BUG: Index.sort_values with key (pandas-dev#54179)

Author: jbrockmendel

doc/source/whatsnew/v2.1.0.rst

@@ -633,11 +633,13 @@ Other
 - Bug in :meth:`DataFrame.reindex` with a ``fill_value`` that should be inferred with a :class:`ExtensionDtype` incorrectly inferring ``object`` dtype (:issue:`52586`)
 - Bug in :meth:`DataFrame.shift` and :meth:`Series.shift` and :meth:`DataFrameGroupBy.shift` when passing both "freq" and "fill_value" silently ignoring "fill_value" instead of raising ``ValueError`` (:issue:`53832`)
 - Bug in :meth:`DataFrame.shift` with ``axis=1`` on a :class:`DataFrame` with a single :class:`ExtensionDtype` column giving incorrect results (:issue:`53832`)
+- Bug in :meth:`Index.sort_values` when a ``key`` is passed (:issue:`52764`)
 - Bug in :meth:`Series.align`, :meth:`DataFrame.align`, :meth:`Series.reindex`, :meth:`DataFrame.reindex`, :meth:`Series.interpolate`, :meth:`DataFrame.interpolate`, incorrectly failing to raise with method="asfreq" (:issue:`53620`)
 - Bug in :meth:`Series.map` when giving a callable to an empty series, the returned series had ``object`` dtype. It now keeps the original dtype (:issue:`52384`)
 - Bug in :meth:`Series.memory_usage` when ``deep=True`` throw an error with Series of objects and the returned value is incorrect, as it does not take into account GC corrections (:issue:`51858`)
 - Bug in :meth:`period_range` the default behavior when freq was not passed as an argument was incorrect(:issue:`53687`)
 - Fixed incorrect ``__name__`` attribute of ``pandas._libs.json`` (:issue:`52898`)
+-
 
 .. ***DO NOT USE THIS SECTION***
 

pandas/core/indexes/base.py

@@ -5820,15 +5820,14 @@ def sort_values(
         >>> idx.sort_values(ascending=False, return_indexer=True)
         (Index([1000, 100, 10, 1], dtype='int64'), array([3, 1, 0, 2]))
         """
-        idx = cast(Index, ensure_key_mapped(self, key))
-
         # GH 35584. Sort missing values according to na_position kwarg
         # ignore na_position for MultiIndex
         if not isinstance(self, ABCMultiIndex):
             _as = nargsort(
-                items=idx, ascending=ascending, na_position=na_position, key=key
+                items=self, ascending=ascending, na_position=na_position, key=key
             )
         else:
+            idx = cast(Index, ensure_key_mapped(self, key))
             _as = idx.argsort(na_position=na_position)
             if not ascending:
                 _as = _as[::-1]

pandas/tests/indexes/ranges/test_range.py

@@ -590,13 +590,18 @@ def test_isin_range(self, base):
         tm.assert_numpy_array_equal(result, expected)
 
     def test_sort_values_key(self):
-        # GH#43666
+        # GH#43666, GH#52764
         sort_order = {8: 2, 6: 0, 4: 8, 2: 10, 0: 12}
         values = RangeIndex(0, 10, 2)
         result = values.sort_values(key=lambda x: x.map(sort_order))
-        expected = Index([4, 8, 6, 0, 2], dtype="int64")
+        expected = Index([6, 8, 4, 2, 0], dtype="int64")
         tm.assert_index_equal(result, expected, check_exact=True)
 
+        # check this matches the Series.sort_values behavior
+        ser = values.to_series()
+        result2 = ser.sort_values(key=lambda x: x.map(sort_order))
+        tm.assert_series_equal(result2, expected.to_series(), check_exact=True)
+
     def test_range_index_rsub_by_const(self):
         # GH#53255
         result = 3 - RangeIndex(0, 4, 1)