BUG: DataFrame.__setitem__ silently dropping columns of a 2D value (GH#46544)#66001
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BUG: DataFrame.__setitem__ silently dropping columns of a 2D value (GH#46544)#66001jbrockmendel wants to merge 1 commit into
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…H#46544) Setting an existing column label with a multi-column 2D array silently kept only the first column. iset now raises, matching the new-column and DataFrame.loc paths. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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closes #46544
Setting an existing column label with a multi-column 2D array silently kept only the first column and discarded the rest:
Every sibling path already rejected this — the new-column case (GH-46545), the list-key case, and
DataFrame.loc/DataFrame.ilocall raise. The existing-column__setitem__path was the lone outlier becauseBlockManager.isetvalidated only the row count of the (transposed) value, never its column count.isetnow raises the sameExpected a 1D arrayValueErrorthatinsertalready does.Single-column 2D values (shape
(N, 1), e.g. GH-42376) and keys that map to multiple columns (duplicate labels / partial MultiIndex keys, where an(N, k)value fills thekmatched columns) are unaffected and still work.