bpo-38840: Incorrect __all__ in multiprocessing.managers (GH-18034) (GH-27684)

This was causing test___all__ to fail on platforms lacking a shared
memory implementation.

Co-Authored-By: Xavier de Gaye <[email protected]>
Co-authored-by: Łukasz Langa <[email protected]>
(cherry picked from commit d097876)

Co-authored-by: Zackery Spytz <[email protected]>

Author: miss-islington

Lib/multiprocessing/managers.py

@@ -8,8 +8,7 @@
 # Licensed to PSF under a Contributor Agreement.
 #
 
-__all__ = [ 'BaseManager', 'SyncManager', 'BaseProxy', 'Token',
-            'SharedMemoryManager' ]
+__all__ = [ 'BaseManager', 'SyncManager', 'BaseProxy', 'Token' ]
 
 #
 # Imports
@@ -35,9 +34,11 @@
 from . import get_context
 try:
     from . import shared_memory
-    HAS_SHMEM = True
 except ImportError:
     HAS_SHMEM = False
+else:
+    HAS_SHMEM = True
+    __all__.append('SharedMemoryManager')
 
 #
 # Register some things for pickling

Misc/NEWS.d/next/Library/2020-01-16-23-41-16.bpo-38840.VzzYZz.rst

@@ -0,0 +1 @@
+Fix ``test___all__`` on platforms lacking a shared memory implementation.