feat: raise NotImplementedError for not supported parameters in `ew…

…m_mean` for cuDF (#1449)

narwhals/_pandas_like/series.py

@@ -188,9 +188,21 @@ def ewm_mean(
     ) -> PandasLikeSeries:
         ser = self._native_series
         mask_na = ser.isna()
-        result = ser.ewm(
-            com, span, half_life, alpha, min_periods, adjust, ignore_na=ignore_nulls
-        ).mean()
+        if self._implementation is Implementation.CUDF:
+            if (min_periods == 0 and not ignore_nulls) or (not mask_na.any()):
+                result = ser.ewm(
+                    com=com, span=span, halflife=half_life, alpha=alpha, adjust=adjust
+                ).mean()
+            else:
+                msg = (
+                    "cuDF only supports `ewm_mean` when there are no missing values "
+                    "or when both `min_period=0` and `ignore_nulls=False`"
+                )
+                raise NotImplementedError(msg)
+        else:
+            result = ser.ewm(
+                com, span, half_life, alpha, min_periods, adjust, ignore_na=ignore_nulls
+            ).mean()
         result[mask_na] = None
         return self._from_native_series(result)
 

tests/expr_and_series/ewm_test.py

@@ -16,9 +16,9 @@
     "ignore:`Expr.ewm_mean` is being called from the stable API although considered an unstable feature."
 )
 def test_ewm_mean_expr(request: pytest.FixtureRequest, constructor: Constructor) -> None:
-    if any(
-        x in str(constructor) for x in ("pyarrow_table_", "dask", "modin", "cudf")
-    ) or ("polars" in str(constructor) and POLARS_VERSION < (1,)):
+    if any(x in str(constructor) for x in ("pyarrow_table_", "dask", "modin")) or (
+        "polars" in str(constructor) and POLARS_VERSION < (1,)
+    ):
         request.applymarker(pytest.mark.xfail)
 
     df = nw.from_native(constructor(data))
@@ -36,7 +36,7 @@ def test_ewm_mean_expr(request: pytest.FixtureRequest, constructor: Constructor)
 def test_ewm_mean_series(
     request: pytest.FixtureRequest, constructor_eager: ConstructorEager
 ) -> None:
-    if any(x in str(constructor_eager) for x in ("pyarrow_table_", "modin", "cudf")) or (
+    if any(x in str(constructor_eager) for x in ("pyarrow_table_", "modin")) or (
         "polars" in str(constructor_eager) and POLARS_VERSION < (1,)
     ):
         request.applymarker(pytest.mark.xfail)
@@ -75,9 +75,9 @@ def test_ewm_mean_expr_adjust(
     adjust: bool,  # noqa: FBT001
     expected: dict[str, list[float]],
 ) -> None:
-    if any(
-        x in str(constructor) for x in ("pyarrow_table_", "dask", "modin", "cudf")
-    ) or ("polars" in str(constructor) and POLARS_VERSION < (1,)):
+    if any(x in str(constructor) for x in ("pyarrow_table_", "dask", "modin")) or (
+        "polars" in str(constructor) and POLARS_VERSION < (1,)
+    ):
         request.applymarker(pytest.mark.xfail)
 
     df = nw.from_native(constructor(data))
@@ -187,3 +187,18 @@ def test_ewm_mean_params(
 
     with pytest.raises(ValueError, match="mutually exclusive"):
         df.select(nw.col("a").ewm_mean(span=1.5, half_life=0.75, ignore_nulls=False))
+
+
+@pytest.mark.filterwarnings(
+    "ignore:`Expr.ewm_mean` is being called from the stable API although considered an unstable feature."
+)
+def test_ewm_mean_cudf_raise() -> None:  # pragma: no cover
+    pytest.importorskip("cudf")
+    import cudf
+
+    df = nw.from_native(cudf.DataFrame({"a": [2.0, 4.0, None, 3.0]}))
+    with pytest.raises(
+        NotImplementedError,
+        match="cuDF only supports `ewm_mean` when there are no missing values",
+    ):
+        df.select(nw.col("a").ewm_mean(com=1))