API / CoW: Add ChainedAssignmentError for inplace ops (#54313)

Author: phofl

doc/source/whatsnew/v2.1.0.rst

@@ -121,6 +121,12 @@ Copy-on-Write improvements
   - DataFrame.update / Series.update
   - DataFrame.fillna / Series.fillna
   - DataFrame.replace / Series.replace
+  - DataFrame.clip / Series.clip
+  - DataFrame.where / Series.where
+  - DataFrame.mask / Series.mask
+  - DataFrame.interpolate / Series.interpolate
+  - DataFrame.ffill / Series.ffill
+  - DataFrame.bfill / Series.bfill
 
 .. _whatsnew_210.enhancements.map_na_action:
 

pandas/core/generic.py

@@ -7385,6 +7385,15 @@ def ffill(
         dtype: float64
         """
         downcast = self._deprecate_downcast(downcast, "ffill")
+        inplace = validate_bool_kwarg(inplace, "inplace")
+        if inplace:
+            if not PYPY and using_copy_on_write():
+                if sys.getrefcount(self) <= REF_COUNT:
+                    warnings.warn(
+                        _chained_assignment_method_msg,
+                        ChainedAssignmentError,
+                        stacklevel=2,
+                    )
 
         return self._pad_or_backfill(
             "ffill",
@@ -7523,6 +7532,15 @@ def bfill(
         3   4.0   7.0
         """
         downcast = self._deprecate_downcast(downcast, "bfill")
+        inplace = validate_bool_kwarg(inplace, "inplace")
+        if inplace:
+            if not PYPY and using_copy_on_write():
+                if sys.getrefcount(self) <= REF_COUNT:
+                    warnings.warn(
+                        _chained_assignment_method_msg,
+                        ChainedAssignmentError,
+                        stacklevel=2,
+                    )
         return self._pad_or_backfill(
             "bfill",
             axis=axis,
@@ -8047,6 +8065,16 @@ def interpolate(
             raise ValueError("downcast must be either None or 'infer'")
 
         inplace = validate_bool_kwarg(inplace, "inplace")
+
+        if inplace:
+            if not PYPY and using_copy_on_write():
+                if sys.getrefcount(self) <= REF_COUNT:
+                    warnings.warn(
+                        _chained_assignment_method_msg,
+                        ChainedAssignmentError,
+                        stacklevel=2,
+                    )
+
         axis = self._get_axis_number(axis)
 
         if self.empty:
@@ -8619,6 +8647,15 @@ def clip(
         """
         inplace = validate_bool_kwarg(inplace, "inplace")
 
+        if inplace:
+            if not PYPY and using_copy_on_write():
+                if sys.getrefcount(self) <= REF_COUNT:
+                    warnings.warn(
+                        _chained_assignment_method_msg,
+                        ChainedAssignmentError,
+                        stacklevel=2,
+                    )
+
         axis = nv.validate_clip_with_axis(axis, (), kwargs)
         if axis is not None:
             axis = self._get_axis_number(axis)
@@ -10500,6 +10537,15 @@ def where(
         3  True  True
         4  True  True
         """
+        inplace = validate_bool_kwarg(inplace, "inplace")
+        if inplace:
+            if not PYPY and using_copy_on_write():
+                if sys.getrefcount(self) <= REF_COUNT:
+                    warnings.warn(
+                        _chained_assignment_method_msg,
+                        ChainedAssignmentError,
+                        stacklevel=2,
+                    )
         other = common.apply_if_callable(other, self)
         return self._where(cond, other, inplace, axis, level)
 
@@ -10558,6 +10604,15 @@ def mask(
         level: Level | None = None,
     ) -> Self | None:
         inplace = validate_bool_kwarg(inplace, "inplace")
+        if inplace:
+            if not PYPY and using_copy_on_write():
+                if sys.getrefcount(self) <= REF_COUNT:
+                    warnings.warn(
+                        _chained_assignment_method_msg,
+                        ChainedAssignmentError,
+                        stacklevel=2,
+                    )
+
         cond = common.apply_if_callable(cond, self)
 
