Logging module not writing to file

I add the following lines before the logging.basicConfig() and it worked for me.

for handler in logging.root.handlers[:]:
    logging.root.removeHandler(handler)

You can try running this snippet in your main file.

import logging 
logging.basicConfig(
    level=logging.INFO, 
    format='%(asctime)s [%(levelname)s] - %(message)s',
    filename='filename.txt')  # pass explicit filename here 
logger = logging.get_logger()  # get the root logger
logger.warning('This should go in the file.')
print logger.handlers   # you should have one FileHandler object

If you are using 'root' logger which is by default has name "", than you can do this trick:

logging.getLogger().setLevel(logging.INFO)    
logger = logging.getLogger('')
logger.handlers = []

In addition you may want to specify logging level as in code above, this level will persist for all descendant loggers.

If instead, you specified particular logger, than do

logger = logging.getLogger('my_service')
logger.handlers = []
fh = logging.FileHandler(log_path)
fh.setLevel(logging.INFO)
# create console handler
ch = logging.StreamHandler()
ch.setLevel(logging.INFO)
logger.addHandler(fh)
logger.addHandler(ch)
logger.info('service started')

The above code will create new logger 'my_service'. In case that logger has been already created it clears all handles. That it adds handles for writing in specified file and console. See official documentation as well.

You can also use hierarchical loggers. It is done directly.

logger = logging.getLogger('my_service.update')
logger.info('updated successfully')

In addition to Forge's answer on using logging.basicConfig(), as of Python 3.8 a parameter to basicConfig() got added. To quote the docs:

"""
This function does nothing if the root logger already has handlers 
configured, unless the keyword argument *force* is set to ``True``.
...
force     If this keyword  is specified as true, any existing handlers
          attached to the root logger are removed and closed, before
          carrying out the configuration as specified by the other
          arguments.
"""

This is why yue dong's answer to remove all handlers has worked for some as well as Alexander's answer to reset logger.handlers to [].

Debugging logger.handlers (as Forge's answer suggests) led me to see a single StreamHandler there (so basicConfig() did nothing for me until I used force=True as parameter).

Hope that helps in addition to all the other answers here too!