logging getLogger does not output - python-3.x

Question

I am trying to setup a logging mechanism for a python module.
Following is the example code that I have written to setup logging
import logging
def init_logger(logger):
formatter = logging.Formatter('%(asctime)s - %(levelname)s - %(filename)s - %(funcName)s - %(message)s')
ch = logging.StreamHandler()
ch.setFormatter(formatter)
ch.setLevel(logging.INFO)
logger.addHandler(ch)
file_handler = logging.FileHandler('test_logging.log')
file_handler.setFormatter(formatter)
file_handler.setLevel(logging.DEBUG)
logger.addHandler(file_handler)
def foo1():
logger = logging.getLogger(__name__)
logger.info('Test Info')
logger.debug('Test Debug')
logger.error('Test Error')
def foo2():
logger = logging.getLogger(__name__)
logger.info('Test Info')
logger.debug('Test Debug')
logger.error('Test Error')
if __name__ == '__main__':
logger = logging.getLogger(__name__)
init_logger(logger)
foo1()
foo2()
I expect the logging to print info level and above to stdout and debug level and above to be written to the log file. But what I see is that only error level is outputted to both stdout and log file.
2019-08-13 11:20:07,775 - ERROR - test_logger.py - foo1 - Test Error
2019-08-13 11:20:07,776 - ERROR - test_logger.py - foo2 - Test Error
As per the documentation getLogger should return the same instance of logger. I even tried to create a new instance for the first time like logger = logging.Logger(__name__) but no luck with that. I am not understanding what am I missing here.

Answer 1

Short answer: you must use logging.basicConfig(level=...) or logger.setLevel in your code.
When you use logging.getLogger('some_name') for the first time you create a new logger with level = NOTSET = 0.
# logging module source code
class Logger(Filterer):
def __init__(self, name, level=NOTSET):
...
logging.NOTSET seems to be a valid level value but it is not. Actually it is illegal value that says that logger is not enabled to log anything and forces logger to use level from parent logger (root logger). This logic is defined in Looger.getEffectiveLevel method:
# logging module source code
def getEffectiveLevel(self):
logger = self
while logger:
if logger.level: # 0 gives False here
return logger.level
logger = logger.parent # 0 makes this line reachable
return NOTSET
Root logger has level=WARNING so newly created loggers inherit this level:
# logging module source code
root = RootLogger(WARNING)
logging.getLogger does not allow you to specify logging level. So you have to use logging.basicConfig to modify root logger or logger.setLevel to modify newly created logger somewhere in the very beginning of the script.
I guess the feature should be documented in logging module guides/documentation.