BUG / CoW: ensure ser[...] (indexing with Ellipsis) returns new object (#56314)

Author: jorisvandenbossche

pandas/core/series.py

@@ -1065,6 +1065,8 @@ def __getitem__(self, key):
         key = com.apply_if_callable(key, self)
 
         if key is Ellipsis:
+            if using_copy_on_write() or warn_copy_on_write():
+                return self.copy(deep=False)
             return self
 
         key_is_scalar = is_scalar(key)

pandas/tests/copy_view/test_indexing.py

@@ -846,6 +846,31 @@ def test_series_getitem_slice(backend, using_copy_on_write, warn_copy_on_write):
         assert s.iloc[0] == 0
 
 
+def test_series_getitem_ellipsis(using_copy_on_write, warn_copy_on_write):
+    # Case: taking a view of a Series using Ellipsis + afterwards modifying the subset
+    s = Series([1, 2, 3])
+    s_orig = s.copy()
+
+    subset = s[...]
+    assert np.shares_memory(get_array(subset), get_array(s))
+
+    with tm.assert_cow_warning(warn_copy_on_write):
+        subset.iloc[0] = 0
+
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(subset), get_array(s))
+
+    expected = Series([0, 2, 3])
+    tm.assert_series_equal(subset, expected)
+
+    if using_copy_on_write:
+        # original parent series is not modified (CoW)
+        tm.assert_series_equal(s, s_orig)
+    else:
+        # original parent series is actually updated
+        assert s.iloc[0] == 0
+
+
 @pytest.mark.parametrize(
     "indexer",
     [slice(0, 2), np.array([True, True, False]), np.array([0, 1])],

pandas/tests/series/indexing/test_indexing.py

@@ -101,14 +101,16 @@ def test_basic_getitem_dt64tz_values():
     assert result == expected
 
 
-def test_getitem_setitem_ellipsis():
+def test_getitem_setitem_ellipsis(using_copy_on_write, warn_copy_on_write):
     s = Series(np.random.default_rng(2).standard_normal(10))
 
     result = s[...]
     tm.assert_series_equal(result, s)
 
-    s[...] = 5
-    assert (result == 5).all()
+    with tm.assert_cow_warning(warn_copy_on_write):
+        s[...] = 5
+    if not using_copy_on_write:
+        assert (result == 5).all()
 
 
 @pytest.mark.parametrize(