fix: Preserve pandas column name attribute

narwhals/_pandas_like/dataframe.py

@@ -422,6 +422,8 @@ def select(self: PandasLikeDataFrame, *exprs: PandasLikeExpr) -> PandasLikeDataF
         new_series = align_series_full_broadcast(*new_series)
         namespace = self.__narwhals_namespace__()
         df = namespace._concat_horizontal([s.native for s in new_series])
+        # `concat` creates a new object, so fine to modify `.columns.name` inplace.
+        df.columns.name = self.native.columns.name
         return self._with_native(df, validate_column_names=True)
 
     def drop_nulls(
@@ -480,6 +482,8 @@ def with_columns(
         to_concat.extend(self._extract_comparand(s) for s in name_columns.values())
         namespace = self.__narwhals_namespace__()
         df = namespace._concat_horizontal(to_concat)
+        # `concat` creates a new object, so fine to modify `.columns.name` inplace.
+        df.columns.name = self.native.columns.name
         return self._with_native(df, validate_column_names=False)
 
     def rename(self, mapping: Mapping[str, str]) -> Self:

tests/frame/join_test.py

@@ -9,7 +9,6 @@
 import pytest
 
 import narwhals as nw
-from narwhals.utils import Implementation
 from tests.utils import DUCKDB_VERSION
 from tests.utils import PANDAS_VERSION
 from tests.utils import POLARS_VERSION
@@ -242,11 +241,14 @@ def test_cross_join_suffix(constructor: Constructor, suffix: str) -> None:
 
 
 def test_cross_join_non_pandas() -> None:
+    _ = pytest.importorskip("modin")
+
+    import modin.pandas as mpd
+
     data = {"antananarivo": [1, 3, 2]}
-    df = nw.from_native(pd.DataFrame(data))
-    # HACK to force testing for a non-pandas codepath
-    df._compliant_frame._implementation = Implementation.MODIN
-    result = df.join(df, how="cross")
+    df1 = nw.from_native(mpd.DataFrame(pd.DataFrame(data)), eager_only=True)
+    df2 = nw.from_native(mpd.DataFrame(pd.DataFrame(data)), eager_only=True)
+    result = df1.join(df2, how="cross")
     expected = {
         "antananarivo": [1, 1, 1, 3, 3, 3, 2, 2, 2],
         "antananarivo_right": [1, 3, 2, 1, 3, 2, 1, 3, 2],

tests/preserve_pandas_like_columns_name_attr_test.py

@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+
+import narwhals.stable.v1 as nw
+
+if TYPE_CHECKING:
+    from tests.utils import Constructor
+
+
+def test_ops_preserve_column_index_name(
+    constructor: Constructor, request: pytest.FixtureRequest
+) -> None:
+    if not any(x in str(constructor) for x in ("pandas", "modin", "cudf", "dask")):
+        pytest.skip(
+            reason="Dataframe columns is a list and do not have a `name` like a pandas Index does"
+        )
+    if "dask" in str(constructor):
+        # https://github.com/dask/dask/issues/11874
+        request.applymarker(pytest.mark.xfail)
+
+    data = {"a": [1, 3, 2], "b": [4, 4, 6], "z": [7.0, 8.0, 9.0]}
+    df_native = constructor(data)
+    df_native.columns.name = "foo"  # type: ignore[union-attr]
+
+    df = nw.from_native(df_native)
+
+    result = df.with_columns(b=nw.col("a") + 1, c=nw.col("a") * 2).select("c", "b")
+
+    assert result.to_native().columns.name == "foo"  # type: ignore[union-attr]
+    assert result.lazy().collect(backend="pandas").to_native().columns.name == "foo"