code: def load_jupyter_server_extension(app):  # pragma: no cover
      
      if isinstance(app.contents_manager_class, TextFileContentsManager):
          app.log.info("[Jupytext Server Extension] NotebookApp.contents_manager_class is "
                       "(a subclass of) jupytext.TextFileContentsManager already - OK")
          return
  
      # The server extension call is too late!
      # The contents manager was set at NotebookApp.init_configurables
  
      # Let's change the contents manager class
      app.log.info('[Jupytext Server Extension] Changing NotebookApp.contents_manager_class '
                   'from {} to jupytext.TextFileContentsManager'.format(app.contents_manager_class.__name__))
      app.contents_manager_class = TextFileContentsManager
  
      try:
          # And rerun selected init steps from https://github.com/jupyter/notebook/blob/
          # 132f27306522b32fa667a6b208034cb7a04025c9/notebook/notebookapp.py#L1634-L1638
  
          # app.init_configurables()
          app.contents_manager = app.contents_manager_class(parent=app, log=app.log)
          app.session_manager.contents_manager = app.contents_manager
  
          # app.init_components()
          # app.init_webapp()
          app.web_app.settings['contents_manager'] = app.contents_manager
  
          # app.init_terminals()
          # app.init_signal()
  
      except Exception:
          app.log.error("""[Jupytext Server Extension] An error occured. Please deactivate the server extension with
      jupyter serverextension disable jupytext
  and configure the contents manager manually by adding
      c.NotebookApp.contents_manager_class = "jupytext.TextFileContentsManager"
  to your .jupyter/jupyter_notebook_config.py file.
  """)
          raise
docs: Use Jupytext's contents manager