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FIX: derive csv header from the union of row keys - #3304
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Roman Lutz (romanlutz) merged 4 commits intoOct 19, 2026
Merged
Roman Lutz (romanlutz) merged 4 commits into
Roman Lutz (romanlutz) merged 4 commits into
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Roman Lutz (romanlutz)
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Oct 19, 2026
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Description
Fixes #3303
write_csvtook its fieldnames from the first row only, so a later row with an extra key madeDictWriterraise after the header and earlier rows were already written, leaving a truncated file behind. The header now comes from the union of keys across all rows in first-seen order, and rows missing a key get an empty cell.Tests and Documentation
Added two tests: one writes rows where only the second row has an extra key and asserts the round trip keeps every value, and one asserts the header keeps first seen column order when later rows introduce keys. Both fail on main and pass with the change. The full csv helper suite passes (9 tests).
No documentation change needed, so I did not run JupyText.