fix: .list namespace should preserve pandas index (#1538)

* fix: .list preserve index

* use set_axis helper function and check pandas versions

* no cover?

narwhals/_pandas_like/series.py

@@ -1170,7 +1170,18 @@ def len(self: Self) -> PandasLikeSeries:
         from narwhals.utils import import_dtypes_module
 
         native_series = self._compliant_series._native_series
-        native_result = native_series.list.len().rename(native_series.name, copy=False)
+        native_result = native_series.list.len()
+
+        if (
+            self._compliant_series._implementation is Implementation.PANDAS
+            and self._compliant_series._backend_version < (3, 0)
+        ):  # pragma: no cover
+            native_result = set_axis(
+                native_result.rename(native_series.name, copy=False),
+                index=native_series.index,
+                implementation=self._compliant_series._implementation,
+                backend_version=self._compliant_series._backend_version,
+            )
         dtype = narwhals_to_native_dtype(
             dtype=import_dtypes_module(self._compliant_series._version).UInt32(),
             starting_dtype=native_result.dtype,

tests/expr_and_series/list/len_test.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import pandas as pd
 import pytest
 
 import narwhals.stable.v1 as nw
@@ -43,3 +44,15 @@ def test_len_series(
 
     result = df["a"].cast(nw.List(nw.Int32())).list.len()
     assert_equal_data({"a": result}, expected)
+
+
+def test_pandas_preserve_index(request: pytest.FixtureRequest) -> None:
+    if PANDAS_VERSION < (2, 2):
+        request.applymarker(pytest.mark.xfail)
+
+    index = pd.Index(["a", "b", "c", "d", "e"])
+    df = nw.from_native(pd.DataFrame(data, index=index), eager_only=True)
+
+    result = df["a"].cast(nw.List(nw.Int32())).list.len()
+    assert_equal_data({"a": result}, expected)
+    assert (result.to_native().index == index).all()