TST: Test CoW with DataFrame.items() (#50595)

Author: lithomas1

pandas/tests/copy_view/test_methods.py

@@ -1099,6 +1099,28 @@ def test_squeeze(using_copy_on_write):
         assert df.loc[0, "a"] == 0
 
 
+def test_items(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6], "c": [7, 8, 9]})
+    df_orig = df.copy()
+
+    # Test this twice, since the second time, the item cache will be
+    # triggered, and we want to make sure it still works then.
+    for i in range(2):
+        for name, ser in df.items():
+
+            assert np.shares_memory(get_array(ser, name), get_array(df, name))
+
+            # mutating df triggers a copy-on-write for that column / block
+            ser.iloc[0] = 0
+
+            if using_copy_on_write:
+                assert not np.shares_memory(get_array(ser, name), get_array(df, name))
+                tm.assert_frame_equal(df, df_orig)
+            else:
+                # Original frame will be modified
+                assert df.loc[0, name] == 0
+
+
 @pytest.mark.parametrize(
     "replace_kwargs",
     [