API / CoW: detect and raise error for chained assignment under Copy-on-Write

.github/workflows/ubuntu.yml

@@ -73,7 +73,6 @@ jobs:
             env_file: actions-pypy-38.yaml
             pattern: "not slow and not network and not single_cpu"
             test_args: "--max-worker-restart 0"
-            error_on_warnings: "0"
           - name: "Numpy Dev"
             env_file: actions-310-numpydev.yaml
             pattern: "not slow and not network and not single_cpu"

doc/source/reference/testing.rst

@@ -28,6 +28,7 @@ Exceptions and warnings
    errors.AccessorRegistrationWarning
    errors.AttributeConflictWarning
    errors.CategoricalConversionWarning
+   errors.ChainedAssignmentError
    errors.ClosedFileError
    errors.CSSWarning
    errors.DatabaseError

doc/source/whatsnew/v2.0.0.rst

@@ -125,6 +125,14 @@ Copy-on-Write improvements
   a modification to the data happens) when constructing a Series from an existing
   Series with the default of ``copy=False`` (:issue:`50471`)
 
+- Trying to set values using chained assignment (for example, ``df["a"][1:3] = 0``)
+  will now always raise an exception when Copy-on-Write is enabled. In this mode,
+  chained assignment can never work because we are always setting into a temporary
+  object that is the result of an indexing operation (getitem), which under
+  Copy-on-Write always behaves as a copy. Thus, assigning through a chain
+  can never update the original Series or DataFrame. Therefore, an informative
+  error is raised to the user instead of silently doing nothing (:issue:`49467`)
+
 Copy-on-Write can be enabled through
 
 .. code-block:: python

pandas/_testing/__init__.py

@@ -103,6 +103,7 @@
     decompress_file,
     ensure_clean,
     ensure_safe_environment_variables,
+    raises_chained_assignment_error,
     set_timezone,
     use_numexpr,
     with_csv_dialect,
@@ -1125,6 +1126,7 @@ def shares_memory(left, right) -> bool:
     "rands",
     "reset_display_options",
     "RNGContext",
+    "raises_chained_assignment_error",
     "round_trip_localpath",
     "round_trip_pathlib",
     "round_trip_pickle",

pandas/_testing/contexts.py

@@ -14,6 +14,9 @@
 
 import numpy as np
 
+from pandas.compat import PYPY
+from pandas.errors import ChainedAssignmentError
+
 from pandas import set_option
 
 from pandas.io.common import get_handle
@@ -227,3 +230,21 @@ def __exit__(
     ) -> None:
 
         np.random.set_state(self.start_state)
+
+
+def raises_chained_assignment_error():
+
+    if PYPY:
+        from contextlib import nullcontext
+
+        return nullcontext()
+    else:
+        import pytest
+
+        return pytest.raises(
+            ChainedAssignmentError,
+            match=(
+                "A value is trying to be set on a copy of a DataFrame or Series "
+                "through chained assignment"
+            ),
+        )

pandas/core/frame.py

@@ -16,6 +16,7 @@
 import functools
 from io import StringIO
 import itertools
+import sys
 from textwrap import dedent
 from typing import (
     TYPE_CHECKING,
@@ -91,12 +92,17 @@
     WriteBuffer,
     npt,
 )
+from pandas.compat import PYPY
 from pandas.compat._optional import import_optional_dependency
 from pandas.compat.numpy import (
     function as nv,
     np_percentile_argname,
 )
-from pandas.errors import InvalidIndexError
+from pandas.errors import (
+    ChainedAssignmentError,
+    InvalidIndexError,
+    _chained_assignment_msg,
+)
 from pandas.util._decorators import (
     Appender,
     Substitution,
@@ -3862,6 +3868,10 @@ def isetitem(self, loc, value) -> None:
         self._iset_item_mgr(loc, arraylike, inplace=False)
 
     def __setitem__(self, key, value):
+        if not PYPY and using_copy_on_write():
+            if sys.getrefcount(self) <= 3:
+                raise ChainedAssignmentError(_chained_assignment_msg)
+
         key = com.apply_if_callable(key, self)
 
