Fix class_labels mutation across multi-metric write_metrics_reports

[aymuos15]

Description

write_metrics_reports rebinds and appends to its class_labels parameter inside the metric_details loop. After the first metric's CSV is written, class_labels is no longer None. It holds ["class0", ..., "classN", "mean"]. On every subsequent metric the else branch runs and appends another "mean", producing headers like class0,class1,mean,mean,mean.

This hits any evaluation with two or more metrics in metric_details, for example Dice + IoU + Hausdorff. The only real caller MetricsSaver always passes class_labels=None. The first metric's CSV is correct, but every one after is structurally corrupt with mismatched header and data columns.

The fix uses a local labels variable per iteration so the original class_labels parameter is never modified. The existing test only verified existence of subsequent metric files, not their headers. A regression test is included.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.

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ℹ️ Recent review info

⚙️ Run configuration

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Review profile: CHILL

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Run ID: 019d4ba5-27f4-4b60-9c7d-8b39e1aef859

📥 Commits

Reviewing files that changed from the base of the PR and between 2a7d0cf and e609d0a.

📒 Files selected for processing (2)

• monai/handlers/utils.py
• tests/handlers/test_write_metrics_reports.py

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📝 Walkthrough

Walkthrough

Renamed the class label variable from class_labels to labels in the write_metrics_reports function. Updated both the raw CSV header generation (lines 125–133) and summary CSV row writing (line 167) to reference the new name. Added a test case that verifies correct header structure in raw CSV files when writing multiple metrics with differing class counts.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	Title clearly describes the main fix: preventing class_labels mutation in write_metrics_reports across multiple metrics.
Description check	✅ Passed	Description covers the bug, root cause, solution, and test additions; missing only optional integration/documentation checkboxes.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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