fix: markdown export under -O

[Guiforge]

Fix Markdown export under Python optimized mode (-O)

Context

Issue: docling-project/docling-core#460

Markdown export could crash when running Python with optimizations enabled (e.g. python -O or PYTHONOPTIMIZE=1).

Symptom

UnboundLocalError: cannot access local variable 'list_group' where it is not associated with a value

This surfaced during Markdown serialization of list items.

Root cause

In docling_core/transforms/serializer/markdown.py (Markdown list item serialization), list_group was assigned via the walrus operator inside an assert:

• assert item.parent and isinstance((list_group := item.parent.resolve(doc)), ListGroup)

Under -O, Python strips assert statements entirely, so the assignment never executes. Since Python still treats list_group as a local variable in the function, later references to list_group raise UnboundLocalError.

(Conceptually the same as Ruff rule RUF018: assignment in assert.)

Fix

• Remove the assignment from the assert.
• Resolve list_group explicitly (outside of assert) and guard access with an is not None check.
• If the parent is unexpectedly missing or not a ListGroup, fall back to the safe Markdown marker - rather than crashing.

This preserves existing behavior for valid documents and prevents optimized-mode crashes.

Tests

• Added test/test_markdown_optimized_mode.py which spawns a subprocess with sys.executable -O and calls DoclingDocument.export_to_markdown() on a minimal document that triggers list marker normalization.

Notes

• No public APIs changed.
• Behavior is strictly more robust: malformed list structures no longer crash serialization in optimized mode.

[github-actions]

❌ DCO Check Failed

Hi @Guiforge, your pull request has failed the Developer Certificate of Origin (DCO) check.

This repository supports remediation commits, so you can fix this without rewriting history — but you must follow the required message format.

---

🛠 Quick Fix: Add a remediation commit

Run this command:

git commit --allow-empty -s -m "DCO Remediation Commit for Guillaume Pouyat <[email protected]>

I, Guillaume Pouyat <[email protected]>, hereby add my Signed-off-by to this commit: a9ce24f7e9bd440812cb60960bad201e2d1caa56"
git push

---

🔧 Advanced: Sign off each commit directly

For the latest commit:

git commit --amend --signoff
git push --force-with-lease

For multiple commits:

git rebase --signoff origin/main
git push --force-with-lease

More info: DCO check report

[dosubot]

Related Documentation

Checked 10 published document(s) in 1 knowledge base(s). No updates required.

^{How did I do? Any feedback?}  

[mergify]

Merge Protections

Your pull request matches the following merge protections and will not be merged until they are valid.

🔴 Require two reviewer for test updates

This rule is failing.

When test data is updated, we require two reviewers

• #approved-reviews-by >= 2

🟢 Enforce conventional commit

Wonderful, this rule succeeded.

Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?:

[cau-git]

@Guiforge thanks for this fix! The change looks good to me, but I would suggest to remove the test unit you provide, it should not require a test at this level.

Also, please follow the steps to remediate the DCO as posted by the github-bot here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!