old_contents: from flask import Flask
  app = Flask(__name__)
  
  @app.route('/')
  def hello_world():
      return 'Hello World!'
  
  if __name__ == '__main__':
      app.run(debug=True)
  
new_contents: """
  Main app module
  """
  from flask import Flask, render_template
  
  from app.config import ProdConfig
  from app.extensions import db, login_manager
  
  
  def create_app(config_object=ProdConfig):
      app = Flask(__name__)
      app.config.from_object(config_object)
      register_extensions(app)
      register_blueprints(app)
      register_error_handlers(app)
      return app
  
  
  def register_extensions(app):
      db.init_app(app)
      login_manager.init_app(app)
  
  
  def register_blueprints(app):
      pass
  
  
  def register_error_handlers(app):
      """Register error handlers."""
      def render_error(error):
          """Render error template."""
          # If a HTTPException, pull the `code` attribute; default to 500
          error_code = getattr(error, 'code', 500)
          return render_template('errors/{0}.html'.format(error_code)), error_code
      for errcode in [401, 404, 500]:
          app.errorhandler(errcode)(render_error)
  
subject: Define main entry point for creating Flask app