python3 - custom logger not working without basicConfig method?

Question

I've got this code:

    def __init__(self, log_level: str = None):
        self.log_level = log_level if log_level and log_level.lower() in ['debug', 'warning', 'info',
                                                                          'error'] else 'error'
        self.logger = logging.getLogger(__name__)
        self.logger.setLevel(getattr(logging, self.log_level.upper()))

And i want to create simple logger without configurations. This code is not working without this line: logging.basicConfig()

Why? I can't really understand. So if you try something like this:

self.logger = logging.getLogger(__name__)
self.logger.setLevel(logging.DEBUG)
self.logger.debug('test')

it would not print anything. But this will:

logging.basicConfig()
self.logger = logging.getLogger(__name__)
self.logger.setLevel(logging.DEBUG)
self.logger.debug('test')

or this:

self.logger = logging.getLogger(__name__)
self.logger.setLevel(logging.DEBUG)
logging.debug("don't care")
self.logger.debug('test')

Answer 1

Logger must have defined handler. If you create logger using logging.getLogger(__name__) you must addHandler. Adding handler is part of basicConfig configuration.

import logging

logger = logging.getLogger('test')
logger.addHandler(logging.StreamHandler())  # if you comment this line, logger will not print out message
logger.setLevel(logging.DEBUG)
logger.info('hi')

You can add multiple handles to one logger. This example will print 2 messages (becasue there are two StreamHandlers).

import logging

logger = logging.getLogger('test')
logger.addHandler(logging.StreamHandler()) 
logger.addHandler(logging.StreamHandler()) 
logger.setLevel(logging.DEBUG)
logger.info('hi')

Each hadnler can have different formater, different logging_level, different output. This is usefull if you want to print in console logs from all levels, but to file only wirte critical and error level messages.