         # see if we can slice the rows

pandas/core/indexing.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from contextlib import suppress
+import sys
 from typing import (
     TYPE_CHECKING,
     Hashable,
@@ -12,17 +13,22 @@
 
 import numpy as np
 
+from pandas._config import using_copy_on_write
+
 from pandas._libs.indexing import NDFrameIndexerBase
 from pandas._libs.lib import item_from_zerodim
 from pandas._typing import (
     Axis,
     AxisInt,
 )
+from pandas.compat import PYPY
 from pandas.errors import (
     AbstractMethodError,
+    ChainedAssignmentError,
     IndexingError,
     InvalidIndexError,
     LossySetitemError,
+    _chained_assignment_msg,
 )
 from pandas.util._decorators import doc
 
@@ -830,6 +836,10 @@ def _ensure_listlike_indexer(self, key, axis=None, value=None) -> None:
 
     @final
     def __setitem__(self, key, value) -> None:
+        if not PYPY and using_copy_on_write():
+            if sys.getrefcount(self.obj) <= 2:
+                raise ChainedAssignmentError(_chained_assignment_msg)
+
         check_dict_or_set_indexers(key)
         if isinstance(key, tuple):
             key = tuple(list(x) if is_iterator(x) else x for x in key)

pandas/core/series.py

@@ -3,6 +3,7 @@
 """
 from __future__ import annotations
 
+import sys
 from textwrap import dedent
 from typing import (
     IO,
@@ -67,8 +68,13 @@
     WriteBuffer,
     npt,
 )
+from pandas.compat import PYPY
 from pandas.compat.numpy import function as nv
-from pandas.errors import InvalidIndexError
+from pandas.errors import (
+    ChainedAssignmentError,
+    InvalidIndexError,
+    _chained_assignment_msg,
+)
 from pandas.util._decorators import (
     Appender,
     Substitution,
@@ -1074,6 +1080,10 @@ def _get_value(self, label, takeable: bool = False):
             return self.iloc[loc]
 
     def __setitem__(self, key, value) -> None:
+        if not PYPY and using_copy_on_write():
+            if sys.getrefcount(self) <= 3:
+                raise ChainedAssignmentError(_chained_assignment_msg)
+
         check_dict_or_set_indexers(key)
         key = com.apply_if_callable(key, self)
         cacher_needs_updating = self._check_is_chained_assignment_possible()

pandas/errors/__init__.py

@@ -320,6 +320,42 @@ class SettingWithCopyWarning(Warning):
     """
 
 
+class ChainedAssignmentError(ValueError):
+    """
+    Exception raised when trying to set using chained assignment.
+
+    When the ``mode.copy_on_write`` option is enabled, chained assignment can
+    never work. In such a situation, we are always setting into a temporary
+    object that is the result of an indexing operation (getitem), which under
+    Copy-on-Write always behaves as a copy. Thus, assigning through a chain
+    can never update the original Series or DataFrame.
+
+    For more information on view vs. copy,
+    see :ref:`the user guide<indexing.view_versus_copy>`.
+
+    Examples
+    --------
+    >>> pd.options.mode.copy_on_write = True
+    >>> df = pd.DataFrame({'A': [1, 1, 1, 2, 2]}, columns=['A'])
+    >>> df["A"][0:3] = 10 # doctest: +SKIP
+    ... # ChainedAssignmentError: ...
+    """
+
+
+_chained_assignment_msg = (
+    "A value is trying to be set on a copy of a DataFrame or Series "
+    "through chained assignment.\n"
+    "When using the Copy-on-Write mode, such chained assignment never works "
+    "to update the original DataFrame or Series, because the intermediate "
+    "object on which we are setting values always behaves as a copy.\n\n"
+    "Try using '.loc[row_indexer, col_indexer] = value' instead, to perform "
+    "the assignment in a single step.\n\n"
+    "See the caveats in the documentation: "
+    "https://pandas.pydata.org/pandas-docs/stable/user_guide/"
+    "indexing.html#returning-a-view-versus-a-copy"
+)
+
+
 class NumExprClobberingError(NameError):
     """
     Exception raised when trying to use a built-in numexpr name as a variable name.

pandas/tests/frame/indexing/test_setitem.py

@@ -1245,12 +1245,15 @@ def test_setitem_column_update_inplace(self, using_copy_on_write):
         df = DataFrame({col: np.zeros(len(labels)) for col in labels}, index=labels)
         values = df._mgr.blocks[0].values
 
