From 53011302f8d8156462ab4a758d92cc74bebd48ca Mon Sep 17 00:00:00 2001
From: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>
Date: Fri, 8 Mar 2024 15:46:59 -0800
Subject: [PATCH 1/2] PERF: Categorical(range).categories returns RangeIndex
 instead of Index

---
 doc/source/whatsnew/v3.0.0.rst                    |  1 +
 pandas/core/arrays/categorical.py                 |  4 ++++
 .../tests/arrays/categorical/test_constructors.py | 15 +++++++++++++++
 3 files changed, 20 insertions(+)

diff --git a/doc/source/whatsnew/v3.0.0.rst b/doc/source/whatsnew/v3.0.0.rst
index ca2ca07ff2fae..8291b6417fb07 100644
--- a/doc/source/whatsnew/v3.0.0.rst
+++ b/doc/source/whatsnew/v3.0.0.rst
@@ -259,6 +259,7 @@ Performance improvements
 - :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`)
+- :attr:`Categorical.categories` returns a :class:`RangeIndex` columns instead of an :class:`Index` if the constructed ``values`` was a ``range``. (:issue:`?`)
 - Performance improvement in :class:`DataFrame` when ``data`` is a ``dict`` and ``columns`` is specified (:issue:`24368`)
 - Performance improvement in :meth:`DataFrame.join` for sorted but non-unique indexes (:issue:`56941`)
 - Performance improvement in :meth:`DataFrame.join` when left and/or right are non-unique and ``how`` is ``"left"``, ``"right"``, or ``"inner"`` (:issue:`56817`)
diff --git a/pandas/core/arrays/categorical.py b/pandas/core/arrays/categorical.py
index af8dc08c1ec26..7710d2930ddae 100644
--- a/pandas/core/arrays/categorical.py
+++ b/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:
diff --git a/pandas/tests/arrays/categorical/test_constructors.py b/pandas/tests/arrays/categorical/test_constructors.py
index 03678fb64d3e9..857b14e2a2558 100644
--- a/pandas/tests/arrays/categorical/test_constructors.py
+++ b/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)

From 3aa8cf370a9d0475f8022cd80b7f89a66dc96f35 Mon Sep 17 00:00:00 2001
From: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>
Date: Fri, 8 Mar 2024 15:48:38 -0800
Subject: [PATCH 2/2] add whatsnew number

---
 doc/source/whatsnew/v3.0.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v3.0.0.rst b/doc/source/whatsnew/v3.0.0.rst
index 8291b6417fb07..94501c4fddfd9 100644
--- a/doc/source/whatsnew/v3.0.0.rst
+++ b/doc/source/whatsnew/v3.0.0.rst
@@ -256,10 +256,10 @@ 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`)
-- :attr:`Categorical.categories` returns a :class:`RangeIndex` columns instead of an :class:`Index` if the constructed ``values`` was a ``range``. (:issue:`?`)
 - Performance improvement in :class:`DataFrame` when ``data`` is a ``dict`` and ``columns`` is specified (:issue:`24368`)
 - Performance improvement in :meth:`DataFrame.join` for sorted but non-unique indexes (:issue:`56941`)
 - Performance improvement in :meth:`DataFrame.join` when left and/or right are non-unique and ``how`` is ``"left"``, ``"right"``, or ``"inner"`` (:issue:`56817`)