fix: when-then-otherwise lit string for arrow backend (#1137)

* fix: when-then-otherwise lit string for arrow backend

* rename test

narwhals/_arrow/namespace.py

@@ -11,6 +11,7 @@
 from narwhals._arrow.expr import ArrowExpr
 from narwhals._arrow.selectors import ArrowSelectorNamespace
 from narwhals._arrow.series import ArrowSeries
+from narwhals._arrow.utils import broadcast_series
 from narwhals._arrow.utils import horizontal_concat
 from narwhals._arrow.utils import vertical_concat
 from narwhals._expression_parsing import combine_root_names
@@ -353,7 +354,8 @@ def __call__(self, df: ArrowDataFrame) -> list[ArrowSeries]:
                 self._otherwise_value, namespace=plx
             )._call(df)[0]  # type: ignore[arg-type]
         except TypeError:
-            # `self._otherwise_value` is a scalar and can't be converted to an expression
+            # `self._otherwise_value` is a scalar and can't be converted to an expression.
+            # Remark that string values _are_ converted into expressions!
             return [
                 value_series._from_native_series(
                     pc.if_else(
@@ -364,7 +366,14 @@ def __call__(self, df: ArrowDataFrame) -> list[ArrowSeries]:
         else:
             otherwise_series = cast(ArrowSeries, otherwise_series)
             condition = cast(ArrowSeries, condition)
-            return [value_series.zip_with(condition, otherwise_series)]
+            condition_native, otherwise_native = broadcast_series(
+                [condition, otherwise_series]
+            )
+            return [
+                value_series._from_native_series(
+                    pc.if_else(condition_native, value_series_native, otherwise_native)
+                )
+            ]
 
     def then(self, value: ArrowExpr | ArrowSeries | Any) -> ArrowThen:
         self._then_value = value

tests/expr_and_series/when_test.py

@@ -138,3 +138,10 @@ def test_when_then_otherwise_into_expr(constructor: Constructor) -> None:
     result = df.select(nw.when(nw.col("a") > 1).then("c").otherwise("e"))
     expected = {"c": [7, 5, 6]}
     compare_dicts(result, expected)
+
+
+def test_when_then_otherwise_lit_str(constructor: Constructor) -> None:
+    df = nw.from_native(constructor(data))
+    result = df.select(nw.when(nw.col("a") > 1).then(nw.col("b")).otherwise(nw.lit("z")))
+    expected = {"b": ["z", "b", "c"]}
+    compare_dicts(result, expected)