feat: support dataframe.columns for interchange level (#1285)

docs/extending.md

@@ -69,6 +69,7 @@ libraries which have interchange only support can access the following methods a
 converting to Narwhals DataFrame:
 
 - `.schema`, hence column names via `.schema.names()` and column types via `.schema.dtypes()`
+- `.columns`
 - `.to_pandas()` and `.to_arrow()`, for converting to Pandas and Arrow, respectively.
 - `.select(names)` (Ibis and DuckDB), where `names` is a list of (string) column names. This is useful for
   selecting columns before converting to another library.

narwhals/_duckdb/dataframe.py

@@ -116,6 +116,8 @@ def __getattr__(self, attr: str) -> Any:
                     self._native_frame.columns, self._native_frame.types
                 )
             }
+        elif attr == "columns":
+            return self._native_frame.columns
 
         msg = (  # pragma: no cover
             f"Attribute {attr} is not supported for metadata-only dataframes.\n\n"

narwhals/_ibis/dataframe.py

@@ -109,6 +109,8 @@ def __getattr__(self, attr: str) -> Any:
                 column_name: map_ibis_dtype_to_narwhals_dtype(ibis_dtype, self._dtypes)
                 for column_name, ibis_dtype in self._native_frame.schema().items()
             }
+        elif attr == "columns":
+            return self._native_frame.columns
         msg = (
             f"Attribute {attr} is not supported for metadata-only dataframes.\n\n"
             "If you would like to see this kind of object better supported in "

tests/frame/interchange_schema_test.py

@@ -156,6 +156,7 @@ def test_interchange_schema_ibis(
         }
     assert result == expected
     assert df["a"].dtype == nw.Int64
+    assert df.columns == list(expected.keys())
 
 
 def test_interchange_schema_duckdb() -> None:
@@ -220,6 +221,7 @@ def test_interchange_schema_duckdb() -> None:
     }
     assert result == expected
     assert df["a"].dtype == nw.Int64
+    assert df.columns == list(expected.keys())
 
 
 def test_invalid() -> None: