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GH-55295: Reject negative num_rows in Dataset head #55612

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GH-55295: Reject negative num_rows in Dataset head #55612

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4 changes: 4 additions & 0 deletions cpp/src/arrow/dataset/scanner.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -593,6 +593,10 @@ Result<std::shared_ptr<Table>> AsyncScanner::TakeRows(const Array& indices) {
}

Result<std::shared_ptr<Table>> AsyncScanner::Head(int64_t num_rows) {
if (num_rows < 0) {
return Status::Invalid("num_rows must be non-negative");
}

if (num_rows == 0) {
return Table::FromRecordBatches(options()->projected_schema, {});
}
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13 changes: 13 additions & 0 deletions python/pyarrow/tests/test_dataset.py
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Expand Up @@ -661,6 +661,19 @@ def test_head(dataset, dataset_reader):
result = fragment.head(1024, columns=['i64']).to_pydict()
assert result == {'i64': list(range(5))}

# ARROW-#55295: Check for negative num_rows
@pytest.mark.parquet
def test_head_negative_num_rows(dataset):
with pytest.raises(pa.ArrowInvalid, match="num_rows must be non-negative"):
dataset.head(-1)

with pytest.raises(pa.ArrowInvalid, match="num_rows must be non-negative"):
dataset.scanner().head(-1)

fragment = next(dataset.get_fragments())
with pytest.raises(pa.ArrowInvalid, match="num_rows must be non-negative"):
fragment.head(-1)


@pytest.mark.parquet
def test_take(dataset, dataset_reader):
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