CoW: Ensure that iterrows does not allow mutating parent

doc/source/whatsnew/v2.0.0.rst

@@ -222,6 +222,7 @@ Copy-on-Write improvements
   - :meth:`DataFrame.to_timestamp` / :meth:`Series.to_timestamp`
   - :meth:`DataFrame.to_period` / :meth:`Series.to_period`
   - :meth:`DataFrame.truncate`
+  - :meth:`DataFrame.iterrows`
   - :meth:`DataFrame.tz_convert` / :meth:`Series.tz_localize`
   - :meth:`DataFrame.fillna` / :meth:`Series.fillna`
   - :meth:`DataFrame.interpolate` / :meth:`Series.interpolate`

pandas/core/frame.py

@@ -1392,8 +1392,14 @@ def iterrows(self) -> Iterable[tuple[Hashable, Series]]:
         """
         columns = self.columns
         klass = self._constructor_sliced
+        using_cow = using_copy_on_write()
         for k, v in zip(self.index, self.values):
             s = klass(v, index=columns, name=k).__finalize__(self)
+            if using_cow and self._mgr.is_single_block:
+                s._mgr.blocks[0].refs = self._mgr.blocks[0].refs  # type: ignore[union-attr]  # noqa
+                s._mgr.blocks[0].refs.add_reference(  # type: ignore[union-attr]
+                    s._mgr.blocks[0]  # type: ignore[arg-type, union-attr]
+                )
             yield k, s
 
     def itertuples(

pandas/tests/copy_view/test_methods.py

@@ -1329,6 +1329,16 @@ def test_asfreq_noop(using_copy_on_write):
     tm.assert_frame_equal(df, df_orig)
 
 
+def test_iterrows(using_copy_on_write):
+    df = DataFrame({"a": 0, "b": 1}, index=[1, 2, 3])
+    df_orig = df.copy()
+
+    for _, sub in df.iterrows():
+        sub.iloc[0] = 100
+    if using_copy_on_write:
+        tm.assert_frame_equal(df, df_orig)
+
+
 def test_interpolate_creates_copy(using_copy_on_write):
     # GH#51126
     df = DataFrame({"a": [1.5, np.nan, 3]})