Python Logging.get_Logger(name) With FileHandler Does Not Write To File

If I get the logger with a name and add a FileHandler it does not write to the file. This works and writes correctly to the file: log = logging.getLogger() fh = logging.FileHandler

Solution 1:

This is a rather old question, but I believe I found the underlying problem and solution, at least with a newer version of Python.

The second code example starts with log = logging.getLogger(name), where name is presumed to be a string representing the name of the logger. Since a name is provided, this log will not be the root logger. According to Logger.setLevel(level) docs for Python 3.6+,

When a logger is created, the level is set to NOTSET (which causes all messages to be processed when the logger is the root logger, or delegation to the parent when the logger is a non-root logger).

This tells us that we have to set the level of our logger so that it will actually process the messages instead of passing it to the root logger.

This is a code example I wrote (in Python 3.7) that does not work:

from pathlib import Path
import logging

formatter = logging.Formatter('%(name)s [%(levelname)s] %(message)s')
log_file_dir = Path('./log/')
config_file = 'config_file.txt'
config_file_path = log_file_dir / config_file

logger = logging.getLogger('example_logger')
fh = logging.FileHandler(config_file_path, mode='w')
fh.setLevel(logging.INFO)
fh.setFormatter(formatter)
logger.addHandler(fh)
logger.info('Start Configuration Log')

And this one works by adding one line:

from pathlib import Path
import logging

formatter = logging.Formatter('%(name)s [%(levelname)s] %(message)s')
log_file_dir = Path('./log/')
config_file = 'config_file_2.txt'
config_file_path = log_file_dir / config_file

logger = logging.getLogger('example_logger')
logger.setLevel(logging.INFO) # <------ Or the applicable level for your use-case
fh = logging.FileHandler(config_file_path, mode='w')
fh.setLevel(logging.INFO)
fh.setFormatter(formatter)
logger.addHandler(fh)
logger.info('Start Configuration Log')

Note: The first code example does create the chosen log file, but does not write 'Start Configuration Log' to the file.