PDEP-8 inplace: return self for inplace=True in methods that will keep the keyword (#63195)

Author: jorisvandenbossche

doc/source/whatsnew/v3.0.0.rst

@@ -753,6 +753,10 @@ Other API changes
 - Arithmetic operations between a :class:`Series`, :class:`Index`, or :class:`ExtensionArray` with a ``list`` now consistently wrap that list with an array equivalent to ``Series(my_list).array``. To do any other kind of type inference or casting, do so explicitly before operating (:issue:`62552`)
 - Comparison operations between :class:`Index` and :class:`Series` now consistently return :class:`Series` regardless of which object is on the left or right (:issue:`36759`)
 - Numpy functions like ``np.isinf`` that return a bool dtype when called on a :class:`Index` object now return a bool-dtype :class:`Index` instead of ``np.ndarray`` (:issue:`52676`)
+- Methods that can operate in-place (:meth:`~DataFrame.replace`, :meth:`~DataFrame.fillna`,
+  :meth:`~DataFrame.ffill`, :meth:`~DataFrame.bfill`, :meth:`~DataFrame.interpolate`,
+  :meth:`~DataFrame.where`, :meth:`~DataFrame.mask`, :meth:`~DataFrame.clip`) now return
+  the modified DataFrame or Series (``self``) instead of ``None`` when ``inplace=True`` (:issue:`63207`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_300.deprecations:

pandas/core/frame.py

@@ -6109,19 +6109,9 @@ def pop(self, item: Hashable) -> Series:
         """
         return super().pop(item=item)
 
-    @overload
-    def _replace_columnwise(
-        self, mapping: dict[Hashable, tuple[Any, Any]], inplace: Literal[True], regex
-    ) -> None: ...
-
-    @overload
-    def _replace_columnwise(
-        self, mapping: dict[Hashable, tuple[Any, Any]], inplace: Literal[False], regex
-    ) -> Self: ...
-
     def _replace_columnwise(
         self, mapping: dict[Hashable, tuple[Any, Any]], inplace: bool, regex
-    ) -> Self | None:
+    ) -> Self:
         """
         Dispatch to Series.replace column-wise.
 
@@ -6134,7 +6124,7 @@ def _replace_columnwise(
 
         Returns
         -------
-        DataFrame or None
+        DataFrame
         """
         # Operate column-wise
         res = self if inplace else self.copy(deep=False)
@@ -6149,9 +6139,7 @@ def _replace_columnwise(
 
                 res._iset_item(i, newobj, inplace=inplace)
 
-        if inplace:
-            return None
-        return res.__finalize__(self)
+        return res if inplace else res.__finalize__(self)
 
     @doc(NDFrame.shift, klass=_shared_doc_kwargs["klass"])
     def shift(