CoW: Add warning for slicing a Series with a MultiIndex (#56214)

Co-authored-by: Joris Van den Bossche <[email protected]>

Author: phofl

pandas/core/series.py

@@ -1175,7 +1175,7 @@ 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)
         new_ser = self._constructor(self._values[indexer], index=new_index, copy=False)
-        if using_copy_on_write() and isinstance(indexer, slice):
+        if isinstance(indexer, slice):
             new_ser._mgr.add_references(self._mgr)  # type: ignore[arg-type]
         return new_ser.__finalize__(self)
 
@@ -1217,7 +1217,7 @@ 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

@@ -1141,16 +1141,18 @@ 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):
+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]]))
+    ser_orig = ser.copy()
     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:
+        tm.assert_series_equal(ser, ser_orig)
+    else:
         expected = Series(
-            [1, 2, 3], index=pd.MultiIndex.from_arrays([[1, 1, 2], [3, 4, 5]])
+            [100, 2, 3], index=pd.MultiIndex.from_arrays([[1, 1, 2], [3, 4, 5]])
         )
         tm.assert_series_equal(ser, expected)
 
@@ -1181,16 +1183,15 @@ def test_getitem_midx_slice(
         assert df.iloc[0, 0] == 100
 
 
-def test_series_midx_tuples_slice(using_copy_on_write):
+def test_series_midx_tuples_slice(using_copy_on_write, warn_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))
-    # 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],