logging: added QueueListener and documentation.

Doc/library/logging.rst

@@ -2651,12 +2651,18 @@ The :class:`QueueHandler` class, located in the :mod:`logging.handlers` module,
 supports sending logging messages to a queue, such as those implemented in the
 :mod:`queue` or :mod:`multiprocessing` modules.
 
+Along with the :class:`QueueListener` class, :class:`QueueHandler` can be used
+to let handlers do their work on a separate thread from the one which does the
+logging. This is important in Web applications and also other service
+applications where threads servicing clients need to respond as quickly as
+possible, while any potentially slow operations (such as sending an email via
+:class:`SMTPHandler`) are done on a separate thread.
 
 .. class:: QueueHandler(queue)
 
    Returns a new instance of the :class:`QueueHandler` class. The instance is
    initialized with the queue to send messages to. The queue can be any queue-
-   like object; it's passed as-is to the :meth:`enqueue` method, which needs
+   like object; it's used as-is by the :meth:`enqueue` method, which needs
    to know how to send messages to it.
 
 
@@ -2688,8 +2694,80 @@ supports sending logging messages to a queue, such as those implemented in the
 
 The :class:`QueueHandler` class was not present in previous versions.
 
+.. queue-listener:
+
+QueueListener
+^^^^^^^^^^^^^
+
+The :class:`QueueListener` class, located in the :mod:`logging.handlers`
+module, supports receiving logging messages from a queue, such as those
+implemented in the :mod:`queue` or :mod:`multiprocessing` modules. The
+messages are received from a queue in an internal thread and passed, on
+the same thread, to one or more handlers for processing.
+
+Along with the :class:`QueueHandler` class, :class:`QueueListener` can be used
+to let handlers do their work on a separate thread from the one which does the
+logging. This is important in Web applications and also other service
+applications where threads servicing clients need to respond as quickly as
+possible, while any potentially slow operations (such as sending an email via
+:class:`SMTPHandler`) are done on a separate thread.
+
+.. class:: QueueListener(queue, *handlers)
+
+   Returns a new instance of the :class:`QueueListener` class. The instance is
+   initialized with the queue to send messages to and a list of handlers which
+   will handle entries placed on the queue. The queue can be any queue-
+   like object; it's passed as-is to the :meth:`dequeue` method, which needs
+   to know how to get messages from it.
+
+   .. method:: dequeue(block)
+
+      Dequeues a record and return it, optionally blocking.
+
+      The base implementation uses ``get()``. You may want to override this
+      method if you want to use timeouts or work with custom queue
+      implementations.
+
+   .. method:: prepare(record)
+
+      Prepare a record for handling.
+
+      This implementation just returns the passed-in record. You may want to
+      override this method if you need to do any custom marshalling or
+      manipulation of the record before passing it to the handlers.
+
+   .. method:: handle(record)
+
+      Handle a record.
+
+      This just loops through the handlers offering them the record
+      to handle. The actual object passed to the handlers is that which
+      is returned from :meth:`prepare`.
+
+   .. method:: start()
+
+      Starts the listener.
+
+      This starts up a background thread to monitor the queue for
+      LogRecords to process.
+
+   .. method:: stop()
+
+      Stops the listener.
+
+      This asks the thread to terminate, and then waits for it to do so.
+      Note that if you don't call this before your application exits, there
+      may be some records still left on the queue, which won't be processed.
+
+.. versionadded:: 3.2
+
+The :class:`QueueListener` class was not present in previous versions.
+
 .. _zeromq-handlers:
 
+Subclassing QueueHandler
+^^^^^^^^^^^^^^^^^^^^^^^^
+
 You can use a :class:`QueueHandler` subclass to send messages to other kinds
 of queues, for example a ZeroMQ "publish" socket. In the example below,the
 socket is created separately and passed to the handler (as its 'queue')::
@@ -2716,6 +2794,7 @@ data needed by the handler to create the socket::
         def __init__(self, uri, socktype=zmq.PUB, ctx=None):
             self.ctx = ctx or zmq.Context()
             socket = zmq.Socket(self.ctx, socktype)
+            socket.bind(uri)
             QueueHandler.__init__(self, socket)
 
         def enqueue(self, record):
@@ -2725,6 +2804,23 @@ data needed by the handler to create the socket::
         def close(self):
             self.queue.close()
 
