Allow passing lambdas as init arguments in multiprocessing (#1416)

* Use NoReturn in multiprocessing init functions
* Use Any for init functions

stdlib/3/multiprocessing/context.pyi

@@ -49,14 +49,13 @@ class BaseContext(object):
     def JoinableQueue(self, maxsize: int = ...) -> Any: ...
     # TODO: change return to SimpleQueue once a stub exists in multiprocessing.queues
     def SimpleQueue(self) -> Any: ...
-    # TODO: change return to Pool once a stub exists in multiprocessing.pool
     def Pool(
         self,
         processes: Optional[int] = ...,
         initializer: Optional[Callable[..., Any]] = ...,
         initargs: Tuple = ...,
         maxtasksperchild: Optional[int] = ...
-    ) -> Any: ...
+    ) -> multiprocessing.Pool: ...
     # TODO: typecode_or_type param is a ctype with a base class of _SimpleCData or array.typecode Need to figure out
     # how to handle the ctype
     # TODO: change return to RawValue once a stub exists in multiprocessing.sharedctypes

stdlib/3/multiprocessing/managers.pyi

@@ -19,7 +19,7 @@ _Namespace = Namespace
 class BaseManager:
     def register(self, typeid: str, callable: Any = ...) -> None: ...
     def shutdown(self) -> None: ...
-    def start(self, initializer: Optional[Callable[..., None]] = ...,
+    def start(self, initializer: Optional[Callable[..., Any]] = ...,
               initargs: Iterable[Any] = ...) -> None: ...
     def __enter__(self) -> 'BaseManager': ...
     def __exit__(self, exc_type, exc_value, tb) -> None: ...

stdlib/3/multiprocessing/pool.pyi

@@ -12,7 +12,7 @@ class AsyncResult():
 
 class ThreadPool():
     def __init__(self, processes: Optional[int] = None,
-                 initializer: Optional[Callable[..., None]] = None,
+                 initializer: Optional[Callable[..., Any]] = None,
                  initargs: Iterable[Any] = ...) -> None: ...
     def apply(self,
               func: Callable[..., Any],