logger configuration to log to file and print to stdout

Question

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!

Answer 1

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 

Answer 2

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().