DEPR: deprecate element-wise operations in (Series|DataFrame).transform

Author: topper-123

pandas/conftest.py

@@ -2005,3 +2005,11 @@ def warsaw(request) -> str:
 @pytest.fixture()
 def arrow_string_storage():
     return ("pyarrow", "pyarrow_numpy")
+
+
+@pytest.fixture(params=[True, False])
+def series_ops_only(request):
+    """
+    Parameter used in Series.transform and DataFrame.transform. Remove in pandas v3.0.
+    """
+    return request.param

pandas/core/apply.py

@@ -206,12 +206,14 @@ def transform(self) -> DataFrame | Series:
         axis = self.axis
         args = self.args
         kwargs = self.kwargs
+        by_row = self.by_row
 
         is_series = obj.ndim == 1
 
         if obj._get_axis_number(axis) == 1:
             assert not is_series
-            return obj.T.transform(func, 0, *args, **kwargs).T
+            soo = False if by_row else True
+            return obj.T.transform(func, 0, *args, series_ops_only=soo, **kwargs).T
 
         if is_list_like(func) and not is_dict_like(func):
             func = cast(list[AggFuncTypeBase], func)
@@ -225,14 +227,17 @@ def transform(self) -> DataFrame | Series:
             func = cast(AggFuncTypeDict, func)
             return self.transform_dict_like(func)
 
-        # func is either str or callable
-        func = cast(AggFuncTypeBase, func)
-        try:
-            result = self.transform_str_or_callable(func)
-        except TypeError:
-            raise
-        except Exception as err:
-            raise ValueError("Transform function failed") from err
+        if not self.by_row:
+            result = obj.apply(func, by_row=by_row, args=args, **kwargs)
+        else:
+            # func is either str or callable
+            func = cast(AggFuncTypeBase, func)
+            try:
+                result = self.transform_str_or_callable(func)
+            except TypeError:
+                raise
+            except Exception as err:
+                raise ValueError("Transform function failed") from err
 
         # Functions that transform may return empty Series/DataFrame
         # when the dtype is not appropriate
@@ -262,6 +267,7 @@ def transform_dict_like(self, func) -> DataFrame:
         obj = self.obj
         args = self.args
         kwargs = self.kwargs
+        soo = False if self.by_row else True
 
         # transform is currently only for Series/DataFrame
         assert isinstance(obj, ABCNDFrame)
@@ -274,7 +280,7 @@ def transform_dict_like(self, func) -> DataFrame:
         results: dict[Hashable, DataFrame | Series] = {}
         for name, how in func.items():
             colg = obj._gotitem(name, ndim=1)
-            results[name] = colg.transform(how, 0, *args, **kwargs)
+            results[name] = colg.transform(how, 0, *args, series_ops_only=soo, **kwargs)
         return concat(results, axis=1)
 
     def transform_str_or_callable(self, func) -> DataFrame | Series:
@@ -591,7 +597,10 @@ def apply_list_or_dict_like(self) -> DataFrame | Series:
             Result when self.func is a list-like or dict-like, None otherwise.
         """
         if self.axis == 1 and isinstance(self.obj, ABCDataFrame):
-            return self.obj.T.apply(self.func, 0, args=self.args, **self.kwargs).T
+            soo = False if self.by_row else True
+            return self.obj.T.apply(
+                self.func, 0, args=self.args, series_ops_only=soo, **self.kwargs
+            ).T
 
         func = self.func
         kwargs = self.kwargs

pandas/core/frame.py

@@ -9902,11 +9902,30 @@ def aggregate(self, func=None, axis: Axis = 0, *args, **kwargs):
         axis=_shared_doc_kwargs["axis"],
     )
     def transform(
-        self, func: AggFuncType, axis: Axis = 0, *args, **kwargs
+        self,
+        func: AggFuncType,
+        axis: Axis = 0,
+        *args,
+        series_ops_only: bool = False,
+        **kwargs,
     ) -> DataFrame:
         from pandas.core.apply import frame_apply
 
-        op = frame_apply(self, func=func, axis=axis, args=args, kwargs=kwargs)
+        if not series_ops_only and is_list_like(func):
+            cls_name = type(self).__name__
+            warnings.warn(
+                f"{cls_name}.transform will in the future only operate on "
+                "whole series. Set series_ops_only = True to opt into the new behavior "
+                f"or use {cls_name}.map to continue operating on series elements.",
+                FutureWarning,
+                stacklevel=find_stack_level(),
+            )
+
+        by_row = False if series_ops_only else "compat"
+
+        op = frame_apply(
+            self, func=func, axis=axis, args=args, by_row=by_row, kwargs=kwargs
+        )
         result = op.transform()
         assert isinstance(result, DataFrame)
         return result

pandas/core/series.py

@@ -4617,12 +4617,30 @@ def aggregate(self, func=None, axis: Axis = 0, *args, **kwargs):
         axis=_shared_doc_kwargs["axis"],
     )
     def transform(
-        self, func: AggFuncType, axis: Axis = 0, *args, **kwargs
+        self,
+        func: AggFuncType,
+        axis: Axis = 0,
+        *args,
+        series_ops_only: bool = False,
+        **kwargs,
     ) -> DataFrame | Series:
         # Validate axis argument
         self._get_axis_number(axis)
+        if not series_ops_only and not isinstance(func, str):
+            cls_name = type(self).__name__
+            warnings.warn(
+                f"{cls_name}.transform will in the future only operate on "
+                "whole series. Set series_ops_only = True to opt into the new behavior "
+                f"or use {cls_name}.map to continue operating on series elements.",
+                FutureWarning,
+                stacklevel=find_stack_level(),
+            )
+
+        by_row = False if series_ops_only else "compat"
         ser = self.copy(deep=False) if using_copy_on_write() else self
-        result = SeriesApply(ser, func=func, args=args, kwargs=kwargs).transform()
+        result = SeriesApply(
+            ser, func=func, by_row=by_row, args=args, kwargs=kwargs
+        ).transform()
         return result
 
     def apply(

pandas/tests/apply/common.py

@@ -1,7 +1,34 @@
+import pandas as pd
+import pandas._testing as tm
+from pandas.api.types import is_list_like
 from pandas.core.groupby.base import transformation_kernels
 
 # There is no Series.cumcount or DataFrame.cumcount
 series_transform_kernels = [
     x for x in sorted(transformation_kernels) if x != "cumcount"
 ]
 frame_transform_kernels = [x for x in sorted(transformation_kernels) if x != "cumcount"]
+
+
+def transform_obj(obj, func, *args, axis=0, series_ops_only=False, **kwargs):
+    """helper function to ease use of series_ops_only and deprecation warning."""
+    if series_ops_only:
+        result = obj.transform(
+            func, axis, *args, series_ops_only=series_ops_only, **kwargs
+        )
+    elif isinstance(obj, pd.DataFrame) and not is_list_like(func):
+        result = obj.transform(func, axis, *args, **kwargs)
+    elif isinstance(func, str):
+        result = obj.transform(func, axis, *args, **kwargs)
+    else:
+        cls_name = type(obj).__name__
+        msg = (
+            f"{cls_name}.transform will in the future only operate on "
+            "whole series. Set series_ops_only = True to opt into the new behavior "
+            f"or use {cls_name}.map to continue operating on series elements."
+        )
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = obj.transform(
+                func, axis, *args, series_ops_only=series_ops_only, **kwargs
+            )
+    return result