add check for column_names-series combination and refactor logic

narwhals/utils.py

@@ -314,33 +314,36 @@ def maybe_set_index(
         msg = "Either `column_names` or `index` should be provided"
         raise ValueError(msg)
 
-    if is_pandas_like_dataframe(native_frame):
-        if column_names is not None:
+    if column_names is not None:
+        if is_pandas_like_dataframe(native_frame):
             return df_any._from_compliant_dataframe(  # type: ignore[no-any-return]
                 df_any._compliant_frame._from_native_frame(
                     native_frame.set_index(column_names)
                 )
             )
+        elif is_pandas_like_series(native_frame):
+            msg = "Cannot set index using column names on a Series"
+            raise ValueError(msg)
 
-        if index is not None:  # pragma: no cover
-            from narwhals.series import Series
-
-            if _is_iterable(index):
-                index = [
-                    idx.to_native() if isinstance(idx, Series) else idx for idx in index
-                ]
-            if isinstance(index, Series):
-                index = index.to_native()
+    if index is not None:  # pragma: no cover
+        from narwhals.series import Series
 
-            if is_pandas_like_series(df_any):
-                native_frame.index = index
-            else:
-                native_frame = native_frame.set_index(index)
+        if _is_iterable(index):
+            index = [idx.to_native() if isinstance(idx, Series) else idx for idx in index]
+        if isinstance(index, Series):
+            index = index.to_native()
 
+        if is_pandas_like_dataframe(native_frame):
             return df_any._from_compliant_dataframe(  # type: ignore[no-any-return]
                 df_any._compliant_frame._from_native_frame(native_frame.set_index(index))
             )
 
+        elif is_pandas_like_series(native_frame):
+            native_frame.index = index
+            return df_any._from_compliant_series(  # type: ignore[no-any-return]
+                df_any._compliant_series._from_native_series(native_frame)
+            )
+
     return df_any  # type: ignore[no-any-return]
 
 

tests/utils_test.py

@@ -143,6 +143,14 @@ def test_maybe_set_index_polars_direct_index(
     assert result is df
 
 
+def test_maybe_set_index_pandas_series_column_names() -> None:
+    df = nw.from_native(pd.Series([0, 1, 2]), allow_series=True)
+    with pytest.raises(
+        ValueError, match="Cannot set index using column names on a Series"
+    ):
+        nw.maybe_set_index(df, column_names=["a"])
+
+
 def test_maybe_set_index_pandas_either_index_or_column_names() -> None:
     df = nw.from_native(pd.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]}))
     column_names = ["a", "b"]