logging: Added QueueHandler.

Lib/logging/handlers.py

@@ -1134,3 +1134,55 @@ class MemoryHandler(BufferingHandler):
         self.flush()
         self.target = None
         BufferingHandler.close(self)
+
+
+class QueueHandler(logging.Handler):
+    """
+    This handler sends events to a queue. Typically, it would be used together
+    with a multiprocessing Queue to centralise logging to file in one process
+    (in a multi-process application), so as to avoid file write contention
+    between processes.
+
+    This code is new in Python 3.2, but this class can be copy pasted into
+    user code for use with earlier Python versions.
+    """
+
+    def __init__(self, queue):
+        """
+        Initialise an instance, using the passed queue.
+        """
+        logging.Handler.__init__(self)
+        self.queue = queue
+
+    def enqueue(self, record):
+        """
+        Enqueue a record.
+
+        The base implementation uses put_nowait. You may want to override
+        this method if you want to use blocking, timeouts or custom queue
+        implementations.
+        """
+        self.queue.put_nowait(record)
+
+    def emit(self, record):
+        """
+        Emit a record.
+
+        Writes the LogRecord to the queue, preparing it for pickling first.
+        """
+        try:
+            # The format operation gets traceback text into record.exc_text
+            # (if there's exception data), and also puts the message into
+            # record.message. We can then use this to replace the original
+            # msg + args, as these might be unpickleable. We also zap the
+            # exc_info attribute, as it's no longer needed and, if not None,
+            # will typically not be pickleable.
+            self.format(record)
+            record.msg = record.message
+            record.args = None
+            record.exc_info = None
+            self.enqueue(record)
+        except (KeyboardInterrupt, SystemExit):
+            raise
+        except:
+            self.handleError(record)

Misc/NEWS

@@ -13,6 +13,9 @@ Core and Builtins
 Library
 -------
 
+- Logging: Added QueueHandler class to facilitate logging usage with
+  multiprocessing.
+
 - Issue #9707: Rewritten reference implementation of threading.local which
   is friendlier towards reference cycles.  This change is not normally
   visible since an optimized C implementation (_thread._local) is used