Got some problems when using logging Filter

[sword]

The logging cookbook gives an Filter example, explainning how to add
contextural info to log. I can't figure out how to filter log from it.

Suppose I have 3 file, a.py, b.py and main.py
#file: a.py
import logging

logger=logging.getLogger(__name__)
def print_log():
    logger.debug("I'm module a")

#file: b.py just like a.py
import logging
logger=logging.getLogger(__name__)
def print_log():
    logger.debug("I'm module b")

#file: main.py
import logging
from logging import Filter
logging.basicConfig(level=logging.DEBUG)
logger=logging.getLogger("main")
logger.debug("This is main process")
logger.addFilter(Filter("a"))

And I expected that the console output would contain main and b module
log only. But it turned out that all logs there.  Is it the problem of
root logger?