div1", "children"),
    Input("b1", "n_clicks"),
    prevent_initial_call=True
)
...
@callback(
    Output("div1", "children"),
    Input("b2", "n_clicks"),
)

Code to reproduce the issue

dash==3.2.0

import dash
from dash import dcc, html, Input, Output, State, callback, ctx

app = dash.Dash(__name__)

app.layout = html.Div([
    html.H3("callback 1: @callback, output=div1 - without duplicate_output=True"),
    html.Button(id="b1", children="Trigger callback 1"),
    html.Div(id="div1", children="callbacks 1 or 2 not triggered yet"),
    ####
    html.H3("callback 2: @app.callback, output=div1"),
    html.Button(id="b2", children="Trigger callback 2"),
    ###
    html.H3("callback 3: @app.callback, output=div3"),
    html.Button(id="b3", children="Trigger callback 3"),
    html.Div(id="div3", children="callback 3 not triggered yet"),
    ####
    html.H3("callback 4: @callback, output=div4"),
    html.Button(id="b4", children="Trigger callback 4"),
    html.Div(id="div4", children="callback 4 not triggered yet"),
])

# this one won't work
@callback(
    Output("div1", "children"),
    Input("b1", "n_clicks"),
    prevent_initial_call=True
)
def update_div1_with_b1(b1):
    return f"Callback 1 triggered by {ctx.triggered_id} and {b1} clicks"

# this one will work
@app.callback(
    Output("div1", "children"),
    Input("b2", "n_clicks"),
    prevent_initial_call=True
)
def update_div1_with_b2(b2):
    return f"Callback 2 triggered by {ctx.triggered_id} and {b2} clicks"

# this one will work
@app.callback(
    Output("div3", "children"),
    Input("b3", "n_clicks"),
    prevent_initial_call=True
)
def update_div3(b3):
    return f"Callback 3 triggered by {ctx.triggered_id} and {b3} clicks"

# this one won't work
@callback(
    Output("div4", "children"),
    Input("b4", "n_clicks"),
    prevent_initial_call=True
)
def update_div4(b4):
    return f"Callback 4 triggered by {ctx.triggered_id} and {b4} clicks"

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