app.py

import os

import dash
import dash_core_components as dcc
import dash_html_components as html


def _set_app():
    external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']

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

    app.layout = html.Div([
        html.H2('Hello World'),
        dcc.Dropdown(
            id='dropdown',
            options=[{'label': i, 'value': i} for i in ['LA', 'NYC', 'MTL']],
            value='LA'
        ),
        html.Div(id='display-value')
    ])

    @app.callback(dash.dependencies.Output('display-value', 'children'),
                  [dash.dependencies.Input('dropdown', 'value')])
    def display_value(value):
        return 'You have selected "{}"'.format(value)

    return app

app = _set_app()
server = app.server

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