Advanced callbacks why is one output getting ignored ? (Docs example)

Dash Python
1

Hi there,

i saw an example here

app.layout = html.Div([
    html.P('Enter a composite number to see its prime factors'),
    dcc.Input(id='num', type='number', debounce=True, min=2, step=1),
    html.P(id='err', style={'color': 'red'}),
    html.P(id='out')
])

@app.callback(
    Output('out', 'children'),
    Output('err', 'children'),
    Input('num', 'value')
)
def show_factors(num):
    if num is None:
        # PreventUpdate prevents ALL outputs updating
        raise dash.exceptions.PreventUpdate

    factors = prime_factors(num)
    if len(factors) == 1:
        # dash.no_update prevents any single output updating
        # (note: it's OK to use for a single-output callback too)
        return dash.no_update, '{} is prime!'.format(num)

    return '{} is {}'.format(num, ' * '.join(str(n) for n in factors)), ''

When i interact with the input, i can see that i get for 20 the whole sentence from the last return. In this case the output with the id = ‘err’ gets ignored. Why is that so? Shouldn’t i see the sentence displayed two times?

Shouldn’t that output update as well?

And how does dash.no_update work? How did it know to ignore the id=‘err’ ?

2

Hit enter.

3

Hitting enter sadly didn’t change anything :S

When i do insert a prime number and the if condition len(factors)==1 is true, then i get an err message and the ‘out’ stays as it is.

What i also don’t understand is why the ‘err’ message gets removed if i now insert a non prime number. Shouldn’t it receive the last return statement and update itself as well?

4

image
If you mean that you don’t want this err msg to be displayed at the same time as the previous Output, then remove dash.no_update.

5

thanks for the quick reply stu. I mean that i expected when typing 6, 2 divs showing 6is 2*3, but as soon as i type a non prime number i get only one div although the output should also appear on the err div since two outputs are declared in the callback decorator.

6

This actually returns a list with two values.
dash.no_update just doesn’t replace the previous value.

7

That was the culprit. Thank you!!!

My eyes completely dismissed the second argument.