Merged documentation update from 3.5.

Doc/howto/logging-cookbook.rst

@@ -2409,3 +2409,105 @@ When this script is run, it should print something like::
 
 showing how the time is formatted both as local time and UTC, one for each
 handler.
+
+
+.. _context-manager:
+
+Using a context manager for selective logging
+---------------------------------------------
+
+There are times when it would be useful to temporarily change the logging
+configuration and revert it back after doing something. For this, a context
+manager is the most obvious way of saving and restoring the logging context.
+Here is a simple example of such a context manager, which allows you to
+optionally change the logging level and add a logging handler purely in the
+scope of the context manager::
+
+    import logging
+    import sys
+
+    class LoggingContext(object):
+        def __init__(self, logger, level=None, handler=None, close=True):
+            self.logger = logger
+            self.level = level
+            self.handler = handler
+            self.close = close
+
+        def __enter__(self):
+            if self.level is not None:
+                self.old_level = self.logger.level
+                self.logger.setLevel(self.level)
+            if self.handler:
+                self.logger.addHandler(self.handler)
+
+        def __exit__(self, et, ev, tb):
+            if self.level is not None:
+                self.logger.setLevel(self.old_level)
+            if self.handler:
+                self.logger.removeHandler(self.handler)
+            if self.handler and self.close:
+                self.handler.close()
+            # implicit return of None => don't swallow exceptions
+
+If you specify a level value, the logger's level is set to that value in the
+scope of the with block covered by the context manager. If you specify a
+handler, it is added to the logger on entry to the block and removed on exit
+from the block. You can also ask the manager to close the handler for you on
+block exit - you could do this if you don't need the handler any more.
+
+To illustrate how it works, we can add the following block of code to the
+above::
+
+    if __name__ == '__main__':
+        logger = logging.getLogger('foo')
+        logger.addHandler(logging.StreamHandler())
+        logger.setLevel(logging.INFO)
+        logger.info('1. This should appear just once on stderr.')
+        logger.debug('2. This should not appear.')
+        with LoggingContext(logger, level=logging.DEBUG):
+            logger.debug('3. This should appear once on stderr.')
+        logger.debug('4. This should not appear.')
+        h = logging.StreamHandler(sys.stdout)
+        with LoggingContext(logger, level=logging.DEBUG, handler=h, close=True):
+            logger.debug('5. This should appear twice - once on stderr and once on stdout.')
+        logger.info('6. This should appear just once on stderr.')
+        logger.debug('7. This should not appear.')
+
+We initially set the logger's level to ``INFO``, so message #1 appears and
+message #2 doesn't. We then change the level to ``DEBUG`` temporarily in the
+following ``with`` block, and so message #3 appears. After the block exits, the
+logger's level is restored to ``INFO`` and so message #4 doesn't appear. In the
+next ``with`` block, we set the level to ``DEBUG`` again but also add a handler
+writing to ``sys.stdout``. Thus, message #5 appears twice on the console (once
+via ``stderr`` and once via ``stdout``). After the ``with`` statement's
+completion, the status is as it was before so message #6 appears (like message
+#1) whereas message #7 doesn't (just like message #2).
+
+If we run the resulting script, the result is as follows::
+
+    $ python logctx.py
+    1. This should appear just once on stderr.
+    3. This should appear once on stderr.
+    5. This should appear twice - once on stderr and once on stdout.
+    5. This should appear twice - once on stderr and once on stdout.
+    6. This should appear just once on stderr.
+
+If we run it again, but pipe ``stderr`` to ``/dev/null``, we see the following,
+which is the only message written to ``stdout``::
+
+    $ python logctx.py 2>/dev/null
+    5. This should appear twice - once on stderr and once on stdout.
+
+Once again, but piping ``stdout`` to ``/dev/null``, we get::
+
+    $ python logctx.py >/dev/null
+    1. This should appear just once on stderr.
+    3. This should appear once on stderr.
+    5. This should appear twice - once on stderr and once on stdout.
+    6. This should appear just once on stderr.
+
+In this case, the message #5 printed to ``stdout`` doesn't appear, as expected.
+
+Of course, the approach described here can be generalised, for example to attach
+logging filters temporarily. Note that the above code works in Python 2 as well
+as Python 3.