Fix groupby.len with null values in cudf.polars (#17671)

closes #17667

Authors:
  - Matthew Roeschke (https://github.com/mroeschke)

Approvers:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

URL: #17671

python/cudf_polars/cudf_polars/dsl/expressions/aggregation.py

@@ -1,4 +1,4 @@
-# SPDX-FileCopyrightText: Copyright (c) 2024 NVIDIA CORPORATION & AFFILIATES.
+# SPDX-FileCopyrightText: Copyright (c) 2024-2025, NVIDIA CORPORATION & AFFILIATES.
 # SPDX-License-Identifier: Apache-2.0
 # TODO: remove need for this
 # ruff: noqa: D101
@@ -69,7 +69,11 @@ def __init__(
             # TODO: handle nans
             req = plc.aggregation.variance(ddof=options)
         elif name == "count":
-            req = plc.aggregation.count(null_handling=plc.types.NullPolicy.EXCLUDE)
+            req = plc.aggregation.count(
+                null_handling=plc.types.NullPolicy.EXCLUDE
+                if not options
+                else plc.types.NullPolicy.INCLUDE
+            )
         elif name == "quantile":
             _, quantile = self.children
             if not isinstance(quantile, Literal):

python/cudf_polars/tests/test_groupby.py

@@ -1,4 +1,4 @@
-# SPDX-FileCopyrightText: Copyright (c) 2024 NVIDIA CORPORATION & AFFILIATES.
+# SPDX-FileCopyrightText: Copyright (c) 2024-2025, NVIDIA CORPORATION & AFFILIATES.
 # SPDX-License-Identifier: Apache-2.0
 from __future__ import annotations
 
@@ -213,3 +213,9 @@ def test_groupby_maintain_order_random(nrows, nkeys, with_nulls):
         )
     q = df.lazy().group_by(key_names, maintain_order=True).agg(pl.col("value").sum())
     assert_gpu_result_equal(q)
+
+
+def test_groupby_len_with_nulls():
+    df = pl.DataFrame({"a": [1, 1, 1, 2], "b": [1, None, 2, 3]})
+    q = df.lazy().group_by("a").agg(pl.col("b").len())
+    assert_gpu_result_equal(q, check_row_order=False)