[pickle] Fix _Pickle.reducer_override annotation (#13545)

Author: srittau

stdlib/@tests/stubtest_allowlists/common.txt

@@ -436,8 +436,11 @@ os._wrap_close.write  # Methods that come from __getattr__() at runtime
 os._wrap_close.writelines  # Methods that come from __getattr__() at runtime
 os.PathLike.__class_getitem__  # PathLike is a protocol; we don't expect all PathLike classes to implement class_getitem
 
+_pickle.Pickler.reducer_override  # Can be added by subclasses
+pickle.Pickler.reducer_override  # Can be added by subclasses
 pickle._Pickler\..*  # Best effort typing for undocumented internals
 pickle._Unpickler\..*  # Best effort typing for undocumented internals
+
 shutil.rmtree  # function with attributes, which we approximate with a callable protocol
 socketserver.BaseServer.get_request  # Not implemented, but expected to exist on subclasses.
 ssl.PROTOCOL_SSLv2  # Depends on the existence and flags of SSL

stdlib/_pickle.pyi

@@ -61,7 +61,6 @@ class PicklerMemoProxy:
 class Pickler:
     fast: bool
     dispatch_table: Mapping[type, Callable[[Any], _ReducedType]]
-    reducer_override: Callable[[Any], Any]
     bin: bool  # undocumented
     def __init__(
         self,
@@ -79,6 +78,10 @@ class Pickler:
 
     # this method has no default implementation for Python < 3.13
     def persistent_id(self, obj: Any, /) -> Any: ...
+    # The following method is not defined on _Pickler, but can be defined on
+    # sub-classes. Should return `NotImplemented` if pickling the supplied
+    # object is not supported and returns the same types as `__reduce__()`.
+    def reducer_override(self, obj: object, /) -> _ReducedType: ...
 
 @type_check_only
 class UnpicklerMemoProxy:

stdlib/pickle.pyi

@@ -208,7 +208,6 @@ class _Pickler:
     dispatch_table: Mapping[type, Callable[[Any], _ReducedType]]
     bin: bool  # undocumented
     dispatch: ClassVar[dict[type, Callable[[Unpickler, Any], None]]]  # undocumented, _Pickler only
-    reducer_override: Callable[[Any], Any]
     def __init__(
         self,
         file: SupportsWrite[bytes],
@@ -220,6 +219,10 @@ class _Pickler:
     def dump(self, obj: Any) -> None: ...
     def clear_memo(self) -> None: ...
     def persistent_id(self, obj: Any) -> Any: ...
+    # The following method is not defined on _Pickler, but can be defined on
+    # sub-classes. Should return `NotImplemented` if pickling the supplied
+    # object is not supported and returns the same types as `__reduce__()`.
+    def reducer_override(self, obj: object, /) -> _ReducedType: ...
 
 class _Unpickler:
     dispatch: ClassVar[dict[int, Callable[[Unpickler], None]]]  # undocumented, _Unpickler only