Add aliases for 'function classes' to multiprocessing (#5346)

Part of #4313

stdlib/multiprocessing/__init__.pyi

@@ -15,7 +15,9 @@ from multiprocessing.context import (
 from multiprocessing.managers import SyncManager
 from multiprocessing.process import active_children as active_children, current_process as current_process
 
-# These are technically functions that return instances of these Queue classes. See #4313 for discussion
+# These are technically functions that return instances of these Queue classes.
+# Using them as annotations is deprecated. Either use imports from
+# multiprocessing.queues or the aliases defined below. See #4266 for discussion.
 from multiprocessing.queues import JoinableQueue as JoinableQueue, Queue as Queue, SimpleQueue as SimpleQueue
 from multiprocessing.spawn import freeze_support as freeze_support
 from typing import Any, Optional, Union, overload
@@ -27,6 +29,26 @@ if sys.version_info >= (3, 8):
 if sys.platform != "win32":
     from multiprocessing.context import ForkContext, ForkServerContext
 
+# The following type aliases can be used to annotate the return values of
+# the corresponding functions. They are not defined at runtime.
+#
+# from multiprocessing import Lock
+# from typing import TYPE_CHECKING
+# if TYPE_CHECKING:
+#     from multiprocessing import _LockType
+# lock: _LockType = Lock()
+
+_QueueType = Queue
+_SimpleQueueType = SimpleQueue
+_JoinableQueueType = JoinableQueue
+_BarrierType = synchronize.Barrier
+_BoundedSemaphoreType = synchronize.BoundedSemaphore
+_ConditionType = synchronize.Condition
+_EventType = synchronize.Event
+_LockType = synchronize.Lock
+_RLockType = synchronize.RLock
+_SemaphoreType = synchronize.Semaphore
+
 # N.B. The functions below are generated at runtime by partially applying
 # multiprocessing.context.BaseContext's methods, so the two signatures should
 # be identical (modulo self).
@@ -38,13 +60,13 @@ RawArray = context._default_context.RawArray
 Value = context._default_context.Value
 Array = context._default_context.Array
 
-def Barrier(parties: int, action: Optional[Callable[..., Any]] = ..., timeout: Optional[float] = ...) -> synchronize.Barrier: ...
-def BoundedSemaphore(value: int = ...) -> synchronize.BoundedSemaphore: ...
-def Condition(lock: Optional[_LockLike] = ...) -> synchronize.Condition: ...
-def Event() -> synchronize.Event: ...
-def Lock() -> synchronize.Lock: ...
-def RLock() -> synchronize.RLock: ...
-def Semaphore(value: int = ...) -> synchronize.Semaphore: ...
+def Barrier(parties: int, action: Optional[Callable[..., Any]] = ..., timeout: Optional[float] = ...) -> _BarrierType: ...
+def BoundedSemaphore(value: int = ...) -> _BoundedSemaphoreType: ...
+def Condition(lock: Optional[_LockLike] = ...) -> _ConditionType: ...
+def Event() -> _EventType: ...
+def Lock() -> _LockType: ...
+def RLock() -> _RLockType: ...
+def Semaphore(value: int = ...) -> _SemaphoreType: ...
 def Pipe(duplex: bool = ...) -> tuple[connection.Connection, connection.Connection]: ...
 def Pool(
     processes: Optional[int] = ...,