old_contents: from flask import Flask, request, render_template
  
  app = Flask(__name__)
  
  @app.route("/")
  def index():
      return render_template('index.html')
  
  if __name__ == "__main__":
      app.run()
  
new_contents: from flask import Flask, render_template
  
  app = Flask(__name__)
  
  @app.route("/")
  def index():
      return render_template('index.html')
  
  if __name__ == "__main__":
      app.run()
  
subject: Update app.py