Backport PR #51950 on branch 2.0.x (BUG: Series.getitem not respecting CoW with MultiIndex)

doc/source/whatsnew/v2.0.0.rst

@@ -209,6 +209,9 @@ Copy-on-Write improvements
 - :meth:`DataFrame.__getitem__` will now respect the Copy-on-Write mechanism when the
   :class:`DataFrame` has :class:`MultiIndex` columns.
 
+- :meth:`Series.__getitem__` will now respect the Copy-on-Write mechanism when the
+   :class:`Series` has a :class:`MultiIndex`.
+
 - :meth:`Series.view` will now respect the Copy-on-Write mechanism.
 
 Copy-on-Write can be enabled through one of

pandas/core/series.py

@@ -1113,6 +1113,8 @@ def _get_value(self, label, takeable: bool = False):
             new_index = mi[loc]
             new_index = maybe_droplevels(new_index, label)
             new_ser = self._constructor(new_values, index=new_index, name=self.name)
+            if using_copy_on_write() and isinstance(loc, slice):
+                new_ser._mgr.add_references(self._mgr)  # type: ignore[arg-type]
             return new_ser.__finalize__(self)
 
         else:

pandas/tests/copy_view/test_indexing.py

@@ -1036,6 +1036,18 @@ def test_set_value_copy_only_necessary_column(
             assert np.shares_memory(get_array(df, "a"), get_array(view, "a"))
 
 
+def test_series_midx_slice(using_copy_on_write):
+    ser = Series([1, 2, 3], index=pd.MultiIndex.from_arrays([[1, 1, 2], [3, 4, 5]]))
+    result = ser[1]
+    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_arrays([[1, 1, 2], [3, 4, 5]])
+        )
+        tm.assert_series_equal(ser, expected)
+
+
 def test_getitem_midx_slice(using_copy_on_write, using_array_manager):
     df = DataFrame({("a", "x"): [1, 2], ("a", "y"): 1, ("b", "x"): 2})
     df_orig = df.copy()