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Avoid reindexing values assigned to a multi-index when cols_droplevel contains empty values (#62404)

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Original file line number	Diff line number	Diff line change
`@@ -999,6 +999,7 @@ MultiIndex`
`999`	`999`	- Bug in :class:`DataFrame` arithmetic operations in case of unaligned MultiIndex columns (:issue:`60498`)
`1000`	`1000`	- Bug in :class:`DataFrame` arithmetic operations with :class:`Series` in case of unaligned MultiIndex (:issue:`61009`)
`1001`	`1001`	- Bug in :meth:`MultiIndex.from_tuples` causing wrong output with input of type tuples having NaN values (:issue:`60695`, :issue:`60988`)
	`1002`	+- Bug in :meth:`DataFrame.__setitem__` where column alignment logic would reindex the assigned value with an empty index, incorrectly setting all values to ``NaN``.(:issue:`61841`)
`1002`	`1003`	- Bug in :meth:`DataFrame.reindex` and :meth:`Series.reindex` where reindexing :class:`Index` to a :class:`MultiIndex` would incorrectly set all values to ``NaN``.(:issue:`60923`)
`1003`	`1004`
`1004`	`1005`	`I/O`

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Original file line number	Diff line number	Diff line change
`@@ -4452,6 +4452,11 @@ def _set_item_frame_value(self, key, value: DataFrame) -> None:`
`4452`	`4452`	`loc, (slice, Series, np.ndarray, Index)`
`4453`	`4453`	`):`
`4454`	`4454`	`cols_droplevel = maybe_droplevels(cols, key)`
	`4455`	`+ if (`
	`4456`	`+ not isinstance(cols_droplevel, MultiIndex)`
	`4457`	`+ and not cols_droplevel.any()`
	`4458`	`+ ):`
	`4459`	`+ return`
`4455`	`4460`	`if len(cols_droplevel) and not cols_droplevel.equals(value.columns):`
`4456`	`4461`	`value = value.reindex(cols_droplevel, axis=1)`
`4457`	`4462`

Lines changed: 22 additions & 0 deletions

Original file line number	Diff line number	Diff line change
`@@ -249,3 +249,25 @@ def test_groupyby_rename_categories_operation_with_multiindex(self, operation):`
`249`	`249`	`expected = getattr(a, operation)(b.sort_index(ascending=False))`
`250`	`250`
`251`	`251`	`tm.assert_series_equal(result, expected)`
	`252`	`+`
	`253`	`+ def test_multiindex_assign_aligns_as_implicit_tuple(self):`
	`254`	`+ # GH 61841`
	`255`	`+ cols = MultiIndex.from_tuples([("A", "B")])`
	`256`	`+ df1 = DataFrame([[i] for i in range(3)], columns=cols)`
	`257`	`+ df2 = df1.copy()`
	`258`	`+ df3 = df1.copy()`
	`259`	`+ s1 = df1["A"].rolling(2).mean()`
	`260`	`+ s2 = s1.copy()`
	`261`	`+ s3 = s1.copy()`
	`262`	`+`
	`263`	`+ df2["C"] = s2`
	`264`	`+ df3[("C", "")] = s3`
	`265`	`+ tm.assert_frame_equal(df2, df3)`
	`266`	`+`
	`267`	`+ df1["C"] = s1`
	`268`	`+ tm.assert_frame_equal(df1, df2)`
	`269`	`+ tm.assert_frame_equal(df1, df3)`
	`270`	`+`
	`271`	`+ df1["C"] = s1`
	`272`	`+ tm.assert_frame_equal(df1, df2)`
	`273`	`+ tm.assert_frame_equal(df1, df3)`

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