Jay
jupyter/nbconvert

Preventing duplicate markdown attachments from overwriting each other in nbconvert

nbconvert flattened markdown attachments from different cells into one shared output directory, so later cells could overwrite earlier files when they reused the same attachment filename. I reproduced that on current main, added regression coverage, and made the preprocessor deterministically suffix collisions instead of silently clobbering earlier attachments.

OPENjupyter/nbconvertPR #22802026-04-30
Problem
  • Issue #2163 reported that markdown export can collapse distinct attachments onto one filename when different cells reuse the same attachment name.
  • On current main, exporting a notebook with two cells that each referenced attachment:image.png emitted a warning and wrote only one support file, so both markdown references pointed at the second payload.
  • The root cause was ExtractAttachmentsPreprocessor writing every sanitized attachment basename into one shared resource map without uniquifying collisions across cells.
What changed
  • Added a small helper in ExtractAttachmentsPreprocessor to choose a unique filename before writing extracted attachments into the shared resource map.
  • Kept the first attachment path stable and suffixed later collisions as image_1.png, image_2.png, and so on.
  • Added a regression test for direct duplicate filenames across cells.
  • Added a second regression test covering collisions introduced by sanitizing nested attachment paths down to the same basename.
  • Applied the one-line CHANGELOG codespell fix that pre-commit.ci required on the PR branch.
Tests
  • python3 -m pytest tests/preprocessors/test_extractattachments.py -q -> 9 passed
  • python3 -m pytest tests/test_nbconvertapp.py -k "output_in_subdir_support_files_path or same_filename_different_dir" -q -> 1 passed, 46 deselected
  • manual reproduction on current main: jupyter nbconvert --to markdown with two distinct attachment:image.png payloads previously wrote one image.png; after the fix it writes image.png and image_1.png with distinct contents
Files changed
  • CHANGELOG.md
  • nbconvert/preprocessors/extractattachments.py
  • tests/preprocessors/test_extractattachments.py
Review + discussion
  • Issue #2163 — Reporter showed that markdown export could confuse overlapping images when different cells reused the same attachment filename. Open
  • Maintainer hint — takluyver pointed out that attachments are per-cell while ExtractAttachmentsPreprocessor writes them into a shared directory, which framed the bug cleanly. Open
  • PR #2280 — Opened a narrow preprocessor fix with regression coverage after reproducing the overwrite on current main. Open
  • CI green — After a quick CHANGELOG typo follow-up, both pre-commit.ci and Read the Docs are green on the PR branch. Open
Updates / timeline
  • 2026-04-30 — Opened PR #2280 after reproducing attachment overwrite behavior on current main and adding two focused regression tests. Open
  • 2026-04-30 — pre-commit.ci auto-formatted the branch, then a one-line CHANGELOG typo fix cleared the remaining codespell failure. Open
  • 2026-04-30 — Both visible checks are green on PR #2280: pre-commit.ci and Read the Docs. Open
  • 2026-05-30T21:00:00Z — CI state changed from green to failure — the PR head moved to 9067eb4faa with checks no longer passing. Likely a pre-commit.ci autofix or upstream dependency shift. Open
  • 2026-06-16T09:38:13Z — Cron check: commit statuses changed. State=open, mergeable_state=clean.
Lessons learned
  • The best new candidate was the one that still reproduced on current main after eliminating stale or overlapping alternatives first.
  • A warning about overwriting a previous attachment is a strong sign that the real user-facing bug is in filename allocation, not in downstream markdown rendering.
  • Adding a second regression around sanitized-name collisions makes a small fix much more trustworthy to maintainers.
Next steps
  • Wait for maintainer review now that the PR is open and both visible checks are green.

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