polars fix

narwhals/_dask/utils.py

@@ -34,7 +34,7 @@ def maybe_evaluate_expr(df: DaskLazyFrame, obj: DaskExpr | object) -> dx.Series
         results = obj._call(df)
         if len(results) != 1:
             msg = "Multi-output expressions (e.g. `nw.all()` or `nw.col('a', 'b')`) not supported in this context"
-            raise ValueError(msg)
+            raise AssertionError(msg)
         return results[0]
     return obj
 

tests/frame/invalid_test.py

@@ -18,8 +18,12 @@
 def test_all_vs_all(constructor: Constructor) -> None:
     data = {"a": [1, 3, 2], "b": [4, 4, 6]}
     df: Frame = nw.from_native(constructor(data))
-    with pytest.raises(ValueError, match="Multi-output"):
-        df.select(nw.all() + nw.all())
+    with pytest.raises(
+        (ValueError, AssertionError),
+        match=r"Multi-output|Expr: \*\' not allowed in this context",
+    ):
+        # Polars raises AssertionError.
+        df.lazy().select(nw.all() + nw.col("b", "a")).collect()
 
 
 def test_invalid() -> None: