From 9bba174327b9d3fd5e431f9761d6b8db5c99ef96 Mon Sep 17 00:00:00 2001 From: Vinay Sajip Date: Mon, 7 Jul 2025 10:01:03 +0100 Subject: [PATCH] gh-94503: Update logging cookbook with an example of uniformly handling newlines in output. (GH-136217) (cherry picked from commit d05423a90ce0ee9ad5207dce3dd06ab2397f3d6e) Co-authored-by: Vinay Sajip --- Doc/howto/logging-cookbook.rst | 62 ++++++++++++++++++++++++++++++++++ 1 file changed, 62 insertions(+) diff --git a/Doc/howto/logging-cookbook.rst b/Doc/howto/logging-cookbook.rst index 7d64a02358adb3..ae2697fbce30ad 100644 --- a/Doc/howto/logging-cookbook.rst +++ b/Doc/howto/logging-cookbook.rst @@ -4078,6 +4078,68 @@ lines. With this approach, you get better output: WARNING:demo: 1/0 WARNING:demo:ZeroDivisionError: division by zero +How to uniformly handle newlines in logging output +-------------------------------------------------- + +Usually, messages that are logged (say to console or file) consist of a single +line of text. However, sometimes there is a need to handle messages with +multiple lines - whether because a logging format string contains newlines, or +logged data contains newlines. If you want to handle such messages uniformly, so +that each line in the logged message appears uniformly formatted as if it was +logged separately, you can do this using a handler mixin, as in the following +snippet: + +.. code-block:: python + + # Assume this is in a module mymixins.py + import copy + + class MultilineMixin: + def emit(self, record): + s = record.getMessage() + if '\n' not in s: + super().emit(record) + else: + lines = s.splitlines() + rec = copy.copy(record) + rec.args = None + for line in lines: + rec.msg = line + super().emit(rec) + +You can use the mixin as in the following script: + +.. code-block:: python + + import logging + + from mymixins import MultilineMixin + + logger = logging.getLogger(__name__) + + class StreamHandler(MultilineMixin, logging.StreamHandler): + pass + + if __name__ == '__main__': + logging.basicConfig(level=logging.DEBUG, format='%(asctime)s %(levelname)-9s %(message)s', + handlers = [StreamHandler()]) + logger.debug('Single line') + logger.debug('Multiple lines:\nfool me once ...') + logger.debug('Another single line') + logger.debug('Multiple lines:\n%s', 'fool me ...\ncan\'t get fooled again') + +The script, when run, prints something like: + +.. code-block:: text + + 2025-07-02 13:54:47,234 DEBUG Single line + 2025-07-02 13:54:47,234 DEBUG Multiple lines: + 2025-07-02 13:54:47,234 DEBUG fool me once ... + 2025-07-02 13:54:47,234 DEBUG Another single line + 2025-07-02 13:54:47,234 DEBUG Multiple lines: + 2025-07-02 13:54:47,234 DEBUG fool me ... + 2025-07-02 13:54:47,234 DEBUG can't get fooled again + .. patterns-to-avoid: pFad - Phonifier reborn

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