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BUG: Index.getitem returning wrong result with negative step for arrow #55832

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Merged
merged 13 commits into from
Nov 22, 2023
Merged

BUG: Index.getitem returning wrong result with negative step for arrow #55832

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BUG: Index.getitem returning wrong result with negative step for arrow
phofl Nov 5, 2023
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1 change: 1 addition & 0 deletions doc/source/whatsnew/v2.1.3.rst
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Expand Up @@ -22,6 +22,7 @@ Fixed regressions
Bug fixes
~~~~~~~~~
- Bug in :meth:`DatetimeIndex.diff` raising ``TypeError`` (:issue:`55080`)
- Bug in :meth:`Index.__getitem__` returning wrong result for Arrow dtypes and negative stepsize (:issue:`55832`)
- Bug in :meth:`Index.isin` raising for Arrow backed string and ``None`` value (:issue:`55821`)

.. ---------------------------------------------------------------------------
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7 changes: 7 additions & 0 deletions pandas/core/arrays/arrow/array.py
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Expand Up @@ -553,6 +553,13 @@ def __getitem__(self, item: PositionalIndexer):
)
# We are not an array indexer, so maybe e.g. a slice or integer
# indexer. We dispatch to pyarrow.
if isinstance(item, slice):
if item.start == item.stop:
pass
elif item.start == -len(self) - 1:
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@lukemanley lukemanley Nov 5, 2023 •
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should this be item.start < -len(self)?

Not sure if you want to mimic numpy behavior here but numpy allows slicing beyond the len of the array:

In [1]: import numpy as np

In [2]: np.array(["a", "b", "c"])[slice(-10, 10)]
Out[2]: array(['a', 'b', 'c'], dtype='<U1')

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Yeah that makes sense, updated

item = slice(None, item.stop, item.step)
elif item.stop == -len(self) - 1:
item = slice(item.start, None, item.step)
value = self._pa_array[item]
if isinstance(value, pa.ChunkedArray):
return type(self)(value)
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14 changes: 11 additions & 3 deletions pandas/tests/indexes/object/test_indexing.py
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Expand Up @@ -4,6 +4,7 @@
import pytest

from pandas._libs.missing import is_matching_na
import pandas.util._test_decorators as td

import pandas as pd
from pandas import Index
Expand Down Expand Up @@ -144,6 +145,13 @@ def test_get_indexer_non_unique_np_nats(self, np_nat_fixture, np_nat_fixture2):


class TestSliceLocs:
@pytest.mark.parametrize(
"dtype",
[
"object",
pytest.param("string[pyarrow_numpy]", marks=td.skip_if_no("pyarrow")),
],
)
@pytest.mark.parametrize(
"in_slice,expected",
[
Expand All @@ -167,12 +175,12 @@ class TestSliceLocs:
(pd.IndexSlice["m":"m":-1], ""), # type: ignore[misc]
],
)
def test_slice_locs_negative_step(self, in_slice, expected):
index = Index(list("bcdxy"))
def test_slice_locs_negative_step(self, in_slice, expected, dtype):
index = Index(list("bcdxy"), dtype=dtype)

s_start, s_stop = index.slice_locs(in_slice.start, in_slice.stop, in_slice.step)
result = index[s_start : s_stop : in_slice.step]
expected = Index(list(expected))
expected = Index(list(expected), dtype=dtype)
tm.assert_index_equal(result, expected)

def test_slice_locs_dup(self):
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