Determining Which Callback Input Changed

Sometimes you’ll have multiple callback inputs and want to update an output differently depending on which input triggered the callback.

You can use the properties of dash.callback_context (or dash.ctx for short in Dash 2.4 and later) to determine which input triggered a callback.

The following examples use dash.callback_context.triggered_id and dash.callback_context.triggered_prop_ids introduced in Dash 2.4.

Capturing the Component ID that Triggered the Callback

In this example, we get the id of the button that triggered the callback and display it each time a button is clicked.
On first load, when no user action has triggered the callback, the value of triggered_id is None. We check for this and use it to display the message: “You haven’t clicked any button yet”.

Note: triggered_id is available in Dash 2.4 and later. In earlier versions of Dash, you can get the triggered id with dash.callback_context.triggered[0]['prop_id'].split('.')[0]

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

app = Dash(__name__)

app.layout = html.Div([
    html.Button('Button 1', id='btn-1-ctx-example'),
    html.Button('Button 2', id='btn-2-ctx-example'),
    html.Button('Button 3', id='btn-3-ctx-example'),

              Input('btn-1-ctx-example', 'n_clicks'),
              Input('btn-2-ctx-example', 'n_clicks'),
              Input('btn-3-ctx-example', 'n_clicks'))
def display(btn1, btn2, btn3):
    button_clicked = ctx.triggered_id
    return html.Div([
            f'''You last clicked button with ID {button_clicked}
            ''' if button_clicked else '''You haven't clicked any button yet''')

if __name__ == '__main__':

With Pattern-Matching Callback IDs

In the above example triggered_id is a string as the id of the input is a string. In cases where a component’s id is a dictionary, for example, with Pattern-Matching Callbacks, triggered_id will also be a dictionary.

For example, with this component:

        ['NYC', 'MTL', 'LA', 'TOKYO'],
            'type': 'filter-dropdown',
            'index': 0

That is used as an input to a callback:

Input({'type': 'filter-dropdown', 'index': ALL}, 'value')

triggered_id returns:

{'index': 0, 'type': 'filter-dropdown'}

Here you can capture the index with: triggered_id.index, or the type with triggered_id.type.

Determining Multiple Input Triggers and Properties

In cases where multiple inputs trigger a callback, you can use triggered_prop_ids. It is also useful if different properties of the same component can trigger the callback and you want to capture the property.

triggered_prop_ids returns a dictionary of inputs that triggered the callback. Each key is a <component_id>.<component_property> and the corresponding value is the <component_id>.

In our first example above, if we used triggered_prop_ids, when Button 2 is clicked, it would return:

{'btn-2-ctx-example.n_clicks': 'btn-2-ctx-example'}

With Pattern-Matching Callbacks

For the pattern-matching callbacks ‘filter-dropdown’ example above, triggered_prop_ids returns:

{'{"index":0,"type":"filter-dropdown"}.value': {'index': 0, 'type': 'filter-dropdown'}}

With Pattern-Matching Callbacks With Multiple Triggers

If we have a series of dropdowns with different indexes (0-2) and they all triggered the callback, triggered_prop_ids returns:

{'{"index":0,"type":"filter-dropdown"}.value': {'index': 0, 'type': 'filter-dropdown'}, '{"index":1,"type":"filter-dropdown"}.value': {'index': 1, 'type': 'filter-dropdown'}, '{"index":2,"type":"filter-dropdown"}.value': {'index': 2, 'type': 'filter-dropdown'}}

In all of these examples, the dictionary keys are in the format `<component_id>.<component_property>, while the dictionary values are the component ids.