Backport PR #52040 on branch 2.0.x (CoW: Optimize Series.reset_index to make lazy copy) (#52050)

Backport PR #52040: CoW: Optimize Series.reset_index to make lazy copy

Co-authored-by: Patrick Hoefler <[email protected]>

Author: meeseeksmachine

pandas/core/series.py

@@ -1584,6 +1584,10 @@ def reset_index(
 
             if inplace:
                 self.index = new_index
+            elif using_copy_on_write():
+                new_ser = self.copy(deep=False)
+                new_ser.index = new_index
+                return new_ser.__finalize__(self, method="reset_index")
             else:
                 return self._constructor(
                     self._values.copy(), index=new_index, copy=False

pandas/tests/copy_view/test_methods.py

@@ -233,6 +233,21 @@ def test_reset_index(using_copy_on_write):
     tm.assert_frame_equal(df, df_orig)
 
 
+@pytest.mark.parametrize("index", [pd.RangeIndex(0, 2), Index([1, 2])])
+def test_reset_index_series_drop(using_copy_on_write, index):
+    ser = Series([1, 2], index=index)
+    ser_orig = ser.copy()
+    ser2 = ser.reset_index(drop=True)
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(ser), get_array(ser2))
+        assert not ser._mgr._has_no_reference(0)
+    else:
+        assert not np.shares_memory(get_array(ser), get_array(ser2))
+
+    ser2.iloc[0] = 100
+    tm.assert_series_equal(ser, ser_orig)
+
+
 def test_rename_columns(using_copy_on_write):
     # Case: renaming columns returns a new dataframe
     # + afterwards modifying the result