CoW: Warn for transform inplace modification (pandas-dev#56381)

Author: phofl

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

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
 

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)