Why can't I change the level of python logger?

The problem is that the default logging handler handles only the default logging levels. You have to add a logging handler that does the actual logging. If you need just console logging, you could use:

logger = logging.getLogger('a')
logger.addHandler(logging.StreamHandler())
logger.setLevel(logging.DEBUG)
...

This gets you:

log level 10
Debug
Info
Warning
Error

logging.basicConfig() adds a handler with some basic formatting, but usually you will define your own format for logging.

One way to solve this is to set the basic config before logging the messages. Like this:

logging.basicConfig()

Although you've set the level of the logger, the message has to pass through a handler which has a log level also. The rule is that the most restrictive log level wins. There are several solutions. Broadly you can use basicConfig to setup the logging root and it will be inherited by child loggers like "a". Or set a handler for "a".

import logging

#50 CRITICAL
#40 ERROR
#30 WARNING <-- default
#20 INFO
#10 DEBUG
# 0 NOTSET 

# option - set a basic config for root logger and its descendents
#logging.basicConfig(level=logging.DEBUG)
logger = logging.getLogger('a')
logger.setLevel(logging.DEBUG)

# option = create a handler for this logger
handler = logging.StreamHandler()
handler.setLevel(logging.DEBUG)
logger.addHandler(handler)

print('log level', logger.getEffectiveLevel())
logger.debug('Debug') # 10
logger.info('Info') # 20
logger.warning('Warning') # 30
logger.error('Error') # 40