Is there a way to prevent a callback from firing in dash?

user_5446749 0
Viewed 8320

I have the following callbacks:

@app.callback(
    [Output("useridPicker", "value"), Output("datePicker", "date"), ],
    [Input("url", "search")]
)
def update_form_default(url_search):
    return 111, dt.today()


@app.callback(
    Output("url", "search"),
    [Input("useridPicker", "value"), Input("datePicker", "date")]
)
def update_current_url(userid, date):
    return f"?userid=111&date=2020-06-29"

I have one text-area to pick a userid (useridPicker), a date picker to pick a date (datePicker) and a url containing as params the current userid and the current date ("YYYY-MM-DD").

When I modify the date or the userid from the url, I would like to update the displayed values inside (useridPicker) and (datePicker). And vice-versa.

For now, I get the following error:

Error: Dependency Cycle Found: useridPicker.value -> url.search -> useridPicker.value

When inside a callback, is there a way to prevent another callback from firing? In order to avoid this circular dependencies error and a possible infinite loop between callbacks.

plotly-dash python
2 Answers

It is possible to abort a Dash callback in two ways. You can either raise a dash.exceptions.PreventUpdate exception to abort the whole callback, or you can return dash.no_update for each of the outputs that you do not wish to update.

user_24288870

@emher is correct about the options for aborting a callback early, but that won't resolve the issue with having circular callbacks (a restriction in React.js underneith dash).

The dash documentation about advanced callbacks has some suggestions how to avoid circular callbacks: https://dash.plotly.com/advanced-callbacks.

As of dash v1.19.0, you can create circular updates within the same callback.

Circular callback chains that involve multiple callbacks are not supported.

Circular callbacks can be used to keep multiple inputs synchronized to each other.

The trick is to replace your two callbacks with a single callback with multiple outputs, and to use dash.callback_context.triggered within the callback to detect which of the inputs were modified to fire the callback. You can use no_update to only update the outputs that need to be changed. Something like this:

from dash import callback_context, no_update

@app.callback(
    Output("url", "search"),
    Output("useridPicker", "value"),
    Output("datePicker", "date"),
    Input("url", "search"),
    Input("useridPicker", "value"),
    Input("datePicker", "date"),
)
def update_form_default(url_search):
    changed_inputs = [
        x["prop_id"]
        for x in callback_context.triggered
    ]
    if "url.search" in changed_inputs:
        return no_update, 111, dt.today()
    else:
        return f"?userid=111&date=2020-06-29", no_update, no_update
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