How to redirect stdout and stderr to logger in Python

I have a logger that has a RotatingFileHandler. I want to redirect all Stdout and Stderr to the logger. How to do so?

Not enough rep to comment, but I wanted to add the version of this that worked for me in case others are in a similar situation.

class LoggerWriter:
    def __init__(self, level):
        # self.level is really like using log.debug(message)
        # at least in my case
        self.level = level

    def write(self, message):
        # if statement reduces the amount of newlines that are
        # printed to the logger
        if message != '\n':
            self.level(message)

    def flush(self):
        # create a flush method so things can be flushed when
        # the system wants to. Not sure if simply 'printing'
        # sys.stderr is the correct way to do it, but it seemed
        # to work properly for me.
        self.level(sys.stderr)

and this would look something like:

log = logging.getLogger('foobar')
sys.stdout = LoggerWriter(log.debug)
sys.stderr = LoggerWriter(log.warning)

UPDATE for Python 3:

• Including a dummy flush function which prevents an error where the function is expected (Python 2 was fine with just linebuf='').
• Note that your output (and log level) appears different if it is logged from an interpreter session vs being run from a file. Running from a file produces the expected behavior (and output featured below).
• We still eliminate extra newlines which other solutions do not.

class StreamToLogger(object):
    """
    Fake file-like stream object that redirects writes to a logger instance.
    """
    def __init__(self, logger, level):
       self.logger = logger
       self.level = level
       self.linebuf = ''

    def write(self, buf):
       for line in buf.rstrip().splitlines():
          self.logger.log(self.level, line.rstrip())

    def flush(self):
        pass

Then test with something like:

import StreamToLogger
import sys
import logging

logging.basicConfig(
        level=logging.DEBUG,
        format='%(asctime)s:%(levelname)s:%(name)s:%(message)s',
        filename='out.log',
        filemode='a'
        )
log = logging.getLogger('foobar')
sys.stdout = StreamToLogger(log,logging.INFO)
sys.stderr = StreamToLogger(log,logging.ERROR)
print('Test to standard out')
raise Exception('Test to standard error')

See below for old Python 2.x answer and the example output:

All of the prior answers seem to have problems adding extra newlines where they aren't needed. The solution that works best for me is from http://www.electricmonk.nl/log/2011/08/14/redirect-stdout-and-stderr-to-a-logger-in-python/, where he demonstrates how send both stdout and stderr to the logger:

import logging
import sys
 
class StreamToLogger(object):
   """
   Fake file-like stream object that redirects writes to a logger instance.
   """
   def __init__(self, logger, log_level=logging.INFO):
      self.logger = logger
      self.log_level = log_level
      self.linebuf = ''
 
   def write(self, buf):
      for line in buf.rstrip().splitlines():
         self.logger.log(self.log_level, line.rstrip())
 
logging.basicConfig(
   level=logging.DEBUG,
   format='%(asctime)s:%(levelname)s:%(name)s:%(message)s',
   filename="out.log",
   filemode='a'
)
 
stdout_logger = logging.getLogger('STDOUT')
sl = StreamToLogger(stdout_logger, logging.INFO)
sys.stdout = sl
 
stderr_logger = logging.getLogger('STDERR')
sl = StreamToLogger(stderr_logger, logging.ERROR)
sys.stderr = sl
 
print "Test to standard out"
raise Exception('Test to standard error')

The output looks like:

2011-08-14 14:46:20,573:INFO:STDOUT:Test to standard out
2011-08-14 14:46:20,573:ERROR:STDERR:Traceback (most recent call last):
2011-08-14 14:46:20,574:ERROR:STDERR:  File "redirect.py", line 33, in 
2011-08-14 14:46:20,574:ERROR:STDERR:raise Exception('Test to standard error')
2011-08-14 14:46:20,574:ERROR:STDERR:Exception
2011-08-14 14:46:20,574:ERROR:STDERR::
2011-08-14 14:46:20,574:ERROR:STDERR:Test to standard error

Note that self.linebuf = '' is where the flush is being handled, rather than implementing a flush function.

