Python Logging Duplicate Output

Question

I am getting duplicate output for Python logging with custom logger. Below is the section of logging code and output. For some reason if I remove the logger.setLevel(logging.DEBUG) line, the logger doesn't seem to respect the console_handler.setLevel(logging.DEBUG) line, and there is no output at all.

Code

...
args = parser.parse_args()

if args.log:
    # Create custom logger
    logger = logging.getLogger('a')
    logger.setLevel(logging.DEBUG)
    # Create handler
    console_handler = logging.StreamHandler()
    console_handler.setLevel(logging.DEBUG)
    # Create formatter and add it to handler
    console_formatter = logging.Formatter('%(asctime)s - %(levelname)s - %(message)s')
    console_handler.setFormatter(console_formatter)
    # Add handler to logger
    logger.addHandler(console_handler)

    try:
        ...
    except IndexError as ie:
        logger.info(f'Test')
        ...
    except Exception as ex:
        logger.error(f'Exception: {ex}')

Output

2022-07-11 11:29:59,409 - INFO - Test
INFO:a:Test

My guess is that some code somewhere (possibly from a module that you are importing) is calling logging.basicConfig. By default, that function uses a formatter that gives logs like INFO:a:Test. — 0x5453
– 0x5453, Commented Jul 11, 2022 at 18:41

ItayB · Accepted Answer · 2022-07-11 18:56:58Z

7

As far as I remember the setting propagates to higher level (ancestor) loggers. You can read about it here.

I would suggest adding logger.propagate = False after initializing logger a:

logger = logging.getLogger('a')
logger.propagate = False
logger.setLevel(logging.DEBUG)

answered Jul 11, 2022 at 18:56

ItayB

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2 Comments

YTKme Over a year ago

thank you for the help! apparently it is the propagate property, which kind of makes me wonder why would this be a default behavior to auto propagate to ancestor loggers.

alanng Over a year ago

Important subtlety: setting logger.propagate = False inside a separate function where the logger is created (containing the logging.getLogger() call) does not work. But setting logger.propagate = False after invoking such a create_logger() function does work.

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