L1ker L1ker - 6 months ago 35
Python Question

Logging, StreamHandler and standard streams

I can't figure out how to log info-level messages to stdout, but everything else to stderr. I already read this http://docs.python.org/library/logging.html. Any suggestion?

Answer

The following script, log1.py:

import logging, sys

class SingleLevelFilter(logging.Filter):
    def __init__(self, passlevel, reject):
        self.passlevel = passlevel
        self.reject = reject

    def filter(self, record):
        if self.reject:
            return (record.levelno != self.passlevel)
        else:
            return (record.levelno == self.passlevel)

h1 = logging.StreamHandler(sys.stdout)
f1 = SingleLevelFilter(logging.INFO, False)
h1.addFilter(f1)
rootLogger = logging.getLogger()
rootLogger.addHandler(h1)
h2 = logging.StreamHandler(sys.stderr)
f2 = SingleLevelFilter(logging.INFO, True)
h2.addFilter(f2)
rootLogger.addHandler(h2)
logger = logging.getLogger("my.logger")
logger.setLevel(logging.DEBUG)
logger.debug("A DEBUG message")
logger.info("An INFO message")
logger.warning("A WARNING message")
logger.error("An ERROR message")
logger.critical("A CRITICAL message")

when run, produces the following results.

C:\temp>log1.py
A DEBUG message
An INFO message
A WARNING message
An ERROR message
A CRITICAL message

As you'd expect, since on a terminal sys.stdout and sys.stderr are the same. Now, let's redirect stdout to a file, tmp:

C:\temp>log1.py >tmp
A DEBUG message
A WARNING message
An ERROR message
A CRITICAL message

So the INFO message has not been printed to the terminal - but the messages directed to sys.stderr have been printed. Let's look at what's in tmp:

C:\temp>type tmp
An INFO message

So that approach appears to do what you want.