If it's an all-Python system (i.e. no C libraries writing to fds directly, as Ignacio Vazquez-Abrams asked about) then you might be able to use an approach as suggested here:

class LoggerWriter:
    def __init__(self, logger, level):
        self.logger = logger
        self.level = level

    def write(self, message):
        if message != '\n':
            self.logger.log(self.level, message)

and then set sys.stdout and sys.stderr to LoggerWriter instances.

You can use redirect_stdout context manager:

import logging
from contextlib import redirect_stdout

logging.basicConfig(stream=sys.stdout, level=logging.DEBUG)
logging.write = lambda msg: logging.info(msg) if msg != '\n' else None

with redirect_stdout(logging):
    print('Test')

or like this

import logging
from contextlib import redirect_stdout


logger = logging.getLogger('Meow')
logger.setLevel(logging.INFO)
formatter = logging.Formatter(
    fmt='[{name}] {asctime} {levelname}: {message}',
    datefmt='%m/%d/%Y %H:%M:%S',
    style='{'
)
ch = logging.StreamHandler()
ch.setLevel(logging.INFO)
ch.setFormatter(formatter)
logger.addHandler(ch)

logger.write = lambda msg: logger.info(msg) if msg != '\n' else None

with redirect_stdout(logger):
    print('Test')

As an evolution to Cameron Gagnon's response, I've improved the LoggerWriterclass to:

class LoggerWriter(object):
    def __init__(self, writer):
        self._writer = writer
        self._msg = ''

    def write(self, message):
        self._msg = self._msg + message
        while '\n' in self._msg:
            pos = self._msg.find('\n')
            self._writer(self._msg[:pos])
            self._msg = self._msg[pos+1:]

    def flush(self):
        if self._msg != '':
            self._writer(self._msg)
            self._msg = ''

now uncontrolled exceptions look nicer:

2018-07-31 13:20:37,482 - ERROR - Traceback (most recent call last):
2018-07-31 13:20:37,483 - ERROR -   File "mf32.py", line 317, in <module>
2018-07-31 13:20:37,485 - ERROR -     main()
2018-07-31 13:20:37,486 - ERROR -   File "mf32.py", line 289, in main
2018-07-31 13:20:37,488 - ERROR -     int('')
2018-07-31 13:20:37,489 - ERROR - ValueError: invalid literal for int() with base 10: ''

Output Redirection Done Right!

The Problem

logger.log and the other functions (.info/.error/etc.) output each call as a separate line, i.e. implicitly add (formatting and) a newline to it.

sys.stderr.write on the other hand just writes its literal input to stream, including partial lines. For example: The output "ZeroDivisionError: division by zero" is actually 4(!) separate calls to sys.stderr.write:

sys.stderr.write('ZeroDivisionError')
sys.stderr.write(': ')
sys.stderr.write('division by zero')
sys.stderr.write('\n')

The 4 most upvoted approaches (1, 2, 3, 4) thus result in extra newlines -- simply put "1/0" into your program and you will get the following:

2021-02-17 13:10:40,814 - ERROR - ZeroDivisionError
2021-02-17 13:10:40,814 - ERROR - : 
2021-02-17 13:10:40,814 - ERROR - division by zero

The Solution

Store the intermediate writes in a buffer. The reason I am using a list as buffer rather than a string is to avoid the Shlemiel the painter’s algorithm. TLDR: It is O(n) instead of potentially O(n^2)

class LoggerWriter:
    def __init__(self, logfct):
        self.logfct = logfct
        self.buf = []

    def write(self, msg):
        if msg.endswith('\n'):
            self.buf.append(msg.removesuffix('\n'))
            self.logfct(''.join(self.buf))
            self.buf = []
        else:
            self.buf.append(msg)

    def flush(self):
        pass

# To access the original stdout/stderr, use sys.__stdout__/sys.__stderr__
sys.stdout = LoggerWriter(logger.info)
sys.stderr = LoggerWriter(logger.error)

2021-02-17 13:15:22,956 - ERROR - ZeroDivisionError: division by zero

For versions below Python 3.9, you could replace replace .removesuffix('\n') with the less accurate .rstrip('\n').

