Fix missing docstring on class, add test for ignoring size when attaching.

Author: applio

Lib/multiprocessing/shared_memory.py

@@ -49,6 +49,20 @@ def _make_filename():
 
 
 class SharedMemory:
+    """Creates a new shared memory block or attaches to an existing
+    shared memory block.
+
+    Every shared memory block is assigned a unique name.  This enables
+    one process to create a shared memory block with a particular name 
+    so that a different process can attach to that same shared memory
+    block using that same name.
+
+    As a resource for sharing data across processes, shared memory blocks
+    may outlive the original process that created them.  When one process
+    no longer needs access to a shared memory block that might still be
+    needed by other processes, the close() method should be called.
+    When a shared memory block is no longer needed by any process, the
+    unlink() method should be called to ensure proper cleanup."""
 
     # Defaults; enables close() and unlink() to run without errors.
     _name = None

Lib/test/_test_multiprocessing.py

@@ -3650,6 +3650,11 @@ def test_shared_memory_basics(self):
         self.assertEqual(also_sms.buf[0], 42)
         also_sms.close()
 
+        # Attach to existing shared memory segment but specify a new size.
+        same_sms = shared_memory.SharedMemory('test01_tsmb', size=20*sms.size)
+        self.assertEqual(same_sms.size, sms.size)  # Size was ignored.
+        same_sms.close()
+
         if shared_memory._USE_POSIX:
             # Posix Shared Memory can only be unlinked once.  Here we
             # test an implementation detail that is not observed across
@@ -3675,9 +3680,6 @@ def test_shared_memory_basics(self):
                 finally:
                     sms_uno.unlink()  # A second shm_unlink() call is bad.
 
-        # Enforcement of `mode` and `read_only` is OS platform dependent
-        # and as such will not be tested here.
-
         with self.assertRaises(FileExistsError):
             # Attempting to create a new shared memory segment with a
             # name that is already in use triggers an exception.