Backport PR #52001 on branch 2.0.x (BUG - CoW: Series with MultiIndex with tuples does not respect CoW) (#52009)

Backport PR #52001: BUG - CoW: Series with MultiIndex with tuples does not respect CoW

Co-authored-by: Patrick Hoefler <61934744+phofl@users.noreply.github.com>

Author: meeseeksmachine

pandas/core/series.py

@@ -1073,9 +1073,10 @@ def _get_values_tuple(self, key: tuple):
 
         # If key is contained, would have returned by now
         indexer, new_index = self.index.get_loc_level(key)
-        return self._constructor(self._values[indexer], index=new_index).__finalize__(
-            self
-        )
+        new_ser = self._constructor(self._values[indexer], index=new_index)
+        if using_copy_on_write() and isinstance(indexer, slice):
+            new_ser._mgr.add_references(self._mgr)  # type: ignore[arg-type]
+        return new_ser.__finalize__(self)
 
     def _get_values(self, indexer: slice | npt.NDArray[np.bool_]) -> Series:
         new_mgr = self._mgr.getitem_mgr(indexer)

pandas/tests/copy_view/test_indexing.py

@@ -1061,3 +1061,19 @@ def test_getitem_midx_slice(using_copy_on_write, using_array_manager):
     if using_copy_on_write:
         new_df.iloc[0, 0] = 100
         tm.assert_frame_equal(df_orig, df)
+
+
+def test_series_midx_tuples_slice(using_copy_on_write):
+    ser = Series(
+        [1, 2, 3],
+        index=pd.MultiIndex.from_tuples([((1, 2), 3), ((1, 2), 4), ((2, 3), 4)]),
+    )
+    result = ser[(1, 2)]
+    assert np.shares_memory(get_array(ser), get_array(result))
+    result.iloc[0] = 100
+    if using_copy_on_write:
+        expected = Series(
+            [1, 2, 3],
+            index=pd.MultiIndex.from_tuples([((1, 2), 3), ((1, 2), 4), ((2, 3), 4)]),
+        )
+        tm.assert_series_equal(ser, expected)