Closes #26559: Allow configuring flush-on-close behaviour of MemoryHandler.

Doc/library/logging.handlers.rst

@@ -806,12 +806,18 @@ should, then :meth:`flush` is expected to do the flushing.
       overridden to implement custom flushing strategies.
 
 
-.. class:: MemoryHandler(capacity, flushLevel=ERROR, target=None)
+.. class:: MemoryHandler(capacity, flushLevel=ERROR, target=None, flushOnClose=True)
 
    Returns a new instance of the :class:`MemoryHandler` class. The instance is
    initialized with a buffer size of *capacity*. If *flushLevel* is not specified,
    :const:`ERROR` is used. If no *target* is specified, the target will need to be
-   set using :meth:`setTarget` before this handler does anything useful.
+   set using :meth:`setTarget` before this handler does anything useful. If
+   *flushOnClose* is specified as ``False``, then the buffer is *not* flushed when
+   the handler is closed. If not specified or specified as ``True``, the previous
+   behaviour of flushing the buffer will occur when the handler is closed.
+
+   .. versionchanged:: 3.6
+      The *flushOnClose* parameter was added.
 
 
    .. method:: close()

Lib/logging/handlers.py

@@ -1,4 +1,4 @@
-# Copyright 2001-2015 by Vinay Sajip. All Rights Reserved.
+# Copyright 2001-2016 by Vinay Sajip. All Rights Reserved.
 #
 # Permission to use, copy, modify, and distribute this software and its
 # documentation for any purpose and without fee is hereby granted,
@@ -18,7 +18,7 @@
 Additional handlers for the logging package for Python. The core package is
 based on PEP 282 and comments thereto in comp.lang.python.
 
-Copyright (C) 2001-2015 Vinay Sajip. All Rights Reserved.
+Copyright (C) 2001-2016 Vinay Sajip. All Rights Reserved.
 
 To use, simply 'import logging.handlers' and log away!
 """
@@ -1238,17 +1238,25 @@ class MemoryHandler(BufferingHandler):
     flushing them to a target handler. Flushing occurs whenever the buffer
     is full, or when an event of a certain severity or greater is seen.
     """
-    def __init__(self, capacity, flushLevel=logging.ERROR, target=None):
+    def __init__(self, capacity, flushLevel=logging.ERROR, target=None,
+                 flushOnClose=True):
         """
         Initialize the handler with the buffer size, the level at which
         flushing should occur and an optional target.
 
         Note that without a target being set either here or via setTarget(),
         a MemoryHandler is no use to anyone!
+
+        The ``flushOnClose`` argument is ``True`` for backward compatibility
+        reasons - the old behaviour is that when the handler is closed, the
+        buffer is flushed, even if the flush level hasn't been exceeded nor the
+        capacity exceeded. To prevent this, set ``flushOnClose`` to ``False``.
         """
         BufferingHandler.__init__(self, capacity)
         self.flushLevel = flushLevel
         self.target = target
+        # See Issue #26559 for why this has been added
+        self.flushOnClose = flushOnClose
 
     def shouldFlush(self, record):
         """
@@ -1282,10 +1290,12 @@ class MemoryHandler(BufferingHandler):
 
     def close(self):
         """
-        Flush, set the target to None and lose the buffer.
+        Flush, if appropriately configured, set the target to None and lose the
+        buffer.
         """
         try:
-            self.flush()
+            if self.flushOnClose:
+                self.flush()
         finally:
             self.acquire()
             try:

Lib/test/test_logging.py

@@ -958,7 +958,7 @@ class MemoryHandlerTest(BaseTest):
     def setUp(self):
         BaseTest.setUp(self)
         self.mem_hdlr = logging.handlers.MemoryHandler(10, logging.WARNING,
-                                                        self.root_hdlr)
+                                                       self.root_hdlr)
         self.mem_logger = logging.getLogger('mem')
         self.mem_logger.propagate = 0
         self.mem_logger.addHandler(self.mem_hdlr)
@@ -995,6 +995,36 @@ class MemoryHandlerTest(BaseTest):
         self.mem_logger.debug(self.next_message())
         self.assert_log_lines(lines)
 
+    def test_flush_on_close(self):
+        """
+        Test that the flush-on-close configuration works as expected.
+        """
+        self.mem_logger.debug(self.next_message())
+        self.assert_log_lines([])
+        self.mem_logger.info(self.next_message())
+        self.assert_log_lines([])
+        self.mem_logger.removeHandler(self.mem_hdlr)
+        # Default behaviour is to flush on close. Check that it happens.
+        self.mem_hdlr.close()
+        lines = [
+            ('DEBUG', '1'),
+            ('INFO', '2'),
+        ]
+        self.assert_log_lines(lines)
+        # Now configure for flushing not to be done on close.
+        self.mem_hdlr = logging.handlers.MemoryHandler(10, logging.WARNING,
+                                                       self.root_hdlr,
+                                                       False)
+        self.mem_logger.addHandler(self.mem_hdlr)
+        self.mem_logger.debug(self.next_message())
+        self.assert_log_lines(lines)  # no change
+        self.mem_logger.info(self.next_message())
+        self.assert_log_lines(lines)  # no change
+        self.mem_logger.removeHandler(self.mem_hdlr)
+        self.mem_hdlr.close()
+        # assert that no new lines have been added
+        self.assert_log_lines(lines)  # no change
+
 
 class ExceptionFormatter(logging.Formatter):
     """A special exception formatter."""