Callback to dropdown options not firing

Dash Python
1

Hi all,

I have a problem with one of my callbacks that should populate a dropdown when a file is selected in the uploader. However, nothing happens. I have created the following minimal example which does not work on my end (the dropdown does not change, nor does the debug message get printed on the console). Is this a bug, or am I doing something wrong?

Thanks for your help!
Daniel

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

external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']


app = dash.Dash(__name__, external_stylesheets=external_stylesheets)

app.layout = html.Div(children=[
    html.H1(children='Dropdown Test'),

    dcc.Dropdown(id='the-dropdown',
                 options=[{'label': 'foo', 'value': 'bar'}]),

    dcc.Upload(id='the-uploader', children=html.Div([
        'Drag and Drop or ', html.A('Select File')
        ]),
        style={
            'width': '100%',
            'height': '60px',
            'lineHeight': '60px',
            'borderWidth': '1px',
            'borderStyle': 'dashed',
            'borderRadius': '5px',
            'textAlign': 'center',
            'margin': '10px'
        }
    )
])


@app.callback(Output('the-dropdown', 'options'),
              [Input('the-uploader', 'contents')])
def load_dropdown(the_file):
    print('load references called', file=sys.stderr)
    if the_file:
        return [{'label': 'bla', 'value': 'bla'}]
    else:
        return None


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