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bpo-32821: Add console logging cookbook recipe #5625

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aec88d7
Add console logging cookbook
mariocj89 Feb 11, 2018
a9cf04e
Add whatsnew entry
mariocj89 Feb 11, 2018
89ab944
Remove unglorious tabs
mariocj89 Feb 12, 2018
7d700fa
Fix Max level filter specification
mariocj89 Apr 23, 2019
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37 changes: 37 additions & 0 deletions Doc/howto/logging-cookbook.rst
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Expand Up @@ -2523,3 +2523,40 @@ 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.


Logging to console stderr and stdout
------------------------------------

Some applications want to log to ``stdout`` and ``stderr`` based on the level
of the ``LogRecord``. To be able to do so we need two ``StreamHandler``
pointing to the two streams, but we want all messages **above** INFO to not
go to ``stderr``, as otherwise ``ERROR`` messages will appear twice in the console.
This can be achieved with a filter as shown below::

import logging

class MaxLevelFilter:
def __init__(self, max_level=None):
self.max_level = max_level

def filter(self, record):
return record.levelno <= self.max_level


import sys
logger = logging.getLogger()
logger.setLevel(logging.DEBUG)
stdout = logging.StreamHandler(sys.stdout)
stdout.setLevel("DEBUG")
stdout.addFilter(MaxLevelFilter(logging.INFO))
stderr = logging.StreamHandler(sys.stderr)
stderr.setLevel(logging.WARNING)
logger.addHandler(stdout)
logger.addHandler(stderr)

logger.info('INFO')
logger.error('ERROR')

This will create an app that logs ``INFO`` and below to ``stdout`` and
``WARN`` and above to ``stderr``.
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Documentation/2018-02-11-17-13-04.bpo-32821.Up7EGI.rst
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@@ -0,0 +1 @@
Add recipe in logging cookbook on configuring a "Console" handler.