fix(rust, python): include offset in arr.get (#5193)

polars/polars-arrow/src/kernels/list.rs

@@ -31,9 +31,11 @@ use crate::utils::CustomIterTools;
 ///
 /// ```
 fn sublist_get_indexes(arr: &ListArray<i64>, index: i64) -> IdxArr {
-    let mut iter = arr.offsets().iter();
+    let offsets = arr.offsets().as_slice();
+    let mut iter = offsets.iter();
 
-    let mut cum_offset: IdxSize = 0;
+    // the indices can be sliced, so we should not start at 0.
+    let mut cum_offset = (*offsets.first().unwrap_or(&0)) as IdxSize;
 
     if let Some(mut previous) = iter.next().copied() {
         let a: IdxArr = iter

py-polars/tests/unit/test_lists.py

@@ -1,7 +1,9 @@
 from __future__ import annotations
 
+import typing
 from datetime import date, datetime, time
 
+import numpy as np
 import pandas as pd
 
 import polars as pl
@@ -505,3 +507,30 @@ def test_list_slice() -> None:
     assert df.select([pl.col("lst").arr.slice(-2, "len")]).to_dict(False) == {
         "lst": [[3, 4], [2, 1]]
     }
+
+
+@typing.no_type_check
+def test_list_sliced_get_5186() -> None:
+    n = 30
+    df = pl.from_dict(
+        {
+            "ind": pl.arange(0, n, eager=True),
+            "inds": np.stack([np.arange(n), -np.arange(n)], axis=-1),
+        }
+    )
+
+    assert df.select(
+        [
+            "ind",
+            pl.col("inds").arr.first().alias("first_element"),
+            pl.col("inds").arr.last().alias("last_element"),
+        ]
+    )[10:20].frame_equal(
+        df[10:20].select(
+            [
+                "ind",
+                pl.col("inds").arr.first().alias("first_element"),
+                pl.col("inds").arr.last().alias("last_element"),
+            ]
+        )
+    )