test: add Expr.skew test (#2557)

* test: add Expr.skew test

* dasks warnings

Author: MarcoGorelli

tests/expr_and_series/skew_test.py

@@ -3,11 +3,10 @@
 import pytest
 
 import narwhals as nw
+from tests.utils import Constructor
 from tests.utils import ConstructorEager
 from tests.utils import assert_equal_data
 
-data = [1, 2, 3, 2, 1]
-
 
 @pytest.mark.parametrize(
     ("data", "expected"),
@@ -24,3 +23,27 @@ def test_skew_series(
 ) -> None:
     result = nw.from_native(constructor_eager({"a": data}), eager_only=True)["a"].skew()
     assert_equal_data({"a": [result]}, {"a": [expected]})
+
+
+@pytest.mark.parametrize(
+    ("data", "expected"),
+    [
+        ([], None),
+        ([1], None),
+        ([1, 2], 0.0),
+        ([0.0, 0.0, 0.0], None),
+        ([1, 2, 3, 2, 1], 0.343622),
+    ],
+)
+@pytest.mark.filterwarnings("ignore:.*invalid value:RuntimeWarning:dask")
+def test_skew_expr(
+    constructor: Constructor,
+    data: list[float],
+    expected: float | None,
+    request: pytest.FixtureRequest,
+) -> None:
+    if "ibis" in str(constructor):
+        # https://github.com/ibis-project/ibis/issues/11176
+        request.applymarker(pytest.mark.xfail)
+    result = nw.from_native(constructor({"a": data})).select(nw.col("a").skew())
+    assert_equal_data(result, {"a": [expected]})