Allow numeric_only=True for simple groupby reductions (#15326)

Adds some simple logic to handle the case that `DataFrameGroupBy._reduce(numeric_only=True)` is called.

## Further Background
This change is needed for some dask_cudf groupby aggregations (e.g. "mean") to work with the latest `dask/dask-expr:main`. Although other workarounds and "fixes" are possible, the easiest solution is probably something like this PR.

Authors:
  - Richard (Rick) Zamora (https://github.com/rjzamora)

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

URL: #15326

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

@@ -22,7 +22,12 @@
 from cudf._lib.types import size_type_dtype
 from cudf._typing import AggType, DataFrameOrSeries, MultiColumnAggType
 from cudf.api.extensions import no_default
-from cudf.api.types import is_bool_dtype, is_float_dtype, is_list_like
+from cudf.api.types import (
+    is_bool_dtype,
+    is_float_dtype,
+    is_list_like,
+    is_numeric_dtype,
+)
 from cudf.core._compat import PANDAS_LT_300
 from cudf.core.abc import Serializable
 from cudf.core.column.column import ColumnBase, StructDtype, as_column
@@ -701,6 +706,11 @@ def agg(self, func):
 
         return result
 
+    def _reduce_numeric_only(self, op: str):
+        raise NotImplementedError(
+            f"numeric_only is not implemented for {type(self)}"
+        )
+
     def _reduce(
         self,
         op: str,
@@ -731,14 +741,12 @@ def _reduce(
 
             The numeric_only, min_count
         """
-        if numeric_only:
-            raise NotImplementedError(
-                "numeric_only parameter is not implemented yet"
-            )
         if min_count != 0:
             raise NotImplementedError(
                 "min_count parameter is not implemented yet"
             )
+        if numeric_only:
+            return self._reduce_numeric_only(op)
         return self.agg(op)
 
     def _scan(self, op: str, *args, **kwargs):
@@ -2648,6 +2656,17 @@ class DataFrameGroupBy(GroupBy, GetAttrGetItemMixin):
 
     _PROTECTED_KEYS = frozenset(("obj",))
 
+    def _reduce_numeric_only(self, op: str):
+        columns = list(
+            name
+            for name in self.obj._data.names
+            if (
+                is_numeric_dtype(self.obj._data[name].dtype)
+                and name not in self.grouping.names
+            )
+        )
+        return self[columns].agg(op)
+
     def __getitem__(self, key):
         return self.obj[key].groupby(
             by=self.grouping.keys,

python/cudf/cudf/tests/test_groupby.py

@@ -1259,7 +1259,7 @@ def test_groupby_unsupported_columns():
     pdg = pdf.groupby("x").sum(numeric_only=True)
     # cudf does not yet support numeric_only, so our default is False (unlike
     # pandas, which defaults to inferring and throws a warning about it).
-    gdg = gdf.groupby("x").sum()
+    gdg = gdf.groupby("x").sum(numeric_only=True)
     assert_groupby_results_equal(pdg, gdg)
 
 
@@ -2158,7 +2158,9 @@ def test_groupby_list_columns_excluded():
     pandas_agg_result = pdf.groupby("a").agg("mean", numeric_only=True)
 
     assert_groupby_results_equal(
-        pandas_result, gdf.groupby("a").mean(), check_dtype=False
+        pandas_result,
+        gdf.groupby("a").mean(numeric_only=True),
+        check_dtype=False,
     )
 
     assert_groupby_results_equal(
@@ -3826,3 +3828,27 @@ def test_groupby_shift_series_multiindex():
     result = ser.groupby(level=0).shift(1)
     expected = ser.to_pandas().groupby(level=0).shift(1)
     assert_eq(expected, result)
+
+
+@pytest.mark.parametrize(
+    "func", ["min", "max", "sum", "mean", "idxmin", "idxmax"]
+)
+@pytest.mark.parametrize(
+    "by,data",
+    [
+        ("a", {"a": [1, 2, 3]}),
+        (["a", "id"], {"id": [0, 0, 1], "a": [1, 2, 3]}),
+        ("a", {"a": [1, 2, 3], "b": ["A", "B", "C"]}),
+        ("id", {"id": [0, 0, 1], "a": [1, 2, 3], "b": ["A", "B", "C"]}),
+        (["b", "id"], {"id": [0, 0, 1], "b": ["A", "B", "C"]}),
+        ("b", {"b": ["A", "B", "C"]}),
+    ],
+)
+def test_group_by_reduce_numeric_only(by, data, func):
+    # Test that simple groupby reductions support numeric_only=True
+    df = cudf.DataFrame(data)
+    expected = getattr(df.to_pandas().groupby(by, sort=True), func)(
+        numeric_only=True
+    )
+    result = getattr(df.groupby(by, sort=True), func)(numeric_only=True)
+    assert_eq(expected, result)