How to Redirect Stdout and Stderr to Logger in Python

Redirect all stdout/stderr globally to logger

While this is not a full answer, it may show you a redirect to adapt to your particular case. This is how I did it a while back. Although I cannot remember why I did it this way, or what the limitation was I was trying to circumvent, the following is redirecting stdout and stderr to a class for print() statements. The class subsequently writes to screen and to file:

import os
import sys
import datetime

class DebugLogger():

    def __init__(self, filename):
        timestamp = datetime.datetime.strftime(datetime.datetime.utcnow(), 
                                               '%Y-%m-%d-%H-%M-%S-%f')
        #build up full path to filename
        logfile = os.path.join(os.path.dirname(sys.executable), 
                               filename + timestamp)
        self.terminal = sys.stdout
        self.log = open(logfile, 'a')

    def write(self, message):
        timestamp = datetime.datetime.strftime(datetime.datetime.utcnow(), 
                                               ' %Y-%m-%d-%H:%M:%S.%f')
        #write to screen
        self.terminal.write(message)
        #write to file
        self.log.write(timestamp + ' - ' + message)      
        self.flush()

    def flush(self):
        self.terminal.flush()
        self.log.flush()
        os.fsync(self.log.fileno())

    def close(self):
        self.log.close()

def main(debug = False):
    if debug:
        filename = 'blabla'
        sys.stdout = DebugLogger(filename)
        sys.stderr = sys.stdout
    print('test')

if __name__ == '__main__':
    main(debug = True)

Redirect Python 'print' output to Logger

You have two options:

1. Open a logfile and replace sys.stdout with it, not a function:

   log = open("myprog.log", "a")
   sys.stdout = log
   
   >>> print("Hello")
   >>> # nothing is printed because it goes to the log file instead.
   

2. Replace print with your log function:

   # If you're using python 2.x, uncomment the next line
   #from __future__ import print_function
   print = log.info
   
   >>> print("Hello!")
   >>> # nothing is printed because log.info is called instead of print
   

logger configuration to log to file and print to stdout

Just get a handle to the root logger and add the StreamHandler. The StreamHandler writes to stderr. Not sure if you really need stdout over stderr, but this is what I use when I setup the Python logger and I also add the FileHandler as well. Then all my logs go to both places (which is what it sounds like you want).

import logging
logging.getLogger().addHandler(logging.StreamHandler())

If you want to output to stdout instead of stderr, you just need to specify it to the StreamHandler constructor.

import sys
# ...
logging.getLogger().addHandler(logging.StreamHandler(sys.stdout))

You could also add a Formatter to it so all your log lines have a common header.

ie:

import logging
logFormatter = logging.Formatter("%(asctime)s [%(threadName)-12.12s] [%(levelname)-5.5s]  %(message)s")
rootLogger = logging.getLogger()

fileHandler = logging.FileHandler("{0}/{1}.log".format(logPath, fileName))
fileHandler.setFormatter(logFormatter)
rootLogger.addHandler(fileHandler)

consoleHandler = logging.StreamHandler()
consoleHandler.setFormatter(logFormatter)
rootLogger.addHandler(consoleHandler)

Prints to the format of:

2012-12-05 16:58:26,618 [MainThread  ] [INFO ]  my message

Making Python loggers output all messages to stdout in addition to log file

All logging output is handled by the handlers; just add a logging.StreamHandler() to the root logger.

Here's an example configuring a stream handler (using stdout instead of the default stderr) and adding it to the root logger:

import logging
import sys

root = logging.getLogger()
root.setLevel(logging.DEBUG)

handler = logging.StreamHandler(sys.stdout)
handler.setLevel(logging.DEBUG)
formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')
handler.setFormatter(formatter)
root.addHandler(handler)

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