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BUG: DataFrame.idxmax wrong row for nullable boolean with NA - #71833
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NA - MaskedArrays
nanops._get_fill_value gave bool the +/-inf fill, which np.putmask casts to True, so a masked slot outranked a real True under argmax. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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DataFrame.idxmaxon a nullablebooleancolumn with missing values returned the firstFalserow instead of the firstTruerow:nanops._get_fill_valuegave bool the-inffill, whichnp.putmaskcasts toTrue. The masked slot then beat any realTrueunder argmax, and the redirect added in GH-66250 turned that into the first unmasked row; before GH-66250 the NA row's label came back. Bool now getsFalse/Truefill values.idxminwas already right.AI disclosure: drafted with Claude Code (
claude opus 5.5 (high)), which found the bug in an audit of GH-66250, wrote the fix and tests, and ran a differential sweep against main.