Fix executor initializer attribute return types

stdlib/@tests/test_cases/check_concurrent_futures.py

@@ -2,7 +2,7 @@
 
 import sys
 from collections.abc import Callable, Iterator
-from concurrent.futures import Future, ThreadPoolExecutor, as_completed
+from concurrent.futures import Future, ProcessPoolExecutor, ThreadPoolExecutor, as_completed
 from typing import Literal
 from typing_extensions import assert_type
 
@@ -32,6 +32,30 @@ def execute_callback(callback: Callable[[], Parent], future: Future[Parent]) ->
     assert isinstance(fut.result(), Child)
 
 
+def _value_initializer(value: int) -> str:
+    return str(value)
+
+
+def check_process_initializer(executor: ProcessPoolExecutor) -> None:
+    ProcessPoolExecutor(initializer=_value_initializer, initargs=(1,))
+    ProcessPoolExecutor(initializer=_value_initializer, initargs=("x",))  # type: ignore
+    assert_type(executor._initializer, Callable[..., object] | None)
+    executor._initializer = _value_initializer
+    executor._initializer = None
+    executor._initializer = 1  # type: ignore
+
+
+if sys.version_info < (3, 14):
+
+    def check_thread_initializer(executor: ThreadPoolExecutor) -> None:
+        ThreadPoolExecutor(initializer=_value_initializer, initargs=(1,))
+        ThreadPoolExecutor(initializer=_value_initializer, initargs=("x",))  # type: ignore
+        assert_type(executor._initializer, Callable[..., object] | None)
+        executor._initializer = _value_initializer
+        executor._initializer = None
+        executor._initializer = 1  # type: ignore
+
+
 if sys.version_info >= (3, 14):
 
     def _initializer(x: int) -> None:

stdlib/concurrent/futures/process.pyi

@@ -168,7 +168,7 @@ class BrokenProcessPool(BrokenExecutor): ...
 
 class ProcessPoolExecutor(Executor):
     _mp_context: BaseContext | None
-    _initializer: Callable[..., None] | None
+    _initializer: Callable[..., object] | None
     _initargs: tuple[Any, ...]
     _executor_manager_thread: _ThreadWakeup
     _processes: MutableMapping[int, Process]

stdlib/concurrent/futures/thread.pyi

@@ -97,7 +97,7 @@ class ThreadPoolExecutor(Executor):
         _create_worker_context: Callable[[], WorkerContext]
         _resolve_work_item_task: _ResolveTaskFunc
     else:
-        _initializer: Callable[..., None] | None
+        _initializer: Callable[..., object] | None
         _initargs: tuple[Any, ...]
     _work_queue: queue.SimpleQueue[_WorkItem[Any]]