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Regression: Loc raising when indexing mi with one level #45785

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Feb 9, 2022
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Regression: Loc raising when indexing mi with one level #45785

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Regression: Loc raising when indexing mi with one level
phofl Feb 2, 2022
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phofl Feb 6, 2022
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1 change: 1 addition & 0 deletions doc/source/whatsnew/v1.4.1.rst
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Expand Up @@ -16,6 +16,7 @@ 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 :func:`.assert_frame_equal` not respecting ``check_flags=False`` (:issue:`45554`)
- Regression in :meth:`DataFrame.loc.__getitem__` raising ``ValueError`` when indexing on a :class:`MultiIndex` with one level (:issue:`45779`)
- Regression in :meth:`Series.fillna` with ``downcast=False`` incorrectly downcasting ``object`` dtype (:issue:`45603`)
- Regression in :meth:`DataFrame.iat` setting values leading to not propagating correctly in subsequent lookups (:issue:`45684`)
- Regression when setting values with :meth:`DataFrame.loc` losing :class:`Index` name if :class:`DataFrame` was empty before (:issue:`45621`)
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7 changes: 6 additions & 1 deletion pandas/core/indexes/multi.py
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Expand Up @@ -3128,7 +3128,12 @@ def maybe_mi_droplevels(indexer, levels):
# e.g. test_partial_string_timestamp_multiindex
return indexer, self[indexer]

return indexer, maybe_mi_droplevels(indexer, [level])
try:
result_index = maybe_mi_droplevels(indexer, [level])
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hmm can we handle this inside?

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oh i c @jbrockmendel moved this. can you document this in the doc-string of maybe_mi_droplevels? (and prob should just move maybe_mi_droplevels) top a non-nested function (ok with these being a followup on main as well).

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Added a note. Would like to move in a separate pr to not backport this

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great, yeah ideally clean in 1.5

except ValueError:
result_index = self[indexer]

return indexer, result_index

def _get_level_indexer(
self, key, level: int = 0, indexer: Int64Index | None = None
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12 changes: 12 additions & 0 deletions pandas/tests/frame/indexing/test_indexing.py
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Expand Up @@ -1546,6 +1546,18 @@ def test_getitem_preserve_object_index_with_dates(self, indexer):

assert ser.index.dtype == object

def test_loc_on_multiindex_one_level(self):
# GH#45779
df = DataFrame(
data=[[0], [1]],
index=MultiIndex.from_tuples([("a",), ("b",)], names=["first"]),
)
expected = DataFrame(
data=[[0]], index=MultiIndex.from_tuples([("a",)], names=["first"])
)
result = df.loc["a"]
tm.assert_frame_equal(result, expected)


class TestDepreactedIndexers:
@pytest.mark.parametrize(
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