+Subclassing QueueListener
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+You can also subclass :class:`QueueListener` to get messages from other kinds
+of queues, for example a ZeroMQ "subscribe" socket. Here's an example::
+
+    class ZeroMQSocketListener(QueueListener):
+        def __init__(self, uri, *handlers, **kwargs):
+            self.ctx = kwargs.get('ctx') or zmq.Context()
+            socket = zmq.Socket(self.ctx, zmq.SUB)
+            socket.setsockopt(zmq.SUBSCRIBE, '') # subscribe to everything
+            socket.connect(uri)
+
+        def dequeue(self):
+            msg = self.queue.recv()
+            return logging.makeLogRecord(json.loads(msg))
+
 .. _formatter-objects:
 
 Formatter Objects

Lib/logging/handlers.py

@@ -1178,8 +1178,8 @@ class QueueHandler(logging.Handler):
 
     def prepare(self, record):
         """
-        Prepares a record for queuing. The object returned by this
-        method is enqueued.
+        Prepares a record for queuing. The object returned by this method is
+        enqueued.
 
         The base implementation formats the record to merge the message
         and arguments, and removes unpickleable items from the record
@@ -1205,7 +1205,7 @@ class QueueHandler(logging.Handler):
         """
         Emit a record.
 
-        Writes the LogRecord to the queue, preparing it first.
+        Writes the LogRecord to the queue, preparing it for pickling first.
         """
         try:
             self.enqueue(self.prepare(record))
@@ -1213,3 +1213,107 @@ class QueueHandler(logging.Handler):
             raise
         except:
             self.handleError(record)
+
+class QueueListener(object):
+    """
+    This class implements an internal threaded listener which watches for
+    LogRecords being added to a queue, removes them and passes them to a
+    list of handlers for processing.
+    """
+    _sentinel = None
+
+    def __init__(self, queue, *handlers):
+        """
+        Initialise an instance with the specified queue and
+        handlers.
+        """
+        self.queue = queue
+        self.handlers = handlers
+        self._stop = threading.Event()
+        self._thread = None
+
+    def dequeue(self, block):
+        """
+        Dequeue a record and return it, optionally blocking.
+
+        The base implementation uses get. You may want to override this method
+        if you want to use timeouts or work with custom queue implementations.
+        """
+        return self.queue.get(block)
+
+    def start(self):
+        """
+        Start the listener.
+
+        This starts up a background thread to monitor the queue for
+        LogRecords to process.
+        """
+        self._thread = t = threading.Thread(target=self._monitor)
+        t.setDaemon(True)
+        t.start()
+
+    def prepare(self , record):
+        """
+        Prepare a record for handling.
+
+        This method just returns the passed-in record. You may want to
+        override this method if you need to do any custom marshalling or
+        manipulation of the record before passing it to the handlers.
+        """
+        return record
+
+    def handle(self, record):
+        """
+        Handle a record.
+
+        This just loops through the handlers offering them the record
+        to handle.
+        """
+        record = self.prepare(record)
+        for handler in self.handlers:
+            handler.handle(record)
+
+    def _monitor(self):
+        """
+        Monitor the queue for records, and ask the handler
+        to deal with them.
+
+        This method runs on a separate, internal thread.
+        The thread will terminate if it sees a sentinel object in the queue.
+        """
+        q = self.queue
+        has_task_done = hasattr(q, 'task_done')
+        while not self._stop.isSet():
+            try:
+                record = self.dequeue(True)
+                if record is self._sentinel:
+                    break
+                self.handle(record)
+                if has_task_done:
+                    q.task_done()
+            except queue.Empty:
+                pass
+        # There might still be records in the queue.
+        while True:
+            try:
+                record = self.dequeue(False)
+                if record is self._sentinel:
+                    break
+                self.handle(record)
+                if has_task_done:
+                    q.task_done()
+            except queue.Empty:
+                break
+
+    def stop(self):
+        """
+        Stop the listener.
+
+        This asks the thread to terminate, and then waits for it to do so.
+        Note that if you don't call this before your application exits, there
+        may be some records still left on the queue, which won't be processed.
+        """
+        self._stop.set()
+        self.queue.put_nowait(self._sentinel)
+        self._thread.join()
+        self._thread = None

Misc/NEWS

@@ -62,6 +62,9 @@ Core and Builtins
 Library
 -------
 
+- Logging: Added QueueListener class to facilitate logging usage for
+  performance-critical threads.
+
 - Issue #9916: Add some missing errno symbols.
 
 - Issue #9877: Expose sysconfig.get_makefile_filename()