Recognize named-aggregation lambdas as scans in GroupBy

python/cudf/cudf/core/groupby/groupby.py

@@ -215,7 +215,28 @@ def _is_all_scan_aggregate(all_aggs: list[list[str]]) -> bool:
     }
 
     def get_name(agg):
-        return agg.__name__ if callable(agg) else agg
+        if not callable(agg):
+            return agg
+        if agg is not list:
+            # A ``lambda x: x.cumsum()``-style aggregation carries its
+            # scan-ness only in the aggregation name it resolves to
+            # (``Aggregation.cumsum`` is an alias of ``sum``; libcudf
+            # separates scan from reduction by the *call*, not the
+            # aggregation object). Probe the callable with a
+            # name-recording stand-in mirroring ``make_aggregation``'s
+            # ``op(Aggregation)`` protocol; true UDFs raise inside the
+            # probe and fall back to ``__name__``.
+            class _NameProbe:
+                def __getattr__(self, name):
+                    return lambda *args, **kwargs: name
+
+            try:
+                name = agg(_NameProbe())
+            except Exception:
+                return agg.__name__
+            if isinstance(name, str):
+                return name
+        return agg.__name__
 
     all_scan = all(
         get_name(agg_name) in groupby_scans

python/cudf/cudf/pandas/scripts/pandas-testing-plugin.py

@@ -1921,8 +1921,6 @@ def pytest_unconfigure(config):
     "tests/groupby/test_reductions.py::test_sum_skipna_object[False]": "Inherent cudf.pandas None-vs-NaN difference for object-dtype null (skipna logic is correct)",
     "tests/groupby/test_timegrouper.py::TestGroupBy::test_groupby_with_timegrouper": "TODO: Add a reason for failure",
     "tests/groupby/test_timegrouper.py::TestGroupBy::test_scalar_call_versus_list_call": "TODO: Add a reason for failure",
-    "tests/groupby/transform/test_transform.py::test_cython_transform_series[cumprod-args0-<lambda>]": "TODO: Add a reason for failure",
-    "tests/groupby/transform/test_transform.py::test_cython_transform_series[cumsum-args1-<lambda>]": "TODO: Add a reason for failure",
     "tests/groupby/transform/test_transform.py::test_nan_in_cumsum_group_label": "AssertionError: Attributes of Series are different",
     "tests/indexes/base_class/test_reshape.py::TestReshape::test_insert_missing[Decimal]": "TODO: Add a reason for failure",
     "tests/indexes/categorical/test_astype.py::TestAstype::test_categorical_date_roundtrip[False]": "TODO: Add a reason for failure",

python/cudf/cudf/tests/groupby/test_transform.py

@@ -119,3 +119,15 @@ def test_transform_cumcount_series(dropna):
     expect = pdf.groupby("A", dropna=dropna).transform("cumcount")
     got = gdf.groupby("A", dropna=dropna).transform("cumcount")
     assert_eq(expect, got)
+
+
+def test_transform_scan_lambda():
+    # a named-aggregation lambda resolving to a scan must scan per group,
+    # not broadcast the group total
+    pdf = pd.DataFrame({"key": [0, 0, 1, 1], "val": [1.0, 2.0, 3.0, 4.0]})
+    gdf = cudf.DataFrame(pdf)
+
+    expect = pdf.groupby("key")["val"].transform(lambda x: x.cumsum())
+    got = gdf.groupby("key")["val"].transform(lambda x: x.cumsum())
+
+    assert_eq(expect, got)