old_contents: from flask import Flask, render_template
  
  from bokeh.embed import server_document
  from bokeh.layouts import column
  from bokeh.models import ColumnDataSource, Slider
  from bokeh.plotting import figure
  from bokeh.server.server import Server
  from bokeh.themes import Theme
  
  from bokeh.sampledata.sea_surface_temperature import sea_surface_temperature
  
  app = Flask(__name__)
  
  def modify_doc(doc):
      df = sea_surface_temperature.copy()
      source = ColumnDataSource(data=df)
  
      plot = figure(x_axis_type='datetime', y_range=(0, 25), y_axis_label='Temperature (Celsius)',
                    title="Sea Surface Temperature at 43.18, -70.43")
      plot.line('time', 'temperature', source=source)
  
      def callback(attr, old, new):
          if new == 0:
              data = df
          else:
              data = df.rolling('{0}D'.format(new)).mean()
          source.data = ColumnDataSource(data=data).data
  
      slider = Slider(start=0, end=30, value=0, step=1, title="Smoothing by N Days")
      slider.on_change('value', callback)
  
      doc.add_root(column(slider, plot))
  
      doc.theme = Theme(filename="theme.yaml")
  
  @app.route('/', methods=['GET'])
  def bkapp_page():
      script = server_document('http://localhost:5006/bkapp')
      return render_template("embed.html", script=script, template="Flask")
  
  def bk_worker():
      # Can't pass num_procs > 1 in this configuration. If you need to run multiple
      # processes, see e.g. flask_gunicorn_embed.py
      server = Server({'/bkapp': modify_doc}, allow_websocket_origin=["localhost:8000"])
      server.start()
      server.io_loop.start()
  
  from threading import Thread
  Thread(target=bk_worker).start()
  
  if __name__ == '__main__':
      print('Opening single process Flask app with embedded Bokeh application on http://localhost:8000/')
      print()
      print('Multiple connections may block the Bokeh app in this configuration!')
      print('See "flask_gunicorn_embed.py" for one way to run multi-process')
      app.run(port=8000)
  
new_contents: from flask import Flask, render_template
  
  from bokeh.embed import server_document
  from bokeh.layouts import column
  from bokeh.models import ColumnDataSource, Slider
  from bokeh.plotting import figure
  from bokeh.server.server import Server
  from bokeh.themes import Theme
  from tornado.ioloop import IOLoop
  
  from bokeh.sampledata.sea_surface_temperature import sea_surface_temperature
  
  app = Flask(__name__)
  
  
  def modify_doc(doc):
      df = sea_surface_temperature.copy()
      source = ColumnDataSource(data=df)
  
      plot = figure(x_axis_type='datetime', y_range=(0, 25), y_axis_label='Temperature (Celsius)',
                    title="Sea Surface Temperature at 43.18, -70.43")
      plot.line('time', 'temperature', source=source)
  
      def callback(attr, old, new):
          if new == 0:
              data = df
          else:
              data = df.rolling('{0}D'.format(new)).mean()
          source.data = ColumnDataSource(data=data).data
  
      slider = Slider(start=0, end=30, value=0, step=1, title="Smoothing by N Days")
      slider.on_change('value', callback)
  
      doc.add_root(column(slider, plot))
  
      doc.theme = Theme(filename="theme.yaml")
  
  
  @app.route('/', methods=['GET'])
  def bkapp_page():
      script = server_document('http://localhost:5006/bkapp')
      return render_template("embed.html", script=script, template="Flask")
  
  
  def bk_worker():
      # Can't pass num_procs > 1 in this configuration. If you need to run multiple
      # processes, see e.g. flask_gunicorn_embed.py
      server = Server({'/bkapp': modify_doc}, io_loop=IOLoop(), allow_websocket_origin=["localhost:8000"])
      server.start()
      server.io_loop.start()
  
  from threading import Thread
  Thread(target=bk_worker).start()
  
  if __name__ == '__main__':
      print('Opening single process Flask app with embedded Bokeh application on http://localhost:8000/')
      print()
      print('Multiple connections may block the Bokeh app in this configuration!')
      print('See "flask_gunicorn_embed.py" for one way to run multi-process')
      app.run(port=8000)
  
subject: Fix io_loop paramenter in examples/howto/server_embed/flask_embed.py (#7614)