CoW: Add ChainedAssignmentError for update

doc/source/whatsnew/v2.1.0.rst

@@ -33,6 +33,7 @@ Copy-on-Write improvements
   operating inplace like this will never work, since the selection behaves
   as a temporary copy. This holds true for:
 
+  - DataFrame.update / Series.update
   - DataFrame.fillna / Series.fillna
 
 .. _whatsnew_210.enhancements.enhancement2:

pandas/compat/_constants.py

@@ -17,7 +17,7 @@
 PY311 = sys.version_info >= (3, 11)
 PYPY = platform.python_implementation() == "PyPy"
 ISMUSL = "musl" in (sysconfig.get_config_var("HOST_GNU_TYPE") or "")
-
+REF_COUNT = 2 if PY311 else 3
 
 __all__ = [
     "IS64",

pandas/core/frame.py

@@ -49,6 +49,7 @@
 from pandas._libs.hashtable import duplicated
 from pandas._libs.lib import is_range_indexer
 from pandas.compat import PYPY
+from pandas.compat._constants import REF_COUNT
 from pandas.compat._optional import import_optional_dependency
 from pandas.compat.numpy import (
     function as nv,
@@ -57,6 +58,7 @@
 from pandas.errors import (
     ChainedAssignmentError,
     InvalidIndexError,
+    _chained_assignment_method_msg,
     _chained_assignment_msg,
 )
 from pandas.util._decorators import (
@@ -8501,6 +8503,14 @@ def update(
         1  2  500
         2  3    6
         """
+        if not PYPY and using_copy_on_write():
+            if sys.getrefcount(self) <= REF_COUNT:
+                warnings.warn(
+                    _chained_assignment_method_msg,
+                    ChainedAssignmentError,
+                    stacklevel=2,
+                )
+
         from pandas.core.computation import expressions
 
         # TODO: Support other joins

pandas/core/generic.py

@@ -90,10 +90,8 @@
     WriteExcelBuffer,
     npt,
 )
-from pandas.compat import (
-    PY311,
-    PYPY,
-)
+from pandas.compat import PYPY
+from pandas.compat._constants import REF_COUNT
 from pandas.compat._optional import import_optional_dependency
 from pandas.compat.numpy import function as nv
 from pandas.errors import (
@@ -7092,8 +7090,7 @@ def fillna(
         inplace = validate_bool_kwarg(inplace, "inplace")
         if inplace:
             if not PYPY and using_copy_on_write():
-                refcount = 2 if PY311 else 3
-                if sys.getrefcount(self) <= refcount:
+                if sys.getrefcount(self) <= REF_COUNT:
                     warnings.warn(
                         _chained_assignment_method_msg,
                         ChainedAssignmentError,

pandas/core/series.py

@@ -37,10 +37,12 @@
 )
 from pandas._libs.lib import is_range_indexer
 from pandas.compat import PYPY
+from pandas.compat._constants import REF_COUNT
 from pandas.compat.numpy import function as nv
 from pandas.errors import (
     ChainedAssignmentError,
     InvalidIndexError,
+    _chained_assignment_method_msg,
     _chained_assignment_msg,
 )
 from pandas.util._decorators import (
@@ -3435,6 +3437,13 @@ def update(self, other: Series | Sequence | Mapping) -> None:
         2    3
         dtype: int64
         """
+        if not PYPY and using_copy_on_write():
+            if sys.getrefcount(self) <= REF_COUNT:
+                warnings.warn(
+                    _chained_assignment_method_msg,
+                    ChainedAssignmentError,
+                    stacklevel=2,
+                )
 
         if not isinstance(other, Series):
             other = Series(other)

pandas/tests/copy_view/test_methods.py

@@ -1731,6 +1731,20 @@ def test_update_series(using_copy_on_write):
         tm.assert_series_equal(view, expected)
 
 
+def test_update_chained_assignment(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3]})
+    ser2 = Series([100.0], index=[1])
+    df_orig = df.copy()
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error():
+            df["a"].update(ser2)
+        tm.assert_frame_equal(df, df_orig)
+
+        with tm.raises_chained_assignment_error():
+            df[["a"]].update(ser2.to_frame())
+        tm.assert_frame_equal(df, df_orig)
+
+
 def test_inplace_arithmetic_series():
     ser = Series([1, 2, 3])
     data = get_array(ser)

pandas/tests/series/indexing/test_indexing.py

@@ -284,11 +284,13 @@ def test_underlying_data_conversion(using_copy_on_write):
     df["val"] = 0
     df_original = df.copy()
     df
-    df["val"].update(s)
 
     if using_copy_on_write:
+        with tm.raises_chained_assignment_error():
+            df["val"].update(s)
         expected = df_original
     else:
+        df["val"].update(s)
         expected = DataFrame(
             {"a": [1, 2, 3], "b": [1, 2, 3], "c": [1, 2, 3], "val": [0, 1, 0]}
         )

pandas/tests/series/methods/test_update.py

@@ -29,10 +29,12 @@ def test_update(self, using_copy_on_write):
         df["c"] = df["c"].astype(object)
         df_orig = df.copy()
 
-        df["c"].update(Series(["foo"], index=[0]))
         if using_copy_on_write:
+            with tm.raises_chained_assignment_error():
+                df["c"].update(Series(["foo"], index=[0]))
             expected = df_orig
         else:
+            df["c"].update(Series(["foo"], index=[0]))
             expected = DataFrame(
                 [[1, np.nan, "foo"], [3, 2.0, np.nan]], columns=["a", "b", "c"]
             )