[issue23010] "unclosed file" warning when defining unused logging FileHandler in dictConfig

[Vinay Sajip]

Vinay Sajip <vinay_sajip at yahoo.co.uk> added the comment:

This had dropped off my radar completely, but I still can't see where there's an actual bug here. This simplified script:

import logging
import sys

LOGGING = {
    'version': 1,
    'handlers': {
        'logfile': {
            'level': 'DEBUG',
            'class': 'logging.FileHandler',
            'filename': '/tmp/debug.log',
        },
    },
}

print('Starting: %s' % sys.version)
from logging.config import dictConfig
dictConfig(LOGGING)
print('After dictconfig')
print('_handlerList 1, initial:', logging._handlerList, len(logging._handlers))
import importlib
print('_handlerList 2, about to import shutil:', logging._handlerList, len(logging._handlers))
import shutil
print('_handlerList 3, just imported shutil:', logging._handlerList, len(logging._handlers))
print('')

when run with Python 3.10, produces

Starting: 3.10.0+ (heads/3.10:7203ecd332, Oct 29 2021, 10:04:19) [GCC 7.5.0]
After dictconfig
_handlerList 1, initial: [<weakref at 0x7f3e25e9e070; to 'FileHandler' at 0x7f3e25e63dc0>] 1
_handlerList 2, about to import shutil: [<weakref at 0x7f3e25e9e070; to 'FileHandler' at 0x7f3e25e63dc0>] 1
/home/vinay/.local/lib/python3.10/_compression.py:33: ResourceWarning: unclosed file <_io.FileIO name='/tmp/debug.log' mode='ab' closefd=True>
  class DecompressReader(io.RawIOBase):
ResourceWarning: Enable tracemalloc to get the object allocation traceback
_handlerList 3, just imported shutil: [] 0

But ... there are no loggers that use this handler, so the only reference would be the weak reference in _handlerList - it gets freed up at some point (in this case, when shutil is imported, but I don't believe that's necessarily relevant) and that causes the ResourceWarning, but where's the problem? If you either add a reference to the handler (by adding it to a logger) or adding "delay: True" to the handler configuration dict to delay opening the file, the ResourceWarning is no longer seen.

I tested with Python 3.4 - 3.10 and all versions behave the same way - it could just be down to where weak references get dumped, which is down to the vagaries of GC. I don't see this as a "leak" - the ResourceWarning is showing the developer that they opened a file for no particular reason and didn't use it.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue23010>
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