Trouble exporting notebooks with otter-grader: Notebook is empty

[NSagan271]

Bug description

Hi! Some EE 120 students are experiencing trouble exporting their notebooks with otter-grader; they're getting "AssertionError: Notebook lab4.ipynb is empty. Please save and checkpoint your notebook and rerun this cell" even after saving and checkpointing, restarting the page, restarting their server, etc.

Here's the lab in question, it's one of our more compute-heavy ones.

According to one student, it worked after "some combination of server restarts, kernel restarts, and refreshing the page."

It could be a problem on our end, or maybe the students are having network connectivity issues, but I'm bringing it up here in case it's something that you all can help with.

Environment & setup

• Hub: EECS Hub
• Language: Python

How to reproduce

1. Go to https://eecs.datahub.berkeley.edu/hub/user-redirect/git-pull?repo=https%3A%2F%2Fgithub.com%2FEE120-Course-Staff%2Flab-fft-heart-rate-analysis&urlpath=tree%2Flab-fft-heart-rate-analysis%2Flab4.ipynb&branch=main
2. Run the first two cells, save and checkpoint, and run the last cell.

It definitely doesn't happen all the time, so I'm not sure how easy it is to reproduce this behavior.

[balajialg]

@NSagan271 Seems like issue was reported in Otter grader repo - ucbds-infra/otter-grader#444
@chrispyles Do you have any suggestions for handling this error now?

[NSagan271]

@balajialg Could increased CPU resources help? If so, I can make a Github issue for that.

I'm asking because I haven't encountered this issue at all locally, but most/all of the students are having issues exporting their submissions.

[balajialg]

Tagging @shaneknapp @ryanlovett if they have input on whether increasing CPU would have any effect. EECS hub has a CPU limit of 2 and a guarantee of 0.1. Should we increase the limit and/or guarantee further to test whether it reduces the save time of notebooks?

@NSagan271 Can you take a look at this code snippet used by Data 8 when a similar error happened to their students - #2107 (comment)

[NSagan271]

BTW might be helpful: it seems to be happening because the lab involves processing a video file. The lab deadline has passed, but we had students do the following and it seemed to work:

1. Download the .ipynb file.
2. Go to datahub.berkeley.edu and make a new folder.
3. Upload the lab4.ipynb to this new folder and open it. Run only the first two cells (otter-grader and imports).
4. Add a cell with grader.export(force_save=True, run_tests=False). Run it.

Since the lab deadline has passed, no changes need to be made at the moment (as long as our next lab doesn't have the same issue).

[balajialg]

@NSagan271 That's an interesting hack. Thanks for sharing it. I will add it to the FAQ documentation

I will close the issue for now but feel free to reopen if this issue persists during the next lab. Thanks!

[chrispyles]

@NSagan271 @balajialg it looks like the EECS hub is using Otter v3.1.4. Otter's force-save functionality was updated to be a bit more robust in ucbds-infra/otter-grader#476, which was included in v3.3.0. Is it possible to upgrade the version of Otter being used on this hub?

[balajialg]

@chrispyles Thanks for the info! Yes - that should not be a problem. Is the version upgrade backward compatible or will that result in instructors rewriting their existing test cases?

[chrispyles]

I don’t think there are any breaking changes