text: import os
  
  import dash
  import dash_core_components as dcc
  import dash_html_components as html
  # pypiwin32==219
  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)
  
  if __name__ == '__main__':
      app.run_server(debug=True)