         # see gh-21891

pandas/tests/copy_view/test_clip.py

@@ -68,3 +68,16 @@ def test_clip_no_op(using_copy_on_write):
         assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
     else:
         assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+
+def test_clip_chained_inplace(using_copy_on_write):
+    df = DataFrame({"a": [1, 4, 2], "b": 1})
+    df_orig = df.copy()
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error():
+            df["a"].clip(1, 2, inplace=True)
+        tm.assert_frame_equal(df, df_orig)
+
+        with tm.raises_chained_assignment_error():
+            df[["a"]].clip(1, 2, inplace=True)
+        tm.assert_frame_equal(df, df_orig)

pandas/tests/copy_view/test_interp_fillna.py

@@ -361,3 +361,17 @@ def test_fillna_chained_assignment(using_copy_on_write):
         with tm.raises_chained_assignment_error():
             df[["a"]].fillna(100, inplace=True)
         tm.assert_frame_equal(df, df_orig)
+
+
+@pytest.mark.parametrize("func", ["interpolate", "ffill", "bfill"])
+def test_interpolate_chained_assignment(using_copy_on_write, func):
+    df = DataFrame({"a": [1, np.nan, 2], "b": 1})
+    df_orig = df.copy()
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error():
+            getattr(df["a"], func)(inplace=True)
+        tm.assert_frame_equal(df, df_orig)
+
+        with tm.raises_chained_assignment_error():
+            getattr(df[["a"]], func)(inplace=True)
+        tm.assert_frame_equal(df, df_orig)

pandas/tests/copy_view/test_methods.py

@@ -1518,6 +1518,20 @@ def test_where_mask_noop_on_single_column(using_copy_on_write, dtype, val, func)
     tm.assert_frame_equal(df, df_orig)
 
 
+@pytest.mark.parametrize("func", ["mask", "where"])
+def test_chained_where_mask(using_copy_on_write, func):
+    df = DataFrame({"a": [1, 4, 2], "b": 1})
+    df_orig = df.copy()
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error():
+            getattr(df["a"], func)(df["a"] > 2, 5, inplace=True)
+        tm.assert_frame_equal(df, df_orig)
+
+        with tm.raises_chained_assignment_error():
+            getattr(df[["a"]], func)(df["a"] > 2, 5, inplace=True)
+        tm.assert_frame_equal(df, df_orig)
+
+
 def test_asfreq_noop(using_copy_on_write):
     df = DataFrame(
         {"a": [0.0, None, 2.0, 3.0]},

pandas/tests/frame/methods/test_interpolate.py

@@ -1,6 +1,7 @@
 import numpy as np
 import pytest
 
+from pandas.errors import ChainedAssignmentError
 import pandas.util._test_decorators as td
 
 from pandas import (
@@ -370,21 +371,31 @@ def test_interp_inplace(self, using_copy_on_write):
         expected = DataFrame({"a": [1.0, 2.0, 3.0, 4.0]})
         expected_cow = df.copy()
         result = df.copy()
-        return_value = result["a"].interpolate(inplace=True)
-        assert return_value is None
+
         if using_copy_on_write:
+            with tm.raises_chained_assignment_error():
+                return_value = result["a"].interpolate(inplace=True)
+            assert return_value is None
             tm.assert_frame_equal(result, expected_cow)
         else:
+            return_value = result["a"].interpolate(inplace=True)
+            assert return_value is None
             tm.assert_frame_equal(result, expected)
 
         result = df.copy()
         msg = "The 'downcast' keyword in Series.interpolate is deprecated"
-        with tm.assert_produces_warning(FutureWarning, match=msg):
-            return_value = result["a"].interpolate(inplace=True, downcast="infer")
-        assert return_value is None
+
         if using_copy_on_write:
+            with tm.assert_produces_warning(
+                (FutureWarning, ChainedAssignmentError), match=msg
+            ):
+                return_value = result["a"].interpolate(inplace=True, downcast="infer")
+            assert return_value is None
             tm.assert_frame_equal(result, expected_cow)
         else:
+            with tm.assert_produces_warning(FutureWarning, match=msg):
+                return_value = result["a"].interpolate(inplace=True, downcast="infer")
+            assert return_value is None
             tm.assert_frame_equal(result, expected.astype("int64"))
 
     def test_interp_inplace_row(self):