logging: Added QueueHandler.prepare and updated documentation.

Doc/library/logging.rst

@@ -944,6 +944,7 @@ Loggers have the following attributes and methods. Note that Loggers are never
 instantiated directly, but always through the module-level function
 ``logging.getLogger(name)``.
 
+.. class:: Logger
 
 .. attribute:: Logger.propagate
 
@@ -2661,7 +2662,20 @@ supports sending logging messages to a queue, such as those implemented in the
 
    .. method:: emit(record)
 
-      Sends the record to the handler's queue.
+      Enqueues the result of preparing the LogRecord.
+
+   .. method:: prepare(record)
+
+      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
+      in-place.
+
+      You might want to override this method if you want to convert
+      the record to a dict or JSON string, or send a modified copy
+      of the record while leaving the original intact.
 
    .. method:: enqueue(record)
 

Lib/logging/handlers.py

@@ -1176,13 +1176,19 @@ class QueueHandler(logging.Handler):
         """
         self.queue.put_nowait(record)
 
-    def emit(self, record):
+    def prepare(self, record):
         """
-        Emit a record.
+        Prepares a record for queuing. The object returned by this
+        method is enqueued.
 
-        Writes the LogRecord to the queue, preparing it for pickling first.
+        The base implementation formats the record to merge the message
+        and arguments, and removes unpickleable items from the record
+        in-place.
+
+        You might want to override this method if you want to convert
+        the record to a dict or JSON string, or send a modified copy
+        of the record while leaving the original intact.
         """
-        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
@@ -1193,7 +1199,16 @@ class QueueHandler(logging.Handler):
         record.msg = record.message
         record.args = None
         record.exc_info = None
-            self.enqueue(record)
+        return record
+
+    def emit(self, record):
+        """
+        Emit a record.
+
+        Writes the LogRecord to the queue, preparing it first.
+        """
+        try:
+            self.enqueue(self.prepare(record))
         except (KeyboardInterrupt, SystemExit):
             raise
         except: