If you define a FileHandler in your logging dictConfig but fail to use it in the same configuration, the file handle is leaked and a ResourceWarning is thrown.

Versions:

    $ python3 -V
    Python 3.4.0
    $ apt-cache show python3.4 | grep ^Version
    Version: 3.4.0-2ubuntu1

Expected:

    $ PYTHONWARNINGS="default,ignore::DeprecationWarning" \
        python3 problem.py 
    imported once

Observed:

    $ PYTHONWARNINGS="default,ignore::DeprecationWarning" \
        python3 problem.py 
    .../lib/python3.4/importlib/_bootstrap.py:656:
        ResourceWarning: unclosed file
          <_io.FileIO name='/tmp/debug.log' mode='ab'>
      code = marshal.loads(data)
    imported once

To reproduce, save below as problem.py:

    if __name__ == '__main__':
        LOGGING = {
            'version': 1,
            'handlers': {
                'logfile': {
                    'level': 'DEBUG',
                    'class': 'logging.FileHandler',
                    'filename': '/tmp/debug.log',
                },
            },
        }
        
        from logging.config import dictConfig
        dictConfig(LOGGING)
        
        # using importlib on a new file triggers the warnings
        import importlib, shutil, os
        shutil.copy(__file__, __file__ + '.new')
        os.unlink(__file__)
        os.rename(__file__ + '.new', __file__)
        importlib.import_module('problem')
    else:
        print('imported once')


Fixed by:

    --- /usr/lib/python3.4/logging/config.py.orig	2014-12-08 14:06:24.911460799 +0100
    +++ /usr/lib/python3.4/logging/config.py	2014-12-08 14:16:17.519460799 +0100
    @@ -637,6 +637,16 @@ class DictConfigurator(BaseConfigurator)
                         except Exception as e:
                             raise ValueError('Unable to configure root '
                                              'logger: %s' % e)
    +
    +                # Close unused loggers
    +                used_handlers = set()
    +                for logger in logging.root.manager.loggerDict.values():
    +                    if hasattr(logger, 'handlers') and logger.handlers:
    +                        used_handlers.add(*logger.handlers)
    +                for name, handler in handlers.items():
    +                    if handler not in used_handlers:
    +                        if hasattr(handler, 'close'):
    +                            handler.close()
             finally:
                 logging._releaseLock()
     

The fix above might not be the best fix, but it proves where the problem is. Perhaps the handlers shouldn't be instantiated until the first user of such a handler is instantiated.

This issue was found by using Django 1.7 with Python 3.4 when enabling the PYTHONWARNINGS.

Cheers,
Walter Doekes
OSSO B.V.