chore: ban dask filtering with boolean mask

narwhals/_dask/dataframe.py

@@ -79,14 +79,15 @@ def filter(
             and isinstance(predicates[0], list)
             and all(isinstance(x, bool) for x in predicates[0])
         ):
-            mask = predicates[0]
-        else:
-            from narwhals._dask.namespace import DaskNamespace
+            msg = "Filtering with boolean mask is not supported for `DaskLazyFrame`"
+            raise NotImplementedError(msg)
+
+        from narwhals._dask.namespace import DaskNamespace
 
-            plx = DaskNamespace(backend_version=self._backend_version)
-            expr = plx.all_horizontal(*predicates)
-            # Safety: all_horizontal's expression only returns a single column.
-            mask = expr._call(self)[0]
+        plx = DaskNamespace(backend_version=self._backend_version)
+        expr = plx.all_horizontal(*predicates)
+        # Safety: all_horizontal's expression only returns a single column.
+        mask = expr._call(self)[0]
         return self._from_native_frame(self._native_frame.loc[mask])
 
     def lazy(self) -> Self:

tests/frame/filter_test.py

@@ -1,5 +1,8 @@
+from contextlib import nullcontext as does_not_raise
 from typing import Any
 
+import pytest
+
 import narwhals.stable.v1 as nw
 from tests.utils import compare_dicts
 
@@ -15,6 +18,16 @@ def test_filter(constructor: Any) -> None:
 def test_filter_with_boolean_list(constructor: Any) -> None:
     data = {"a": [1, 3, 2], "b": [4, 4, 6], "z": [7.0, 8, 9]}
     df = nw.from_native(constructor(data))
-    result = df.filter([False, True, True])
-    expected = {"a": [3, 2], "b": [4, 6], "z": [8.0, 9.0]}
-    compare_dicts(result, expected)
+
+    context = (
+        pytest.raises(
+            NotImplementedError, match="Filtering with boolean mask is not supported"
+        )
+        if "dask" in str(constructor)
+        else does_not_raise()
+    )
+
+    with context:
+        result = df.filter([False, True, True])
+        expected = {"a": [3, 2], "b": [4, 6], "z": [8.0, 9.0]}
+        compare_dicts(result, expected)