old_contents: from flask import Flask, url_for, render_template
  app = Flask(__name__)
  
  @app.route('/')
  def hello_world():
      return render_template('index.html')
  
  # def index(): pass
  
  @app.route('/login')
  def login(): pass
  
  @app.route('/user/<username>')
  def profile(username): pass
  
  @app.errorhandler(404)
  def page_not_found(e):
      return render_template('404.html'), 404
  	
  # with app.test_request_context():
  # 	print (url_for('hello_world'))
  # 	print (url_for('login'))
  # 	print (url_for('login', next='/', other_param='hah'))
  # 	print (url_for('profile', username='John Doe'))
  
  if __name__ == '__main__':
      app.run(debug=True)
      # to make externally visible:
      # app.run(host='0.0.0.0')
  
new_contents: from flask import Flask, url_for, render_template
  app = Flask(__name__)
  
  @app.route('/')
  def hello_world():
      return render_template('index.html')
  
  # def index(): pass
  
  @app.route('/login')
  def login(): pass
  
  @app.route('/user/<username>')
  def profile(username): pass
  
  @app.errorhandler(404)
  def page_not_found(e):
      return render_template('404.html'), 404
  	
  if __name__ == '__main__':
      app.run(debug=True)
      # to make externally visible:
      # app.run(host='0.0.0.0')
  
subject: Remove commented sections