fix: fix skew for duckdb with fewer than 3 elements

narwhals/_duckdb/expr.py

@@ -300,13 +300,25 @@ def mean(self) -> Self:
         )
 
     def skew(self) -> Self:
+        from duckdb import CaseExpression
+        from duckdb import ConstantExpression
         from duckdb import FunctionExpression
 
-        return self._from_call(
-            lambda _input: FunctionExpression("skewness", _input),
-            "skew",
-            returns_scalar=True,
-        )
+        def func(_input: duckdb.Expression) -> duckdb.Expression:
+            count = FunctionExpression("count", _input)
+            return CaseExpression(
+                condition=count == 0, value=ConstantExpression(None)
+            ).otherwise(
+                CaseExpression(
+                    condition=count == 1, value=ConstantExpression(float("nan"))
+                ).otherwise(
+                    CaseExpression(
+                        condition=count == 2, value=ConstantExpression(0.0)
+                    ).otherwise(FunctionExpression("skewness", _input))
+                )
+            )
+
+        return self._from_call(func, "skew", returns_scalar=True)
 
     def median(self) -> Self:
         from duckdb import FunctionExpression

tests/expr_and_series/unary_test.py

@@ -79,11 +79,7 @@ def test_unary_series(constructor_eager: ConstructorEager) -> None:
     assert_equal_data(result, expected)
 
 
-def test_unary_two_elements(
-    constructor: Constructor, request: pytest.FixtureRequest
-) -> None:
-    if "duckdb" in str(constructor):
-        request.applymarker(pytest.mark.xfail)
+def test_unary_two_elements(constructor: Constructor) -> None:
     data = {"a": [1, 2], "b": [2, 10], "c": [2.0, None]}
     result = nw.from_native(constructor(data)).select(
         a_nunique=nw.col("a").n_unique(),