-        for label in df.columns:
-            df[label][label] = 1
-
         if not using_copy_on_write:
+            for label in df.columns:
+                df[label][label] = 1
+
             # diagonal values all updated
             assert np.all(values[np.arange(10), np.arange(10)] == 1)
         else:
+            with tm.raises_chained_assignment_error():
+                for label in df.columns:
+                    df[label][label] = 1
             # original dataframe not updated
             assert np.all(values[np.arange(10), np.arange(10)] == 0)

pandas/tests/frame/indexing/test_xs.py

@@ -124,7 +124,8 @@ def test_xs_view(self, using_array_manager, using_copy_on_write):
         df_orig = dm.copy()
 
         if using_copy_on_write:
-            dm.xs(2)[:] = 20
+            with tm.raises_chained_assignment_error():
+                dm.xs(2)[:] = 20
             tm.assert_frame_equal(dm, df_orig)
         elif using_array_manager:
             # INFO(ArrayManager) with ArrayManager getting a row as a view is

pandas/tests/frame/test_block_internals.py

@@ -340,7 +340,11 @@ def test_stale_cached_series_bug_473(self, using_copy_on_write):
             )
             repr(Y)
             Y["e"] = Y["e"].astype("object")
-            Y["g"]["c"] = np.NaN
+            if using_copy_on_write:
+                with tm.raises_chained_assignment_error():
+                    Y["g"]["c"] = np.NaN
+            else:
+                Y["g"]["c"] = np.NaN
             repr(Y)
             result = Y.sum()  # noqa
             exp = Y["g"].sum()  # noqa

pandas/tests/indexing/multiindex/test_chaining_and_caching.py

@@ -50,11 +50,13 @@ def test_cache_updating(using_copy_on_write):
 
     # setting via chained assignment
     # but actually works, since everything is a view
-    df.loc[0]["z"].iloc[0] = 1.0
-    result = df.loc[(0, 0), "z"]
     if using_copy_on_write:
-        assert result == df_original.loc[0, "z"]
+        with tm.raises_chained_assignment_error():
+            df.loc[0]["z"].iloc[0] = 1.0
+        assert df.loc[(0, 0), "z"] == df_original.loc[0, "z"]
     else:
+        df.loc[0]["z"].iloc[0] = 1.0
+        result = df.loc[(0, 0), "z"]
         assert result == 1
 
     # correct setting

pandas/tests/indexing/multiindex/test_partial.py

@@ -128,20 +128,26 @@ def test_partial_set(
         exp.iloc[65:85] = 0
         tm.assert_frame_equal(df, exp)
 
-        df["A"].loc[2000, 4] = 1
-        if not using_copy_on_write:
-            exp["A"].loc[2000, 4].values[:] = 1
+        if using_copy_on_write:
+            with tm.raises_chained_assignment_error():
+                df["A"].loc[2000, 4] = 1
+            df.loc[(2000, 4), "A"] = 1
+        else:
+            df["A"].loc[2000, 4] = 1
+        exp.iloc[65:85, 0] = 1
         tm.assert_frame_equal(df, exp)
 
         df.loc[2000] = 5
         exp.iloc[:100] = 5
         tm.assert_frame_equal(df, exp)
 
         # this works...for now
-        df["A"].iloc[14] = 5
         if using_copy_on_write:
+            with tm.raises_chained_assignment_error():
+                df["A"].iloc[14] = 5
             df["A"].iloc[14] == exp["A"].iloc[14]
         else:
+            df["A"].iloc[14] = 5
             assert df["A"].iloc[14] == 5
 
     @pytest.mark.parametrize("dtype", [int, float])

pandas/tests/indexing/multiindex/test_setitem.py

@@ -501,8 +501,8 @@ def test_frame_setitem_copy_raises(
     # will raise/warn as its chained assignment
     df = multiindex_dataframe_random_data.T
     if using_copy_on_write:
-        # TODO(CoW) it would be nice if this could still warn/raise
-        df["foo"]["one"] = 2
+        with tm.raises_chained_assignment_error():
+            df["foo"]["one"] = 2
     else:
         msg = "A value is trying to be set on a copy of a slice from a DataFrame"
         with pytest.raises(SettingWithCopyError, match=msg):
@@ -516,7 +516,8 @@ def test_frame_setitem_copy_no_write(
     expected = frame
     df = frame.copy()
     if using_copy_on_write:
-        df["foo"]["one"] = 2
+        with tm.raises_chained_assignment_error():
+            df["foo"]["one"] = 2
     else:
         msg = "A value is trying to be set on a copy of a slice from a DataFrame"
         with pytest.raises(SettingWithCopyError, match=msg):