add dask and test

narwhals/_dask/expr.py

@@ -725,6 +725,15 @@ def func(_input: Any, dtype: DType | type[DType]) -> Any:
             returns_scalar=False,
         )
 
+    def is_finite(self: Self) -> Self:
+        import dask.array as da  # ignore-banned-import
+
+        return self._from_call(
+            lambda _input: da.isfinite(_input),
+            "is_finite",
+            returns_scalar=False,
+        )
+
 
 class DaskExprStringNamespace:
     def __init__(self, expr: DaskExpr) -> None:

tests/expr_and_series/is_finite_test.py

@@ -0,0 +1,27 @@
+from __future__ import annotations
+
+import pytest
+
+import narwhals.stable.v1 as nw
+from tests.utils import Constructor
+from tests.utils import ConstructorEager
+from tests.utils import assert_equal_data
+
+data = {"a": [float("nan"), float("inf"), 2.0]}
+expected = {"a": [False, False, True]}
+
+
+@pytest.mark.filterwarnings("ignore:invalid value encountered in cast")
+def test_is_finite_expr(constructor: Constructor) -> None:
+    df = nw.from_native(constructor(data))
+    result = df.select(nw.col("a").is_finite())
+
+    assert_equal_data(result, expected)
+
+
+@pytest.mark.filterwarnings("ignore:invalid value encountered in cast")
+def test_is_finite_series(constructor_eager: ConstructorEager) -> None:
+    df = nw.from_native(constructor_eager(data), eager_only=True)
+    result = {"a": df["a"].is_finite()}
+
+    assert_equal_data(result, expected)