Improved logging cookbook for logging with multiprocessing.

Doc/howto/logging-cookbook.rst

@@ -807,6 +807,106 @@ the basis for code meeting your own specific requirements::
     if __name__ == '__main__':
         main()
 
+A variant of the above script keeps the logging in the main process, in a
+separate thread::
+
+    import logging
+    import logging.config
+    import logging.handlers
+    from multiprocessing import Process, Queue
+    import random
+    import threading
+    import time
+
+    def logger_thread(q):
+        while True:
+            record = q.get()
+            if record is None:
+                break
+            logger = logging.getLogger(record.name)
+            logger.handle(record)
+
+
+    def worker_process(q):
+        qh = logging.handlers.QueueHandler(q)
+        root = logging.getLogger()
+        root.setLevel(logging.DEBUG)
+        root.addHandler(qh)
+        levels = [logging.DEBUG, logging.INFO, logging.WARNING, logging.ERROR,
+                  logging.CRITICAL]
+        loggers = ['foo', 'foo.bar', 'foo.bar.baz',
+                   'spam', 'spam.ham', 'spam.ham.eggs']
+        for i in range(100):
+            lvl = random.choice(levels)
+            logger = logging.getLogger(random.choice(loggers))
+            logger.log(lvl, 'Message no. %d', i)
+
+    if __name__ == '__main__':
+        q = Queue()
+        d = {
+            'version': 1,
+            'formatters': {
+                'detailed': {
+                    'class': 'logging.Formatter',
+                    'format': '%(asctime)s %(name)-15s %(levelname)-8s %(processName)-10s %(message)s'
+                }
+            },
+            'handlers': {
+                'console': {
+                    'class': 'logging.StreamHandler',
+                    'level': 'INFO',
+                },
+                'file': {
+                    'class': 'logging.FileHandler',
+                    'filename': 'mplog.log',
+                    'mode': 'w',
+                    'formatter': 'detailed',
+                },
+                'foofile': {
+                    'class': 'logging.FileHandler',
+                    'filename': 'mplog-foo.log',
+                    'mode': 'w',
+                    'formatter': 'detailed',
+                },
+                'errors': {
+                    'class': 'logging.FileHandler',
+                    'filename': 'mplog-errors.log',
+                    'mode': 'w',
+                    'level': 'ERROR',
+                    'formatter': 'detailed',
+                },
+            },
+            'loggers': {
+                'foo': {
+                    'handlers' : ['foofile']
+                }
+            },
+            'root': {
+                'level': 'DEBUG',
+                'handlers': ['console', 'file', 'errors']
+            },
+        }
+        workers = []
+        for i in range(5):
+            wp = Process(target=worker_process, name='worker %d' % (i + 1), args=(q,))
+            workers.append(wp)
+            wp.start()
+        logging.config.dictConfig(d)
+        lp = threading.Thread(target=logger_thread, args=(q,))
+        lp.start()
+        # At this point, the main process could do some useful work of its own
+        # Once it's done that, it can wait for the workers to terminate...
+        for wp in workers:
+            wp.join()
+        # And now tell the logging thread to finish up, too
+        q.put(None)
+        lp.join()
+
+This variant shows how you can e.g. apply configuration for particular loggers
+- e.g. the ``foo`` logger has a special handler which stores all events in the
+``foo`` subsystem in a file ``mplog-foo.log``. This will be used by the logging
+machinery in the main process (even though the logging events are generated in
+the worker processes) to direct the messages to the appropriate destinations.
 
 Using file rotation
 -------------------