Backport PR #52565 on branch 2.0.x (REGR: Fix regression in sort_values not resetting index)

doc/source/whatsnew/v2.0.1.rst

@@ -15,6 +15,7 @@ Fixed regressions
 ~~~~~~~~~~~~~~~~~
 - Fixed regression for subclassed Series when constructing from a dictionary (:issue:`52445`)
 - Fixed regression in :meth:`Series.describe` showing ``RuntimeWarning`` for extension dtype :class:`Series` with one element (:issue:`52515`)
+- Fixed regression in :meth:`DataFrame.sort_values` not resetting index when :class:`DataFrame` is already sorted and ``ignore_index=True`` (:issue:`52553`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_201.bug_fixes:

pandas/core/frame.py

@@ -6784,10 +6784,14 @@ def sort_values(
                 return self.copy(deep=None)
 
         if is_range_indexer(indexer, len(indexer)):
+            result = self.copy(deep=(not inplace and not using_copy_on_write()))
+            if ignore_index:
+                result.index = default_index(len(result))
+
             if inplace:
-                return self._update_inplace(self)
+                return self._update_inplace(result)
             else:
-                return self.copy(deep=None)
+                return result
 
         new_data = self._mgr.take(
             indexer, axis=self._get_block_manager_axis(axis), verify=False

pandas/tests/frame/methods/test_sort_values.py

@@ -630,6 +630,13 @@ def test_sort_values_no_by_inplace(self):
         tm.assert_frame_equal(df, expected)
         assert result is None
 
+    def test_sort_values_no_op_reset_index(self):
+        # GH#52553
+        df = DataFrame({"A": [10, 20], "B": [1, 5]}, index=[2, 3])
+        result = df.sort_values(by="A", ignore_index=True)
+        expected = DataFrame({"A": [10, 20], "B": [1, 5]})
+        tm.assert_frame_equal(result, expected)
+
 
 class TestDataFrameSortKey:  # test key sorting (issue 27237)
     def test_sort_values_inplace_key(self, sort_by_key):