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user_id=308438

I just had a further thought: is the approach below any good 
to you? Apart from not being able to use the root logger, it 
seems to meet your need.

import logging

class MyLogger(logging.Logger):
  def makeRecord(self, name, level, fn, lno, msg, args, 
exc_info):
    record = logging.Logger.makeRecord(self, name, level, fn, 
lno, msg, args, exc_info)
    record.magicnumber = 0xDECAFBAD    # special number
    return record

logging._loggerClass = MyLogger

h = logging.StreamHandler()
logger = logging.getLogger("mylogger")
h.setFormatter(logging.Formatter("%(asctime)s %(levelname)
s %(magicnumber)X %(message)s"))
logger.addHandler(h)
logger.warn("There's a custom attribute in my message!")