CoW: Add warning for slicing a Series with a MultiIndex

Author: phofl

pandas/core/series.py

@@ -1213,7 +1213,7 @@ def _get_value(self, label, takeable: bool = False):
             new_ser = self._constructor(
                 new_values, index=new_index, name=self.name, copy=False
             )
-            if using_copy_on_write() and isinstance(loc, slice):
+            if isinstance(loc, slice):
                 new_ser._mgr.add_references(self._mgr)  # type: ignore[arg-type]
             return new_ser.__finalize__(self)
 

pandas/tests/copy_view/test_indexing.py

@@ -1150,13 +1150,12 @@ 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):
+def test_series_midx_slice(using_copy_on_write, warn_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))
-    # TODO(CoW-warn) should warn -> reference is only tracked in CoW mode, so
-    # warning is not triggered
-    result.iloc[0] = 100
+    with tm.assert_cow_warning(warn_copy_on_write):
+        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]])