Fix maxima of categorical column (#15701)

Closes #15641

Applies patch suggested by @wence-

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

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

URL: #15701

python/cudf/cudf/core/column/categorical.py

@@ -515,6 +515,10 @@ class CategoricalColumn(column.ColumnBase):
     dtype: cudf.core.dtypes.CategoricalDtype
     _codes: Optional[NumericalColumn]
     _children: Tuple[NumericalColumn]
+    _VALID_REDUCTIONS = {
+        "max",
+        "min",
+    }
     _VALID_BINARY_OPERATIONS = {
         "__eq__",
         "__ne__",
@@ -699,6 +703,25 @@ def slice(
             ),
         )
 
+    def _reduce(
+        self,
+        op: str,
+        skipna: Optional[bool] = None,
+        min_count: int = 0,
+        *args,
+        **kwargs,
+    ) -> ScalarLike:
+        # Only valid reductions are min and max
+        if not self.ordered:
+            raise TypeError(
+                f"Categorical is not ordered for operation {op} "
+                "you can use .as_ordered() to change the Categorical "
+                "to an ordered one."
+            )
+        return self._decode(
+            self.codes._reduce(op, skipna, min_count, *args, **kwargs)
+        )
+
     def _binaryop(self, other: ColumnBinaryOperand, op: str) -> ColumnBase:
         other = self._wrap_binop_normalization(other)
         # TODO: This is currently just here to make mypy happy, but eventually

python/cudf/cudf/tests/test_categorical.py

@@ -875,3 +875,22 @@ def test_cat_groupby_fillna():
         lfunc_args_and_kwargs=(("d",), {}),
         rfunc_args_and_kwargs=(("d",), {}),
     )
+
+
+@pytest.mark.parametrize("op", ["min", "max"])
+def test_categorical_maxima(op):
+    ser = cudf.Series(
+        ["a", "d", "c", "z", "g"],
+        dtype=cudf.CategoricalDtype(["z", "c", "g", "d", "a"], ordered=False),
+    )
+    assert not ser.cat.ordered
+
+    # Cannot get extrema of unordered Categorical column
+    with pytest.raises(TypeError, match="Categorical is not ordered"):
+        getattr(ser, op)()
+
+    # Max/min should work after converting to "ordered"
+    ser_pd = ser.to_pandas()
+    result = getattr(ser.cat.as_ordered(), op)()
+    result_pd = getattr(ser_pd.cat.as_ordered(), op)()
+    assert_eq(result, result_pd)