CoW: Add warning for update (#56068)

Author: phofl

pandas/core/frame.py

@@ -62,6 +62,7 @@
     InvalidIndexError,
     _chained_assignment_method_msg,
     _chained_assignment_msg,
+    _chained_assignment_warning_method_msg,
 )
 from pandas.util._decorators import (
     Appender,
@@ -8849,6 +8850,13 @@ def update(
                     ChainedAssignmentError,
                     stacklevel=2,
                 )
+        elif not PYPY and not using_copy_on_write():
+            if sys.getrefcount(self) <= REF_COUNT:
+                warnings.warn(
+                    _chained_assignment_warning_method_msg,
+                    FutureWarning,
+                    stacklevel=2,
+                )
 
         from pandas.core.computation import expressions
 

pandas/core/series.py

@@ -44,6 +44,7 @@
     InvalidIndexError,
     _chained_assignment_method_msg,
     _chained_assignment_msg,
+    _chained_assignment_warning_method_msg,
 )
 from pandas.util._decorators import (
     Appender,
@@ -3560,6 +3561,18 @@ def update(self, other: Series | Sequence | Mapping) -> None:
                     ChainedAssignmentError,
                     stacklevel=2,
                 )
+        elif not PYPY and not using_copy_on_write():
+            ctr = sys.getrefcount(self)
+            ref_count = REF_COUNT
+            if hasattr(self, "_cacher"):
+                # see https://github.com/pandas-dev/pandas/pull/56060#discussion_r1399245221
+                ref_count += 1
+            if ctr <= ref_count:
+                warnings.warn(
+                    _chained_assignment_warning_method_msg,
+                    FutureWarning,
+                    stacklevel=2,
+                )
 
         if not isinstance(other, Series):
             other = Series(other)

pandas/tests/copy_view/test_methods.py

@@ -12,6 +12,7 @@
     Series,
     Timestamp,
     date_range,
+    option_context,
     period_range,
 )
 import pandas._testing as tm
@@ -1684,7 +1685,7 @@ def test_get(using_copy_on_write, warn_copy_on_write, key):
             warn = FutureWarning if isinstance(key, str) else None
         else:
             warn = SettingWithCopyWarning if isinstance(key, list) else None
-        with pd.option_context("chained_assignment", "warn"):
+        with option_context("chained_assignment", "warn"):
             with tm.assert_produces_warning(warn):
                 result.iloc[0] = 0
 
@@ -1721,7 +1722,7 @@ def test_xs(
         with tm.assert_cow_warning(single_block or axis == 1):
             result.iloc[0] = 0
     else:
-        with pd.option_context("chained_assignment", "warn"):
+        with option_context("chained_assignment", "warn"):
             with tm.assert_produces_warning(SettingWithCopyWarning):
                 result.iloc[0] = 0
 
@@ -1756,7 +1757,7 @@ def test_xs_multiindex(
         warn = SettingWithCopyWarning
     else:
         warn = None
-    with pd.option_context("chained_assignment", "warn"):
+    with option_context("chained_assignment", "warn"):
         with tm.assert_produces_warning(warn):
             result.iloc[0, 0] = 0
 
@@ -1813,6 +1814,17 @@ def test_update_chained_assignment(using_copy_on_write):
         with tm.raises_chained_assignment_error():
             df[["a"]].update(ser2.to_frame())
         tm.assert_frame_equal(df, df_orig)
+    else:
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            df["a"].update(ser2)
+
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            with option_context("mode.chained_assignment", None):
+                df[["a"]].update(ser2.to_frame())
+
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            with option_context("mode.chained_assignment", None):
+                df[df["a"] > 1].update(ser2.to_frame())
 
 
 def test_inplace_arithmetic_series(using_copy_on_write):

pandas/tests/series/indexing/test_indexing.py

@@ -295,7 +295,8 @@ def test_underlying_data_conversion(using_copy_on_write):
             df["val"].update(s)
         expected = df_original
     else:
-        df["val"].update(s)
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            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

@@ -34,7 +34,8 @@ def test_update(self, using_copy_on_write):
                 df["c"].update(Series(["foo"], index=[0]))
             expected = df_orig
         else:
-            df["c"].update(Series(["foo"], index=[0]))
+            with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+                df["c"].update(Series(["foo"], index=[0]))
             expected = DataFrame(
                 [[1, np.nan, "foo"], [3, 2.0, np.nan]], columns=["a", "b", "c"]
             )