You have already assigned a callback to the output

Dash Python
1

#Timeline comparables table
@app.callback(Output(‘memory-output-comparables’, ‘data’),
[Input(‘my-dropdown’, ‘value’)])
def filter_countries(deal_ids_selected):
if not deal_ids_selected:
# Return all the rows on initial load/no selected.
return comparablesList.iloc[-1:].loc[:, comparablesList.columns != ‘deal_id’].to_dict(‘rows’)

filtered = comparablesList.query('deal_id in @deal_ids_selected')
filtered = filtered.loc[:, filtered.columns != 'deal_id']

return filtered.to_dict('rows')

@app.callback(Output(‘comparablesTable’, ‘data’),
[Input(‘memory-output-comparables’, ‘data’)])
def on_data_set_table(data):
if data is None:
raise PreventUpdate

return data

#calculate delta
@app.callback(
Output(‘comparablesTable’, ‘data’),
[Input(‘comparablesTable’, ‘data_timestamp’)],
[State(‘comparablesTable’, ‘data’)])
def update_columns(timestamp, rows):
for row in rows:
try:
row[‘delta’] = float(row[‘lastPrice’]) ** 2
except:
row[‘delta’] = ‘NA’
return rows

You have already assigned a callback to the output
with ID “memory-output-timeline” and property “data”. An output can only have
a single callback function. Try combining your inputs and
callback functions together into one function.

Is there any way I can output two callbacks towards the same object? As in above… I cannot have two callbacks that assign something to comparablesTable

2

Only one callback per object/parameter pair is allowed. You can have one callback that handles setting the data property of memory-output-timeline and another that handles setting the value of another property of memory-output-timeline. If you have multiple paths that would dictate the data output to memory-output-timeline, you need to set some conditional logic in one callback that would return the values accordingly.

3

Great thanks
Are you able to elaborate on conditional logic?

I have a datatable and for the callback there it fills the table with part of an SQL database
I also want a button to add additional rows afterwards, and I need to have calculations in the datatable as well

All these things require access to the datatable ‘data’ parameter

4

In another forum post (can’t recall which one), I found the following code snippet.

dash.callback_context.triggered[0]['prop_id'].split('.')[0]

This returns the ID of the input that triggered the callback. Here is a snippet that shows this:

@app.callback(
    output=Output('data-container', 'children'),
    inputs=[Input('SubmitButton', 'n_clicks'),
            Input('OptionsButton', 'n_clicks')],
def poll_noaa_website_for_hilo_data(submit_clicks, options_click):
    user_click = dash.callback_context.triggered[0]['prop_id'].split('.')[0]

    callback_states=dash.callback_context.states.values()
    callback_inputs=dash.callback_context.inputs.values()

    if not user_click or user_click != 'SubmitButton':
       # do something
    else:
      # do something else

Also, dash.callback_context.states.values() provides current values of the states when the callback was triggered. dash.callback_context.inputs.values() is the equivalent for inputs. I eventually combined all dash.callback_context calls into a function for readability.

5

This is how it turned out

@app.callback(
    Output('comparablesTable', 'data'),
    [Input('addComparable', 'n_clicks'),
     Input('memory-output-comparables', 'data')],
    [State('comparablesTable', 'data'),
     State('comparablesTable', 'columns')])
def route_comparables(data, n_clicks, rows, columns):
    user_click = dash.callback_context.triggered[0]['prop_id'].split('.')[0]
    
    callback_states = dash.callback_context.states.values()
    callback_inputs = dash.callback_context.inputs.values()
    
    if not user_click or user_click != 'addComparable':
        if n_clicks > 0:
            rows.append({i['id']: '' for i in columns})
        return rows
    else: 
        if data is None:
            raise PreventUpdate
        return data

It works for the most part except the adding of rows below, which is quite buggy. Only adds 5 rows at a time and deletes the data input from memory-output-comparables

if n_clicks > 0:
    rows.append({i['id']: '' for i in columns})
return rows
6

The order of your input parameters are mismatched. Based on the ordering of @app.callback, your function definition should be def route_comparables(n_clicks, data, rows, columns).

Also, if you are not using callback_states and callback_inputs, you can remove these lines…I just included them to illustrate how to check their status should this be helpful.

7

Hmmm doing that prevents the data to be loaded in the first place. I.e. the memory-output-comparables input doesn’t do anything

8

ok…Since your entire app code is not provided I can’t provide any further input. I just know the ordering of the callback params needs to match.

9

Just figured it out… had the incorrect input in my if statement
needed to have memory-output-comparables

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