Separating stdout and stderr in python's logger

Question

I'm using the following code to separate the errors/warnings from the info/debug messages:

import sys, logging

class StdErrFilter(logging.Filter):
    def filter(self, rec):
        return rec.levelno in (logging.ERROR, logging.WARNING)

class StdOutFilter(logging.Filter):
    def filter(self, rec):
        return rec.levelno in (logging.DEBUG, logging.INFO)

logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)

formatter = logging.Formatter('%(process)s - %(asctime)s - %(name)s - %(levelname)s - %(message)s')

h1 = logging.StreamHandler(sys.stdout)
h1.setLevel(logging.DEBUG)
h1.setFormatter(formatter)
h1.addFilter(StdOutFilter())
logger.addHandler(h1)

h2 = logging.StreamHandler(sys.stderr)
h2.setLevel(logging.WARNING)
h2.setFormatter(formatter)
h2.addFilter(StdErrFilter())
logger.addHandler(h2)

unfortunately it looks like the filters are getting ignored. Both files contain INFO/DEBUG messages... Any ideas?

Answer 1

In UNIX terminal sys.stdout and sys.stderr are the same, which means if you run this script, you will get this:

(venv) 🍀  python a.py
5798 - 2017-05-10  - __main__ - DEBUG - A DEBUG message
5798 - 2017-05-10  - __main__ - INFO - An INFO message
5798 - 2017-05-10  - __main__ - WARNING - A WARNING message
5798 - 2017-05-10  - __main__ - ERROR - An ERROR message

As a matter of fact, maybe you need to redirect sys.stderr and sys.stdout to different files by running this command, you will find that they are actually separated:

(venv) ➜ python a.py  2>> error 1>> info
(venv) 🍀 cat error
5895 - 2017-05-10  - __main__ - WARNING - A WARNING message
5895 - 2017-05-10  - __main__ - ERROR - An ERROR message
(venv) 🍀 cat info
5895 - 2017-05-10  - __main__ - DEBUG - A DEBUG message
5895 - 2017-05-10  - __main__ - INFO - An INFO message