logger configuration to log to file and print to stdout

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I’m using Python’s logging module to log some debug strings to a file which works pretty well. Now in addition, I’d like to use this module to also print the strings out to stdout. How do I do this? In order to log my strings to a file I use following code:

import logging
import logging.handlers
logger = logging.getLogger("")
logger.setLevel(logging.DEBUG)
handler = logging.handlers.RotatingFileHandler(
    LOGFILE, maxBytes=(1048576*5), backupCount=7
)
formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
handler.setFormatter(formatter)
logger.addHandler(handler)

and then call a logger function like

logger.debug("I am written to the file")

Thank you for some help here!

8 s
8

logging.basicConfig() can take a keyword argument handlers since Python 3.3, which simplifies logging setup a lot, especially when setting up multiple handlers with the same formatter:

handlers – If specified, this should be an iterable of already created handlers to add to the root logger. Any handlers which don’t already have a formatter set will be assigned the default formatter created in this function.

The whole setup can therefore be done with a single call like this:

import logging

logging.basicConfig(
    level=logging.INFO,
    format="%(asctime)s [%(levelname)s] %(message)s",
    handlers=[
        logging.FileHandler("debug.log"),
        logging.StreamHandler()
    ]
)

(Or with import sys + StreamHandler(sys.stdout) per original question’s requirements – the default for StreamHandler is to write to stderr. Look at LogRecord attributes if you want to customize the log format and add things like filename/line, thread info etc.)

The setup above needs to be done only once near the beginning of the script. You can use the logging from all other places in the codebase later like this:

logging.info('Useful message')
logging.error('Something bad happened')
...

Note: If it doesn’t work, someone else has probably already initialized the logging system differently. Comments suggest doing logging.root.handlers = [] before the call to basicConfig().

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