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REGR: loc.setitem losing index name when df was empty before #45623

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merged 3 commits into from
Jan 26, 2022
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REGR: loc.setitem losing index name when df was empty before #45623

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REGR: loc.setitem losing index name when df was empty before
phofl Jan 25, 2022
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Add ignore
phofl Jan 25, 2022
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Use index
phofl Jan 26, 2022
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1 change: 1 addition & 0 deletions doc/source/whatsnew/v1.4.1.rst
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Expand Up @@ -15,6 +15,7 @@ including other versions of pandas.
Fixed regressions
~~~~~~~~~~~~~~~~~
- Regression in :meth:`Series.mask` with ``inplace=True`` and ``PeriodDtype`` and an incompatible ``other`` coercing to a common dtype instead of raising (:issue:`45546`)
- Regression in :meth:`DataFrame.loc.__setitem__` losing :class:`Index` name if :class:`DataFrame` was empty before (:issue:`45621`)
-

.. ---------------------------------------------------------------------------
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2 changes: 2 additions & 0 deletions pandas/core/indexing.py
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Expand Up @@ -2018,6 +2018,8 @@ def _setitem_with_indexer_missing(self, indexer, value):
# internals.concat logic
df = value.to_frame().T
df.index = [indexer]
# error: "List[Any]" has no attribute "name"
df.index.name = self.obj.index.name # type: ignore[attr-defined]
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hmm this is actually modifying the indexer i think.

maybe
df.index = pd.Index([indexer], name=self.obj.index.name)?

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Good point, changed

if not has_dtype:
# i.e. if we already had a Series or ndarray, keep that
# dtype. But if we had a list or dict, then do inference
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7 changes: 7 additions & 0 deletions pandas/tests/frame/indexing/test_indexing.py
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Expand Up @@ -1279,6 +1279,13 @@ def test_setting_mismatched_na_into_nullable_fails(
with pytest.raises(TypeError, match=msg):
df2.iloc[:2, 0] = [null, null]

def test_loc_expand_empty_frame_keep_index_name(self):
# GH#45621
df = DataFrame(columns=["b"], index=Index([], name="a"))
df.loc[0] = 1
expected = DataFrame({"b": [1]}, index=Index([0], name="a"))
tm.assert_frame_equal(df, expected)


class TestDataFrameIndexingUInt64:
def test_setitem(self, uint64_frame):
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