Custom ContentsManager with a NoOpCheckpoints cannot open notebook, raises Unhandled error

[miraculixx]

Description

Implementing a NoOpCheckpoints checkpoints class for a custom ContentsManager results in a Unhandled error being raised when opening a notebook in Jupyter Lab.

Reproduce

1. Create a new ContentsManager class (e.g. subclass FileContentsManager)
2. Create the NoOpCheckpoints as shown in the documentation
3. Set the checkpoints_class = NoOpCheckpoints trait
4. Run Jupyter lab and try creating a new notebook
5. This will result in client error shown below (screenshot) and in the severs' log (traceback in command line output)

Expected behavior

This should work, or the documentation for NoOpCheckpoints should be changed.

Context

This used to work. I did not manage to find the jupyter part and version (client or server) that has introduced the issue.

• Operating System and version: Linux Mint 21.2 Victoria base: Ubuntu 22.04 jammy
• Python 3.10.12,
• Browser and version: Google Chrome Version 118.0.5993.88 (Official Build) (64-bit)
• Jupyter Server version:jupyter_server==2.9.0

Troubleshoot Output

$ pip freeze | grep -i jup
jupyter-events==0.6.3
jupyter-lsp==2.2.0
jupyter-telemetry==0.1.0
jupyter_client==8.4.0
jupyter_core==5.4.0
jupyter_server==2.9.0
jupyter_server_terminals==0.4.4
jupyterhub==4.0.2
jupyterlab==4.0.7
jupyterlab-pygments==0.2.2
jupyterlab_server==2.24.0

Command Line Output

[W 2023-10-25 11:57:18.899 ServerApp] wrote error: 'Unhandled error'                                                                                          
    Traceback (most recent call last):                                                                                                                        
      File "/.../lib/python3.10/site-packages/tornado/web.py", line 1786, in _execute                                     
        result = await result                                                                                                                                 
      File "/.../python3.10/site-packages/jupyter_server/services/contents/handlers.py", line 311, in post            
        url_escape(checkpoint["id"]),                                                                                                                         
    TypeError: 'NoneType' object is not subscriptable                                                                                                         
[E 2023-10-25 11:57:18.900 ServerApp] {                                                                                                                       
      "Host": "localhost:8888",                                                                                                                               
      "Accept": "*/*",                                                                                                                                        
      "Referer": "http://localhost:8888/lab/workspaces/auto-i/tree/Untitled.ipynb",                                                                           
      "User-Agent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/118.0.0.0 Safari/537.36"

Browser Output

Paste the output from your browser Javascript console here, if applicable.

[miraculixx]

Changing the NoOpsCheckpoints implementation slightly removes the issue (added the checkpoint_model() method and called this method in create_file_checkpoint() and create_notebook_checkpoint(), hint taken from FileContentsManager)

from IPython.utils.tz import utcnow
from jupyter_server.services.contents.checkpoints import GenericCheckpointsMixin, Checkpoints


class NoOpCheckpoints(GenericCheckpointsMixin, Checkpoints):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.log.info("**** NoOpCheckpoints initialized")

    def checkpoint_model(self):
        info = {
            "id": "",
            "last_modified": utcnow()
        }
        return info

    def create_file_checkpoint(self, content, format, path):
        """ -> checkpoint model"""
        return self.checkpoint_model()

    def create_notebook_checkpoint(self, nb, path):
        """ -> checkpoint model"""
        return self.checkpoint_model()

    def get_file_checkpoint(self, checkpoint_id, path):
        """ -> {'type': 'file', 'content': <str>, 'format': {'text', 'base64'}}"""

    def get_notebook_checkpoint(self, checkpoint_id, path):
        """ -> {'type': 'notebook', 'content': <output of nbformat.read>}"""

    def delete_checkpoint(self, checkpoint_id, path):
        """deletes a checkpoint for a file"""

    def list_checkpoints(self, path):
        """returns a list of checkpoint models for a given file,
        default just does one per file
        """
        return []

    def rename_checkpoint(self, checkpoint_id, old_path, new_path):
        """renames checkpoint from old path to new path"""