fix: Preserve pandas column name attribute

narwhals/_dask/dataframe.py

@@ -110,7 +110,7 @@ def collect(
             from narwhals._pandas_like.dataframe import PandasLikeDataFrame
 
             return PandasLikeDataFrame(
-                result,
+                result.rename_axis(columns=self.native.columns.name),
                 implementation=Implementation.PANDAS,
                 backend_version=parse_version(pd),
                 version=self._version,

narwhals/_pandas_like/dataframe.py

@@ -117,6 +117,7 @@ def __init__(
         validate_backend_version(self._implementation, self._backend_version)
         if validate_column_names:
             check_column_names_are_unique(native_dataframe.columns)
+        self._native_columns_name = native_dataframe.columns.name
 
     @classmethod
     def from_arrow(cls, data: IntoArrowTable, /, *, context: _FullContext) -> Self:
@@ -248,7 +249,7 @@ def _with_version(self: Self, version: Version) -> Self:
 
     def _with_native(self: Self, df: Any, *, validate_column_names: bool = True) -> Self:
         return self.__class__(
-            df,
+            df.rename_axis(columns=self._native_columns_name),
             implementation=self._implementation,
             backend_version=self._backend_version,
             version=self._version,
@@ -630,7 +631,7 @@ def collect(
             import pandas as pd  # ignore-banned-import
 
             return PandasLikeDataFrame(
-                self.to_pandas(),
+                self.to_pandas().rename_axis(columns=self.native.columns.name),
                 implementation=Implementation.PANDAS,
                 backend_version=parse_version(pd),
                 version=self._version,

narwhals/_pandas_like/namespace.py

@@ -231,43 +231,33 @@ def concat(
     ) -> PandasLikeDataFrame:
         dfs: list[Any] = [item._native_frame for item in items]
         if how == "horizontal":
-            return PandasLikeDataFrame(
-                horizontal_concat(
-                    dfs,
-                    implementation=self._implementation,
-                    backend_version=self._backend_version,
-                ),
+            native_dataframe = horizontal_concat(
+                dfs,
                 implementation=self._implementation,
                 backend_version=self._backend_version,
-                version=self._version,
-                validate_column_names=True,
             )
-        if how == "vertical":
-            return PandasLikeDataFrame(
-                vertical_concat(
-                    dfs,
-                    implementation=self._implementation,
-                    backend_version=self._backend_version,
-                ),
+        elif how == "vertical":
+            native_dataframe = vertical_concat(
+                dfs,
                 implementation=self._implementation,
                 backend_version=self._backend_version,
-                version=self._version,
-                validate_column_names=True,
             )
-
-        if how == "diagonal":
-            return PandasLikeDataFrame(
-                diagonal_concat(
-                    dfs,
-                    implementation=self._implementation,
-                    backend_version=self._backend_version,
-                ),
+        elif how == "diagonal":
+            native_dataframe = diagonal_concat(
+                dfs,
                 implementation=self._implementation,
                 backend_version=self._backend_version,
-                version=self._version,
-                validate_column_names=True,
             )
-        raise NotImplementedError
+        else:
+            raise NotImplementedError
+
+        return PandasLikeDataFrame(
+            native_dataframe,
+            implementation=self._implementation,
+            backend_version=self._backend_version,
+            version=self._version,
+            validate_column_names=True,
+        )
 
     def when(self: Self, predicate: PandasLikeExpr) -> PandasWhen:
         return PandasWhen.from_expr(predicate, context=self)

narwhals/translate.py

@@ -488,8 +488,8 @@ def _from_native_impl(  # noqa: PLR0915
         return DataFrame(
             PandasLikeDataFrame(
                 native_object,
-                backend_version=parse_version(pd),
                 implementation=Implementation.PANDAS,
+                backend_version=parse_version(pd),
                 version=version,
                 validate_column_names=True,
             ),

tests/preserve_pandas_like_columns_name_attr_test.py

@@ -0,0 +1,28 @@
+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) -> 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"
+        )
+
+    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"