With flush added to Vinay Sajip's answer:

class LoggerWriter:
    def __init__(self, logger, level): 
        self.logger = logger
        self.level = level 

    def write(self, message):
        if message != '\n':
            self.logger.log(self.level, message)

    def flush(self): 
        pass

Quick but Fragile One-Liner

sys.stdout.write = logger.info

sys.stderr.write = logger.error

What this does is simply assign the logger functions to the stdout/stderr .write call which means any write call will instead invoke the logger functions.

The downside of this approach is that both calls to .write and the logger functions typically add a newline so you will end up with extra lines in your log file, which may or may not be a problem depending on your use case.

Another pitfall is that if your logger writes to stderr itself we get infinite recursion (a stack overflow error). So only output to a file.

Solving problem where StreamHandler causes infinite Recurison

My logger was causing an infinite recursion, because the Streamhandler was trying to write to stdout, which itself is a logger -> leading to infinite recursion.

Solution

Reinstate the original sys.__stdout__ for the StreamHandler ONLY, so that you can still see the logs showing in the terminal.

class DefaultStreamHandler(logging.StreamHandler):
    def __init__(self, stream=sys.__stdout__):
        # Use the original sys.__stdout__ to write to stdout
        # for this handler, as sys.stdout will write out to logger.
        super().__init__(stream)


class LoggerWriter(io.IOBase):
    """Class to replace the stderr/stdout calls to a logger"""

    def __init__(self, logger_name: str, log_level: int):
        """:param logger_name: Name to give the logger (e.g. 'stderr')
        :param log_level: The log level, e.g. logging.DEBUG / logging.INFO that
                          the MESSAGES should be logged at.
        """
        self.std_logger = logging.getLogger(logger_name)
        # Get the "root" logger from by its name (i.e. from a config dict or at the bottom of this file)
        #  We will use this to create a copy of all its settings, except the name
        app_logger = logging.getLogger("myAppsLogger")
        [self.std_logger.addHandler(handler) for handler in app_logger.handlers]
        self.std_logger.setLevel(app_logger.level)  # the minimum lvl msgs will show at
        self.level = log_level  # the level msgs will be logged at
        self.buffer = []

    def write(self, msg: str):
        """Stdout/stderr logs one line at a time, rather than 1 message at a time.
        Use this function to aggregate multi-line messages into 1 log call."""
        msg = msg.decode() if issubclass(type(msg), bytes) else msg

        if not msg.endswith("\n"):
            return self.buffer.append(msg)

        self.buffer.append(msg.rstrip("\n"))
        message = "".join(self.buffer)
        self.std_logger.log(self.level, message)
        self.buffer = []


def replace_stderr_and_stdout_with_logger():
    """Replaces calls to sys.stderr -> logger.info & sys.stdout -> logger.error"""
    # To access the original stdout/stderr, use sys.__stdout__/sys.__stderr__
    sys.stdout = LoggerWriter("stdout", logging.INFO)
    sys.stderr = LoggerWriter("stderr", logging.ERROR)


if __name__ == __main__():
    # Load the logger & handlers
    logger = logging.getLogger("myAppsLogger")
    logger.setLevel(logging.DEBUG)
    # HANDLER = logging.StreamHandler()
    HANDLER = DefaultStreamHandler()  # <--- replace the normal streamhandler with this
    logger.addHandler(HANDLER)
    logFormatter = logging.Formatter("[%(asctime)s] - %(name)s - %(levelname)s - %(message)s")
    HANDLER.setFormatter(logFormatter)

    # Run this AFTER you load the logger
    replace_stderr_and_stdout_with_logger()

And then finally call the replace_stderr_and_stdout_with_logger() after you've initialised your logger (the last bit of the code)