Dash Error: Duplicate callback outputs even with allow_duplicate=True

Ismael Azehaf

Hi All

Problem

I have recently been confronted to the following issue with Dash:

I have two callbacks which point to the same output curve_type_selection. Here is the code:

@callback(
    Output("graph", "figure"),
    Output("curve_type_selection", "disabled", allow_duplicate=True),
    Output("wait_completion", "children"),
    Input("curve_type_selection", "value"),
    State("url", "pathname"),
    State("graph", "figure"),
    prevent_initial_call=True,
)
def draw_parametric_curve(selected_curve, pathname, figure):
    """Callback function to trigger the computation."""
    project = DashClient[Cosmic_WaveSolution].get_project(pathname)
    step = project.steps.compute_step
    step.selected_curve = selected_curve
    step.compute_parametric_curve()
    figure["data"][0]["x"] = step.x_coords
    figure["data"][0]["y"] = step.y_coords
    return figure, False, True

@callback(
    Output("curve_type_selection", "disabled", allow_duplicate=True),
    Input("curve_type_selection", "value"),
    prevent_initial_call=True,
)
def disable_selection(selected_curve):
    """Callback function to trigger the computation."""
    return True 

I added the allow_duplicate option to get ride of the duplicate outputs error as recommended in Dash documentation. But still I am getting the error message.

Solution

The solution can be found here. The issue was actually not with the outputs, but rather with the inputs. The workaround is to either switch the orders of the inputs wherever possible, or add a dummy input wherever necessary.

In my case, I added a dummy input like this:

@callback(
    Output("graph", "figure"),
    Output("curve_type_selection", "disabled", allow_duplicate=True),
    Output("wait_completion", "children"),
    Input("curve_type_selection", "n_clicks"), # Dummy input to avoid callbacks outputs duplicates
    Input("curve_type_selection", "value"),
    State("url", "pathname"),
    State("graph", "figure"),
    prevent_initial_call=True,
)
def draw_parametric_curve(n_click, selected_curve, pathname, figure):
    """Callback function to trigger the computation."""
    ctx = callback_context
    if "curve_type_selection.value" in ctx.triggered_prop_ids.keys():
        project = DashClient[Cosmic_WaveSolution].get_project(pathname)
        step = project.steps.compute_step
        step.selected_curve = selected_curve
        step.compute_parametric_curve()
        figure["data"][0]["x"] = step.x_coords
        figure["data"][0]["y"] = step.y_coords
        return figure, False, True
    raise PreventUpdate


@callback(
    Output("curve_type_selection", "disabled", allow_duplicate=True),
    Input("curve_type_selection", "value"),
    prevent_initial_call=True,
)
def disable_selection(selected_curve):
    """Callback function to trigger the computation."""
    return True