feat: pyarrow lit

narwhals/_arrow/namespace.py

@@ -7,6 +7,7 @@
 
 from narwhals import dtypes
 from narwhals._arrow.expr import ArrowExpr
+from narwhals._arrow.series import ArrowSeries
 from narwhals._expression_parsing import parse_into_exprs
 from narwhals.dependencies import get_pyarrow
 from narwhals.utils import flatten
@@ -15,8 +16,6 @@
     from typing import Callable
 
     from narwhals._arrow.dataframe import ArrowDataFrame
-    from narwhals._arrow.expr import ArrowExpr
-    from narwhals._arrow.series import ArrowSeries
     from narwhals._arrow.typing import IntoArrowExpr
 
 
@@ -126,6 +125,26 @@ def all(self) -> ArrowExpr:
             backend_version=self._backend_version,
         )
 
+    def lit(self, value: Any, dtype: dtypes.DType | None) -> ArrowExpr:
+        def _lit_arrow_series(df: ArrowDataFrame) -> ArrowSeries:
+            arrow_series = ArrowSeries._from_iterable(
+                data=[value] * len(df),
+                name="lit",
+                backend_version=self._backend_version,
+            )
+            if dtype:
+                return arrow_series.cast(dtype)
+            return arrow_series
+
+        return ArrowExpr(
+            lambda df: [_lit_arrow_series(df)],
+            depth=0,
+            function_name="lit",
+            root_names=None,
+            output_names=["lit"],
+            backend_version=self._backend_version,
+        )
+
     def all_horizontal(self, *exprs: IntoArrowExpr) -> ArrowExpr:
         return reduce(
             lambda x, y: x & y,

narwhals/_arrow/series.py

@@ -5,7 +5,6 @@
 from typing import Iterable
 from typing import Sequence
 
-from narwhals._arrow.namespace import ArrowNamespace
 from narwhals._arrow.utils import reverse_translate_dtype
 from narwhals._arrow.utils import translate_dtype
 from narwhals._arrow.utils import validate_column_comparand
@@ -15,6 +14,7 @@
 if TYPE_CHECKING:
     from typing_extensions import Self
 
+    from narwhals._arrow.namespace import ArrowNamespace
     from narwhals.dtypes import DType
 
 
@@ -164,6 +164,8 @@ def count(self) -> int:
         return pc.count(self._native_series)  # type: ignore[no-any-return]
 
     def __narwhals_namespace__(self) -> ArrowNamespace:
+        from narwhals._arrow.namespace import ArrowNamespace
+
         return ArrowNamespace(backend_version=self._backend_version)
 
     @property

tests/frame/lit_test.py

@@ -18,26 +18,25 @@
     [(None, [2, 2, 2]), (nw.String, ["2", "2", "2"]), (nw.Float32, [2.0, 2.0, 2.0])],
 )
 def test_lit(
-    constructor_with_lazy: Any, dtype: DType | None, expected_lit: list[Any]
+    constructor_with_pyarrow: Any, dtype: DType | None, expected_lit: list[Any]
 ) -> None:
     data = {"a": [1, 3, 2], "b": [4, 4, 6], "z": [7.0, 8, 9]}
-    df_raw = constructor_with_lazy(data)
-    df = nw.from_native(df_raw)
+    df_raw = constructor_with_pyarrow(data)
+    df = nw.from_native(df_raw).lazy()
     result = df.with_columns(nw.lit(2, dtype).alias("lit"))
-    result_native = nw.to_native(result)
     expected = {
         "a": [1, 3, 2],
         "b": [4, 4, 6],
         "z": [7.0, 8.0, 9.0],
         "lit": expected_lit,
     }
-    compare_dicts(result_native, expected)
+    compare_dicts(result, expected)
 
 
-def test_lit_error(constructor_with_lazy: Any) -> None:
+def test_lit_error(constructor_with_pyarrow: Any) -> None:
     data = {"a": [1, 3, 2], "b": [4, 4, 6], "z": [7.0, 8, 9]}
-    df_raw = constructor_with_lazy(data)
-    df = nw.from_native(df_raw)
+    df_raw = constructor_with_pyarrow(data)
+    df = nw.from_native(df_raw).lazy()
     with pytest.raises(
         ValueError, match="numpy arrays are not supported as literal values"
     ):