Figure doesn't load from callback unless another dash_core_component is in the layout

[mbkupfer]

I was trying to create a workable example to solve this community question, but I could only make it work with another dcc component inside. Here is the example:

import time

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


loading_component = html.H1(
    style={},
    children='Loading...'
)

app = dash.Dash(__name__)
app.layout = html.Div([
    dcc.Input(), # callback doesn't work without this line

    html.Div(id='live-graph-container', children=[
        loading_component
    ]),
    # Arbitrary element to fire the callback below
    html.P(
        id='trigger-live-chart',
        children=''),
])

@app.callback(Output('live-graph-container', 'children'),
    [Input('trigger-live-chart','children')])
def generate_live_chart(_):
    time.sleep(3) # simmulate 3 seconds of data processing
    return dcc.Graph(
        id='live-graph',
        figure={
            'data': [
                {'x': [1, 2, 3], 'y': [4, 1, 2], 'type': 'bar', 'name': 'SF'},
                {'x': [1, 2, 3], 'y': [2, 4, 5], 'type': 'bar', 'name': u'Montréal'},
            ],
            'layout': {
                'title': 'Dash Data Visualization'
            }
        }
    )


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

If you remove that dcc.Input() line, the program still runs, but you are just stuck with the loading text. Is this happening for others when they try to use it, or just me? Any thoughts on why this is occurring?

For reference, here are my plotly related packages version:

dash                   0.28.2
dash-core-components   0.33.0
dash-html-components   0.13.2
dash-renderer          0.14.1
plotly                 3.3.0

[chriddyp]

Thanks for reporting and for helping out in the community forum! I believe that this is a repeat of plotly/dash-renderer#46

[mbkupfer]

Sounds good and glad to help out!

Seems like the work around is very simple, but good to know that this issue exists. Perhaps adding an exception that parses the layout after parsing callbacks would be a nice warning to print out for those not in the know.