How can INFO and DEBUG logging message be sent to stdout and higher level message to stderr

import logging
import sys

class LessThanFilter(logging.Filter):
    def __init__(self, exclusive_maximum, name=""):
        super(LessThanFilter, self).__init__(name)
        self.max_level = exclusive_maximum

    def filter(self, record):
        #non-zero return means we log this message
        return 1 if record.levelno < self.max_level else 0

#Get the root logger
logger = logging.getLogger()
#Have to set the root logger level, it defaults to logging.WARNING
logger.setLevel(logging.NOTSET)

logging_handler_out = logging.StreamHandler(sys.stdout)
logging_handler_out.setLevel(logging.DEBUG)
logging_handler_out.addFilter(LessThanFilter(logging.WARNING))
logger.addHandler(logging_handler_out)

logging_handler_err = logging.StreamHandler(sys.stderr)
logging_handler_err.setLevel(logging.WARNING)
logger.addHandler(logging_handler_err)

#demonstrate the logging levels
logger.debug('DEBUG')
logger.info('INFO')
logger.warning('WARNING')
logger.error('ERROR')
logger.critical('CRITICAL')

Implementation aside, I do think it is a good idea to use the logging facilities in python to output to the terminal, in particular because you can add another handler to additionally log to a file. If you set stdout to be INFO instead of DEBUG, you can even include additional DEBUG information that the user wouldn't standardly see in the log file.

Yes. You must define multiple handlers for your logging.

http://docs.python.org/library/logging.html#logging-to-multiple-destinations

http://docs.python.org/library/logging.handlers.html#module-logging.handlers

I had the same problem and wrote a custom logging handler called SplitStreamHandler:

import sys
import logging

class SplitStreamHandler(logging.Handler):
    def __init__(self):
        logging.Handler.__init__(self)

    def emit(self, record):
        # mostly copy-paste from logging.StreamHandler
        try:
            msg = self.format(record)
            if record.levelno < logging.WARNING:
                stream = sys.stdout
            else:
                stream = sys.stderr
            fs = "%s\n"

            try:
                if (isinstance(msg, unicode) and
                    getattr(stream, 'encoding', None)):
                    ufs = fs.decode(stream.encoding)
                    try:
                        stream.write(ufs % msg)
                    except UnicodeEncodeError:
                        stream.write((ufs % msg).encode(stream.encoding))
                else:
                    stream.write(fs % msg)
            except UnicodeError:
                stream.write(fs % msg.encode("UTF-8"))

            stream.flush()
        except (KeyboardInterrupt, SystemExit):
            raise
        except:
            self.handleError(record)