PERF: Categorical(range).categories returns RangeIndex instead of Index (pandas-dev#57787)

Author: mroeschke

doc/source/whatsnew/v3.0.0.rst

@@ -256,6 +256,7 @@ Removal of prior version deprecations/changes
 
 Performance improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
+- :attr:`Categorical.categories` returns a :class:`RangeIndex` columns instead of an :class:`Index` if the constructed ``values`` was a ``range``. (:issue:`57787`)
 - :func:`concat` returns a :class:`RangeIndex` level in the :class:`MultiIndex` result when ``keys`` is a ``range`` or :class:`RangeIndex` (:issue:`57542`)
 - :meth:`RangeIndex.append` returns a :class:`RangeIndex` instead of a :class:`Index` when appending values that could continue the :class:`RangeIndex` (:issue:`57467`)
 - :meth:`Series.str.extract` returns a :class:`RangeIndex` columns instead of an :class:`Index` column when possible (:issue:`57542`)

pandas/core/arrays/categorical.py

@@ -431,6 +431,10 @@ def __init__(
         if isinstance(vdtype, CategoricalDtype):
             if dtype.categories is None:
                 dtype = CategoricalDtype(values.categories, dtype.ordered)
+        elif isinstance(values, range):
+            from pandas.core.indexes.range import RangeIndex
+
+            values = RangeIndex(values)
         elif not isinstance(values, (ABCIndex, ABCSeries, ExtensionArray)):
             values = com.convert_to_list_like(values)
             if isinstance(values, list) and len(values) == 0:

pandas/tests/arrays/categorical/test_constructors.py

@@ -24,6 +24,7 @@
     IntervalIndex,
     MultiIndex,
     NaT,
+    RangeIndex,
     Series,
     Timestamp,
     date_range,
@@ -779,3 +780,17 @@ def test_constructor_preserves_freq(self):
         result = cat.categories.freq
 
         assert expected == result
+
+    @pytest.mark.parametrize(
+        "values, categories",
+        [
+            [range(5), None],
+            [range(4), range(5)],
+            [[0, 1, 2, 3], range(5)],
+            [[], range(5)],
+        ],
+    )
+    def test_range_values_preserves_rangeindex_categories(self, values, categories):
+        result = Categorical(values=values, categories=categories).categories
+        expected = RangeIndex(range(5))
+        tm.assert_index_equal(result, expected, exact=True)