Duplicating cells results in autograding failure because of duplicate IDs, and confusing error message

This is similar to #981, as in that may solve the problem, but I’ll document core problem here and a workaround.

Students have submitted notebooks in which they have copied cells. This duplicates the metadata: of importance here is "nbgrader": {"grade_id": ...}. When this happens, the following error message is given:

[AutogradeApp | WARNING] Cell with id 'Task_3_1_test' exists multiple times!
...
[AutogradeApp | WARNING] Removing failed assignment: /m/jhnas/jupyter/course/mlpython2019/files/autograded/staafv1/R1_Introduction
[AutogradeApp | ERROR] One or more notebooks in the assignment use an old version of the nbgrader metadata format. Please **back up your class files directory** and then update the metadata using: nbgrader update .

The first warning is the key to the problem. However, the error message shows that nbgrader metadata should be updated, which not the real problem here (though I haven’t tested it if actually works). At least, the error message could be improved or the failure could be noticed earlier.

It isn’t a perfect solution, but it would be nice if nbgrader could also continue on in this case. Clearly the student has done something weird (and notebook/jupyterlab could solve it by not duplicating metadata). But could nbgrader also survive this and make some choice one way or the other itself, considering that duplicating metadata is probably a type of problem that will appear again? That way the common case won’t cause a failure stopping all autograding… and we leave it to instructors to tell of the problem later.

Who knows what the right nbgrader-only solution would be - simple would be to pick the last cell with the metadata. But many corner cases could break this.

Workaround for anyone else who gets this problem: My tests showed that you have to delete the whole nbgrader metadata dict from the notebook cell - changing the grade_id is not enough because it can’t find that grade_id in the database, nor is removing just the grade_id cell.

Operating system

Linux

nbgrader --version

Modified version, based on upstream d56f75b (close to current master).

[trevorcampbell]

Just adding a +1 to this – I ran in to this exact issue in my course just now!

[elesiuta]

I experienced this issue as well in the class I recently TA’d (nbgrader 0.5.4 on jupyterhub 0.9.4). My solution was a script that merges cells when it detects duplicate grade_ids by concatenating the cell’s source to the first then removing the duplicate.

I uploaded the script as part of a small collection I wrote for that course in case it’s of use to anyone https://github.com/elesiuta/jupyter-nbgrader-helper (run with --fix AssignName NbName.ipynb). As an aside, if there’s any feature in there you think might be useful to implement properly into nbgrader I’d be happy to open a pull request and work on it.