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

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

right from the updated docs, it cover this case pretty well now.

http://docs.python.org/howto/logging.html#logging-advanced-tutorial

import sys # Add this.
import logging

# create logger
logger = logging.getLogger('simple_example')
logger.setLevel(logging.DEBUG)

# create console handler and set level to debug
ch = logging.StreamHandler( sys.__stdout__ ) # Add this
ch.setLevel(logging.DEBUG)

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

# add formatter to ch
ch.setFormatter(formatter)

# add ch to logger
logger.addHandler(ch)

# 'application' code
logger.debug('debug message')
logger.info('info message')
logger.warn('warn message')
logger.error('error message')
logger.critical('critical message')

i’ve mentioned on comments the two changes required from the example to make the output go to stdout. you may also use filters to redirect depending on the level.

more information to understand the changes is at
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 = "%sn"

            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)

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.

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
import logging
import sys

class LessThenFilter(logging.Filter):
    def __init__(self, level):
        self._level = level
        logging.Filter.__init__(self)

    def filter(self, rec):
        return rec.levelno < self._level

log = logging.getLogger()
log.setLevel(logging.NOTSET)

sh_out = logging.StreamHandler(stream=sys.stdout)
sh_out.setLevel(logging.DEBUG)
sh_out.setFormatter(logging.Formatter('%(levelname)s: %(message)s'))
sh_out.addFilter(LessThenFilter(logging.WARNING))
log.addHandler(sh_out)

sh_err = logging.StreamHandler(stream=sys.stderr)
sh_err.setLevel(logging.WARNING)
sh_err.setFormatter(logging.Formatter('%(levelname)s: %(message)s'))
log.addHandler(sh_err)

logging.critical('x')
logging.error('x')
logging.warning('x')
logging.info('x')
logging.debug('x')

Just for your convenience adding everything together with the formatter in one package:

# shared formatter, but you can use separate ones:
FORMAT = '%(asctime)s - %(name)s - %(levelname)s - %(threadName)s - %(message)s'
formatter = logging.Formatter(FORMAT)

# single app logger:
log = logging.getLogger(__name__)
log.setLevel(logging.INFO)

# 2 handlers for the same logger:
h1 = logging.StreamHandler(sys.stdout)
h1.setLevel(logging.DEBUG)
# filter out everything that is above INFO level (WARN, ERROR, ...)
h1.addFilter(lambda record: record.levelno <= logging.INFO)
h1.setFormatter(formatter)
log.addHandler(h1)

h2 = logging.StreamHandler(sys.stderr)
# take only warnings and error logs
h2.setLevel(logging.WARNING)
h2.setFormatter(formatter)
log.addHandler(h2)

# profit:
log.info(...)
log.debug(...)

My use case was to redirect stdout to a datafile while seeing errors on the screen during processing.

Here’s the compact solution I use (tested with Python 3.10):

import logging
import sys

root_logger = logging.getLogger()

# configure default StreamHandler to stderr
logging.basicConfig(
    format="%(asctime)s | %(levelname)-8s | %(filename)s:%(funcName)s(%(lineno)d) | %(message)s",
    level=logging.INFO,  # overall log level; DEBUG or INFO make most sense
)
stderr_handler = root_logger.handlers[0]  # get default handler
stderr_handler.setLevel(logging.WARNING)  # only print WARNING+

# add another StreamHandler to stdout which only emits below WARNING
stdout_handler = logging.StreamHandler(sys.stdout)
stdout_handler.addFilter(lambda rec: rec.levelno < logging.WARNING)
stdout_handler.setFormatter(stderr_handler.formatter)  # reuse the stderr formatter
root_logger.addHandler(stdout_handler)

You will have to associated a handler to each stream you want logged statements to be sent to.
Python has 5 logging levels – Critical, Error, Warning, Info, and Debug.

Let’s assume you want that Critical and Error logs should go to STDERROR, while others should go to STDOUT – as shown in the table above.

Here are the steps.

1. Init logging module

   import logging, sys
   log = logging.getLogger()

2. Create two streams, one attached to STDERROR,
   other to STDOUT

   hStErr = logging.StreamHandler(sys.stderr)
   hStOut = logging.StreamHandler(sys.stdout)

3. Set log level for each handler. This tells handler to only log statements which have log level equal to or higher than the level we set. eg. if we set this to WARNING, the handler will only handle logging statements for WARNING,ERROR, and CRITICAL levels. Also make sure that the log level is set in handler only, and not with the log object, so that there is no conflict

   hStErr.setLevel(‘ERROR’)
   hStOut.setLevel(‘DEBUG’)
   log.setLevel(‘NOTSET’)

Now here comes a catch. While hStErr will only output logging for ERROR and CRITICAL logging levels, hStOut will output for all 5 levels. Remember that the level only tells the minimum logging level which should be handled, so all levels which are greater will also be handled. To limit hStOut to not handle ERROR and CRITICAL, we use filter.

4. Specify filter so that ERROR, and CRITICAL aren’t handled by hStOut

   hStOut.addFilter(lambda x : x.levelno < logging.ERROR)

5. Add these handlers to logger

   log.addHandler(hStErr)
   log.addHandler(hStOut)

Here are all pieces together.

import logging, sys
log = logging.getLogger()
hStErr = logging.StreamHandler(sys.stderr)
hStOut = logging.StreamHandler(sys.stdout)

hStErr.setLevel('ERROR')
hStOut.setLevel('DEBUG')
log.setLevel('NOTSET')

hStOut.addFilter(lambda x : x.levelno < logging.ERROR)

log.addHandler(hStErr)
log.addHandler(hStOut)

log.error("error log")
log.info("info log")

Output when we run this script.

error log
info log

Pycharm IDE colors output from std error red. The following image shows that the error log statement above was sent to stderr.

If we comment the addFilter line in above script, we will see the following output.

error log
error log
info log

Note that without filter hStOut will output logging statements from both INFO and ERROR, while for INFO hStErr outputs nothing, and hStOut outputs a single statement – info log