dcc.Location url change not triggered if callback has two outputs

@clausimu wrote:

Hello,

I have come across a possible bug (or complete lack of understanding on my part) with dcc.Location. Is there anything I’m doing wrong? In the real app I need this to reload the user-generated data in tables when pages are changed.

What is happening:
A callback in a multi-page app only gets triggered by a url change if the callback has a single output. If there are two outputs, the callback never happens.

What should be happening:
A callback that is triggered by a url change should always fire, independent of the number and type of outputs.

Simple reproducible example:
In the following simple, reproducible example app2 works correctly, app1 does not. The only difference between both apps is that the callback in app2 has a single output (updating a label with the pathname), while the callback in app1 has two outputs (updating two labels with the pathname).

app.py

import dash
app = dash.Dash(__name__)
server = app.server
app.config.suppress_callback_exceptions = True

index.py

import dash_core_components as dcc
import dash_html_components as html
from dash.dependencies import Input, Output

from app import app
import app1
import app2


app.layout = html.Div([
    dcc.Location(id='url', refresh=True),
    html.Div(id='page-content')
])

index_page = html.Div([
    html.Div([
        html.H1('Index Page'),
        dcc.Link('Go to Page 1', href='app1'),
        dcc.Link('Go to Page 2', href='app2'),
    ], className='subpage')
], className='page')

@app.callback(Output('page-content', 'children'),
              [Input('url', 'pathname')])
def display_page(pathname):
    print('index:', pathname)
    if pathname == '/app1':
        return app1.layout
    elif pathname == '/app2':
        return app2.layout
    else:
        return index_page

if __name__ == '__main__':
    app.run_server(debug=True)

app1.py

import dash_html_components as html
import dash_core_components as dcc
from dash.dependencies import Input, Output

from app import app

layout = html.Div([
    html.Div([
        html.H1('Page 1'),
        dcc.Link('Go to Index', href='index_page'),
        dcc.Link('Go to Page 2', href='app2'),
        html.H3(id='label_page1', children='none'),
        html.H3(id='label2_page1', children='none'),
        ], className='subpage')
], className='page')


@app.callback([Output('label_page1', 'children'),
               Output('label2_page1', 'children')],
              [Input('url', 'pathname')])
def update_sample_info(pathname):
    print('page1:', pathname)
    return(pathname, pathname)

app2.py

import dash_html_components as html
import dash_core_components as dcc
from dash.dependencies import Input, Output

from app import app

layout = html.Div([
    html.Div([
        html.H1('Page 2'),
        dcc.Link('Go to Index', href='index_page'),
        dcc.Link('Go to Page 1', href='app1'),
        html.H3(id='label_page2', children='none')
        ], className='subpage')
], className='page')


@app.callback(Output('label_page2', 'children'),
              [Input('url', 'pathname')])
def update_sample_info(pathname):
    print('page2:', pathname)
    return(pathname)

Version
I am using python 3.6 and dash 1.9.0 on openSUSE Leap15.1

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