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feat: Broadcast scalar input for list.slice #27487

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feat: Broadcast scalar input for list.slice #27487

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b8060f1
feat: Slice broadcast
Kevin-Patyk May 3, 2026
8e4dcc7
update: Address comment
Kevin-Patyk May 4, 2026
d8a63ae
Merge branch 'main' into feat/slice_broadcast
Kevin-Patyk May 4, 2026
e122796
fix: Python lint and rebase
Kevin-Patyk May 4, 2026
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7 changes: 7 additions & 0 deletions crates/polars-expr/src/dispatch/list.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -142,6 +142,13 @@ pub(super) fn slice(args: &mut [Column]) -> PolarsResult<Column> {
let offset_s = &args[1];
let length_s = &args[2];

let target_len = offset_s.len().max(length_s.len());
let list_ca = if list_ca.len() == 1 && target_len > 1 {
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@nameexhaustion nameexhaustion May 5, 2026

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It can be prohibitively expensive to be repeating the list values for each row; I'd have expected the implementation to have a kernel that can directly construct the output off of the single input row.

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@Kevin-Patyk Kevin-Patyk May 5, 2026

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Okay I will look into it! 👍

list_ca.new_from_index(0, target_len)
} else {
list_ca.clone()
};

let mut out: ListChunked = match (offset_s.len(), length_s.len()) {
(1, 1) => {
let offset = offset_s.get(0).unwrap().try_extract::<i64>()?;
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17 changes: 17 additions & 0 deletions py-polars/tests/unit/operations/namespaces/list/test_list.py
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Expand Up @@ -1384,3 +1384,20 @@ def test_list_eval_exceed_idx_size() -> None:
"len": [[2147483648], [2147483647], [2147483646]],
"unique": [[None], [None], [None]],
}


@pytest.mark.parametrize(
("offset", "length"),
[
(0, pl.lit(pl.Series([1, 2, 3]))),
(pl.lit(pl.Series([0, 1, 2])), 2),
(pl.lit(pl.Series([0, 1, 2])), pl.lit(pl.Series([3, 2, 1]))),
],
)
def test_list_slice_broadcast_27480(offset: Any, length: Any) -> None:
result = pl.select(pl.lit([0, 1, 2]).list.slice(offset, length).alias("broadcast"))
expected = pl.select(
pl.repeat(pl.lit([0, 1, 2]), 3).list.slice(offset, length).alias("broadcast")
)

assert_frame_equal(result, expected)
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