code: def fancy_error_template_middleware(app):
      
      def application(environ, start_response):
          try:
              return app(environ, start_response)
          except Exception as exc:
              sio = StringIO()
              traceback.print_exc(file=sio)
              sio.seek(0)
              response =  Response(
                  status=500,
                  body=render_error_page(500, exc, traceback=sio.read()),
                  content_type="text/html"
              )
              return response(environ, start_response)
  
      return application
docs: WGSI middleware for catching errors and rendering the error page.