"manually graded task" cells cause metadata error

Operating system

MacOS 10.15.2

nbgrader --version

Python version 3.7.4 (default, Aug 13 2019, 15:17:50) [Clang 4.0.1 (tags/RELEASE_401/final)] nbgrader version 0.6.1

jupyterhub --version (if used with JupyterHub)

Not using JupyterHub.

jupyter notebook --version

6.0.1

Expected behavior

With the “create assignment” cell toolbar, I should be able to set a code cell’s type to “Manually graded task” and then have the notebook pass a Validation button test.

Actual behavior

Whenever I set any code cell in the notebook to “Manually graded task” – even the only cell in the notebook, even if it has no content or just contains pass – clicking the Validate button throws the following error:

Traceback (most recent call last):
  File "/Users/ibeatty/anaconda3/lib/python3.7/site-packages/nbgrader/server_extensions/validate_assignment/handlers.py", line 46, in validate_notebook
    result = validator.validate(fullpath)
  File "/Users/ibeatty/anaconda3/lib/python3.7/site-packages/nbgrader/validator.py", line 311, in validate
    nb = self._preprocess(nb)
  File "/Users/ibeatty/anaconda3/lib/python3.7/site-packages/nbgrader/validator.py", line 290, in _preprocess
    nb, resources = pp.preprocess(nb, resources)
  File "/Users/ibeatty/anaconda3/lib/python3.7/site-packages/nbgrader/preprocessors/checkcellmetadata.py", line 16, in preprocess
    raise ValidationError(msg)
jsonschema.exceptions.ValidationError: Notebook failed to validate; the nbgrader metadata may be corrupted.

Markdown cells do not have this problem.

Steps to reproduce the behavior

1. Start jupyter notebook with jupyter notebook from the command line.
2. Create a new Python 3 notebook.
3. Turn on the Create Assignment toolbar from the View menu -> Cell Toolbar
4. Use the cell toolbar to change the automatically created, empty code cell’s type to Manually Graded Task. (It doesn’t matter whether the cell is empty or alone.)
5. Click the Validate button.

Boom.

[AnnabellBrocker]

So it is intended that a “manually graded task” cell should not be a code cell?

And thanks for the second link, that was a little more understandable.

[perllaghu]

The two cells act differently.

See https://nbgrader.readthedocs.io/en/stable/user_guide/creating_and_grading_assignments.html … and https://noteable.edina.ac.uk/nbguide/#section_ca - which may be slightly clearer