chore: validate predicates in nw.when one level higher (#1756)

* chore: validate predicates in `nw.when` one level higher

* sort out fail

narwhals/_arrow/namespace.py

@@ -359,12 +359,7 @@ def when(
         *predicates: IntoArrowExpr,
     ) -> ArrowWhen:
         plx = self.__class__(backend_version=self._backend_version, version=self._version)
-        if predicates:
-            condition = plx.all_horizontal(*predicates)
-        else:
-            msg = "at least one predicate needs to be provided"
-            raise TypeError(msg)
-
+        condition = plx.all_horizontal(*predicates)
         return ArrowWhen(condition, self._backend_version, version=self._version)
 
     def concat_str(

narwhals/_dask/namespace.py

@@ -310,12 +310,7 @@ def when(
         *predicates: IntoDaskExpr,
     ) -> DaskWhen:
         plx = self.__class__(backend_version=self._backend_version, version=self._version)
-        if predicates:
-            condition = plx.all_horizontal(*predicates)
-        else:
-            msg = "at least one predicate needs to be provided"
-            raise TypeError(msg)
-
+        condition = plx.all_horizontal(*predicates)
         return DaskWhen(
             condition, self._backend_version, returns_scalar=False, version=self._version
         )

narwhals/_pandas_like/namespace.py

@@ -371,12 +371,7 @@ def when(
         plx = self.__class__(
             self._implementation, self._backend_version, version=self._version
         )
-        if predicates:
-            condition = plx.all_horizontal(*predicates)
-        else:
-            msg = "at least one predicate needs to be provided"
-            raise TypeError(msg)
-
+        condition = plx.all_horizontal(*predicates)
         return PandasWhen(
             condition, self._implementation, self._backend_version, version=self._version
         )

narwhals/expr.py

@@ -7643,6 +7643,9 @@ def max_horizontal(*exprs: IntoExpr | Iterable[IntoExpr]) -> Expr:
 class When:
     def __init__(self, *predicates: IntoExpr | Iterable[IntoExpr]) -> None:
         self._predicates = flatten([predicates])
+        if not self._predicates:
+            msg = "At least one predicate needs to be provided to `narwhals.when`."
+            raise TypeError(msg)
 
     def _extract_predicates(self, plx: Any) -> Any:
         return [extract_compliant(plx, v) for v in self._predicates]

tests/expr_and_series/when_test.py

@@ -54,11 +54,7 @@ def test_multiple_conditions(
     assert_equal_data(result, expected)
 
 
-def test_no_arg_when_fail(
-    constructor: Constructor, request: pytest.FixtureRequest
-) -> None:
-    if "duckdb" in str(constructor):
-        request.applymarker(pytest.mark.xfail)
+def test_no_arg_when_fail(constructor: Constructor) -> None:
     df = nw.from_native(constructor(data))
     with pytest.raises((TypeError, ValueError)):
         df.select(nw.when().then(value=3).alias("a_when"))