Python Logging Cookbook

A LogRecord propagates up the logger hierarchy (unless propagate=False). Each handler gates by its own level + filters; the logger's level is only an initial cutoff.

Handler routing with level caps

logger = logging.getLogger('app'); logger.setLevel(logging.DEBUG)
fh = logging.FileHandler('debug.log');   fh.setLevel(logging.DEBUG)
ch = logging.StreamHandler();            ch.setLevel(logging.ERROR)

class MaxLevel(logging.Filter):
    def __init__(self, lvl): self.lvl = lvl
    def filter(self, r):     return r.levelno <= self.lvl
fh.addFilter(MaxLevel(logging.WARNING))   # file: DEBUG–WARNING only
logger.addHandler(fh); logger.addHandler(ch)

QueueHandler + QueueListener (non-blocking)