Logging, StreamHandler and standard streams

Question

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?

Lyrism

If you like an answer, the normal way of responding is to accept it - i.e. mark it as accepted :-) – Judaism

Answer 1

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.

Judaism

Thank you,that is exactly what I need. By the way eclipe highlight the std's. – Lyrism

An answer from the author himself! Neat-o! – Roundlet

Answer 2

Generally, I think it makes sense to redirect messages lower than WARNING to stdout, instead of only INFO messages.

Based on Vinay Sajip's excellent answer, I came up with this:

class MaxLevelFilter(Filter):
    '''Filters (lets through) all messages with level < LEVEL'''
    def __init__(self, level):
        self.level = level

    def filter(self, record):
        return record.levelno < self.level # "<" instead of "<=": since logger.setLevel is inclusive, this should be exclusive


MIN_LEVEL= DEBUG
#...
stdout_hdlr = StreamHandler(sys.stdout)
stderr_hdlr = StreamHandler(sys.stderr)
lower_than_warning= MaxLevelFilter(WARNING)
stdout_hdlr.addFilter( lower_than_warning )     #messages lower than WARNING go to stdout
stdout_hdlr.setLevel( MIN_LEVEL )
stderr_hdlr.setLevel( max(MIN_LEVEL, WARNING) ) #messages >= WARNING ( and >= STDOUT_LOG_LEVEL ) go to stderr
#...

Tun

Answer 3

Since my edit was rejected, here's my answer. @goncalopp's answer is good but doesn't stand alone or work out of the box. Here's my improved version:

import sys, logging


class LogFilter(logging.Filter):
    """Filters (lets through) all messages with level < LEVEL"""
    # https://mcmap.net/q/259390/-logging-streamhandler-and-standard-streams
    def __init__(self, level):
        self.level = level

    def filter(self, record):
        # "<" instead of "<=": since logger.setLevel is inclusive, this should
        # be exclusive
        return record.levelno < self.level

MIN_LEVEL = logging.DEBUG
stdout_hdlr = logging.StreamHandler(sys.stdout)
stderr_hdlr = logging.StreamHandler(sys.stderr)
log_filter = LogFilter(logging.WARNING)
stdout_hdlr.addFilter(log_filter)
stdout_hdlr.setLevel(MIN_LEVEL)
stderr_hdlr.setLevel(max(MIN_LEVEL, logging.WARNING))
# messages lower than WARNING go to stdout
# messages >= WARNING (and >= STDOUT_LOG_LEVEL) go to stderr

rootLogger = logging.getLogger()
rootLogger.addHandler(stdout_hdlr)
rootLogger.addHandler(stderr_hdlr)
logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)

# Example Usage
>>> 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")

Aliform

Answer 4

Simplest handler to send colored output to stderr:

class ColorStderr(logging.StreamHandler):
    def __init__(self):
        class AddColor(logging.Formatter):
            def format(self, record: logging.LogRecord):
                msg = super().format(record)
                # Green/Cyan/Yellow/Red/Redder based on log level:
                color = '\033[1;' + ('32m', '36m', '33m', '31m', '41m')[
                   min(4,int(4 * record.levelno / logging.FATAL))]
                return color + record.levelname + '\033[1;0m: ' + msg
        super().__init__(sys.stderr)
        self.setFormatter(AddColor())

Use with:

logging.basicConfig(level=logging.INFO, handlers=[ColorStderr()])

Or you can even apply the formatter directly to the current log handler, with no need for ColorStderr:

logging.getLogger().handlers[0].setFormatter(AddColor())

Hardigg

What does this notation with the colon do here record: logging.LogRecord? – Celie

@Celie a year later: It's a type hint :) – Celie

Answer 5

Here is my answer (combination of the others):

logger = logging.getLogger(__name__)

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

    def filter(self, record):
        return record.levelno < self.level

info_handler = logging.StreamHandler(stdout)
error_handler = logging.StreamHandler(stderr)

info_handler.addFilter(LogFilter(logging.WARNING))
error_handler.setLevel(max(logging.DEBUG, logging.WARNING))

logger.handlers = [info_handler, error_handler]
logger.setLevel(logging.DEBUG)

Legra

Answer 6

Try This Monkey Patch ~~

import sys
import logging
import threading

def _logging_handle(self, record):
    self.STREAM_LOCKER = getattr(self, "STREAM_LOCKER", threading.RLock())
    if self.stream in (sys.stdout, sys.stderr) and record.levelname in self.FIX_LEVELS:
        try:
            self.STREAM_LOCKER.acquire()
            self.stream = sys.stdout
            self.old_handle(record)
            self.stream = sys.stderr
        finally:
            self.STREAM_LOCKER.release()
    else:
        self.old_handle(record)


def patch_logging_stream(*levels):
    """
    writing some logging level message to sys.stdout

    example:
    patch_logging_stream(logging.INFO, logging.DEBUG)
    logging.getLogger('root').setLevel(logging.DEBUG)

    logging.getLogger('root').debug('test stdout')
    logging.getLogger('root').error('test stderr')
    """
    stream_handler = logging.StreamHandler
    levels = levels or [logging.DEBUG, logging.INFO]
    stream_handler.FIX_LEVELS = [logging.getLevelName(i) for i in levels]
    if hasattr(stream_handler, "old_handle"):
        stream_handler.handle = stream_handler.old_handle
    stream_handler.old_handle = stream_handler.handle
    stream_handler.handle = _logging_handle

Test

#
patch_logging_stream(logging.INFO, logging.DEBUG)
logging.getLogger('root').setLevel(logging.DEBUG)

logging.getLogger('root').debug('test root stdout')
logging.getLogger('root').error('test root stderr')

Test Output

$ python3 test_patch_logging.py 2>/dev/null
DEBUG:root:test root stdout

$ python3 test_patch_logging.py 1>/dev/null
ERROR:root:test root stderr

Turgot