CoW: Add warning for replace with inplace (pandas-dev#56060)

Author: phofl

pandas/core/generic.py

@@ -7797,6 +7797,22 @@ def replace(
                         ChainedAssignmentError,
                         stacklevel=2,
                     )
+            elif not PYPY and not using_copy_on_write():
+                ctr = sys.getrefcount(self)
+                ref_count = REF_COUNT
+                if isinstance(self, ABCSeries) and hasattr(self, "_cacher"):
+                    # in non-CoW mode, chained Series access will populate the
+                    # `_item_cache` which results in an increased ref count not below
+                    # the threshold, while we still need to warn. We detect this case
+                    # of a Series derived from a DataFrame through the presence of
+                    # `_cacher`
+                    ref_count += 1
+                if ctr <= ref_count:
+                    warnings.warn(
+                        _chained_assignment_warning_method_msg,
+                        FutureWarning,
+                        stacklevel=2,
+                    )
 
         if not is_bool(regex) and to_replace is not None:
             raise ValueError("'to_replace' must be 'None' if 'regex' is not a bool")

pandas/tests/copy_view/test_replace.py

@@ -4,6 +4,7 @@
 from pandas import (
     Categorical,
     DataFrame,
+    option_context,
 )
 import pandas._testing as tm
 from pandas.tests.copy_view.util import get_array
@@ -395,6 +396,17 @@ def test_replace_chained_assignment(using_copy_on_write):
         with tm.raises_chained_assignment_error():
             df[["a"]].replace(1, 100, inplace=True)
         tm.assert_frame_equal(df, df_orig)
+    else:
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            with option_context("mode.chained_assignment", None):
+                df[["a"]].replace(1, 100, inplace=True)
+
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            with option_context("mode.chained_assignment", None):
+                df[df.a > 5].replace(1, 100, inplace=True)
+
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            df["a"].replace(1, 100, inplace=True)
 
 
 def test_replace_listlike(using_copy_on_write):