`jupyter nbconvert --to script nb.ipynb --output nb 2>/dev/null` was
silently exiting 0 without producing the output file in some
environments (likely because jupyter/jupyter_core wasn't on PATH or
nbconvert's --output handling differed across versions). The 2>/dev/null
hid the underlying error, and `set -e` did not catch the missing-output
case because nbconvert itself returned 0.
Switch to a direct nbformat-based conversion:
pip install -q nbformat
python -c "import nbformat; nb=nbformat.read('nb.ipynb', as_version=4);
code='\n\n'.join(c.source for c in nb.cells if c.cell_type == 'code')
open('nb.py','w').write(code + '\n')"
Smaller dep set, no shell-out to a jupyter wrapper script, and an
explicit `test -s nb.py` afterwards catches any silent failure
before downstream steps try to read the file.
Reproduces the failure on RTX PRO 6000 Blackwell (sm_120, docker
29.2.1, ubuntu 24.04) where nbconvert's CLI silently no-op'd.
8344fa0a56