From 335b6874bfc717a4662ff10ee3dd9b98c287b923 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date: Thu, 7 Dec 2023 20:20:05 +0100
Subject: [PATCH 1/2] CoW: Warn for transform inplace modification

---
 pandas/core/series.py                  | 6 +++++-
 pandas/tests/copy_view/test_methods.py | 4 ++--
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/pandas/core/series.py b/pandas/core/series.py
index 464e066b4e86a..de4b3e750a7e8 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -4740,7 +4740,11 @@ def transform(
     ) -> DataFrame | Series:
         # Validate axis argument
         self._get_axis_number(axis)
-        ser = self.copy(deep=False) if using_copy_on_write() else self
+        ser = (
+            self.copy(deep=False)
+            if using_copy_on_write() or warn_copy_on_write()
+            else self
+        )
         result = SeriesApply(ser, func=func, args=args, kwargs=kwargs).transform()
         return result
 
diff --git a/pandas/tests/copy_view/test_methods.py b/pandas/tests/copy_view/test_methods.py
index 1e3c95dbc1ca3..558b483933f25 100644
--- a/pandas/tests/copy_view/test_methods.py
+++ b/pandas/tests/copy_view/test_methods.py
@@ -1938,8 +1938,8 @@ def func(ser):
         ser.iloc[0] = 100
         return ser
 
-    # TODO(CoW-warn) should warn?
-    ser.transform(func)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        ser.transform(func)
     if using_copy_on_write:
         tm.assert_series_equal(ser, ser_orig)
 

From c7d1eec138f82cf4b8c7f4adf7273bedc8c45598 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date: Thu, 7 Dec 2023 20:31:59 +0100
Subject: [PATCH 2/2] Remove todo

---
 pandas/core/frame.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 1630c2c31920d..179279cc08bab 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -4895,7 +4895,6 @@ def eval(self, expr: str, *, inplace: bool = False, **kwargs) -> Any | None:
 
         inplace = validate_bool_kwarg(inplace, "inplace")
         kwargs["level"] = kwargs.pop("level", 0) + 1
-        # TODO(CoW) those index/column resolvers create unnecessary refs to `self`
         index_resolvers = self._get_index_resolvers()
         column_resolvers = self._get_cleaned_column_resolvers()
         resolvers = column_resolvers, index_resolvers