fenfeng9 opened a new issue, #50105:
URL: https://github.com/apache/arrow/issues/50105
### Describe the bug, including details regarding any error messages,
version, and platform.
### Description
Slicing a sparse union array produces incorrect logical values.
`to_pylist()` and scalar access on the slice return wrong results —a
non-null element is reported as null, or a value from the wrong position is
returned.
### Reproduce
```python
import pyarrow as pa
def main():
arr = pa.UnionArray.from_sparse(
pa.array([0, 0, 0], type=pa.int8()),
[pa.array([None, 20, 30])],
)
sliced = arr.slice(2)
print(f"pyarrow: {pa.__version__}")
print()
# Direct access on the original array is still correct.
expected_arr_2 = 30
actual_arr_2 = arr[2].as_py()
print(f"arr[2].as_py() expected: {expected_arr_2}")
print(f"arr[2].as_py() actual: {actual_arr_2}")
print()
# The sliced array should preserve the same logical value in to_pylist().
expected_sliced_pylist = [30]
actual_sliced_pylist = sliced.to_pylist()
print(f"sliced.to_pylist() expected: {expected_sliced_pylist}")
print(f"sliced.to_pylist() actual: {actual_sliced_pylist}")
print()
# Scalar access on the sliced array should also return the same value.
expected_sliced_0 = 30
actual_sliced_0 = sliced[0].as_py()
print(f"sliced[0].as_py() expected: {expected_sliced_0}")
print(f"sliced[0].as_py() actual: {actual_sliced_0}")
if __name__ == "__main__":
main()
```
### Result
```python
pyarrow: 24.0.0
arr[2].as_py() expected: 30
arr[2].as_py() actual: 30
sliced.to_pylist() expected: [30]
sliced.to_pylist() actual: [None]
sliced[0].as_py() expected: 30
sliced[0].as_py() actual: None
```
### Component(s)
C++
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