[New-bugs-announce] [issue42378] logging reopens file with same mode, possibly truncating

[Ed Catmur]

New submission from Ed Catmur <ed at catmur.uk>:

If a logging.FileHandler is configured with mode='w', or if logging.basicConfig is called with filemode='w' (as suggested by the Basic Logging Tutorial https://docs.python.org/3/howto/logging.html#logging-basic-tutorial)
and if some code logs during shutdown, after logging has closed its handlers (asyncio is prone to do this, e.g.),
then logging.FileHandler._open will reopen the log file with the same mode as it was originally opened with, potentially truncating it and losing valuable information:

import atexit
atexit.register(lambda: logging.info("so long"))
import logging
logging.basicConfig(filename='test.log', filemode='w', level=logging.INFO)
logging.info("super important stuff")

$ python truncate.py
$ cat test.log
INFO:root:so long

I have a hunch that the fix to issue 26789 will potentially make things worse, as previously at least there was a chance that logging had been fully unloaded so that the call to open would fail.

https://stackoverflow.com/questions/39838616/why-is-python-logging-framework-losing-messages

----------
components: Library (Lib)
messages: 381146
nosy: ecatmur2
priority: normal
severity: normal
status: open
title: logging reopens file with same mode, possibly truncating
type: behavior
versions: Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42378>
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