How do I write log messages to a log file and the

Question

Does this code write to both a log file and the console at the same time?

logFile = open("logfile.log",a)
print >>logFile,message
logFile.close()

Answer 1

No, it will not write to both. print() will write to the console only. One quick note on your original code. I presume you define message somewhere, but the code is still incorrect. You need quotes around the a in the open statement, like this:

open("logfile.log", "a")

since I presume you meant to append to the file. Otherwise, you code throws a NameError since a is not a defined variable.

However, as others have said, you should strongly consider using the logging module. Here is a simple example of how to write to both the console and a log file. The code is partially derived from here and here:

import inspect
import logging

def function_logger(file_level, console_level = None):
    function_name = inspect.stack()[1][3]
    logger = logging.getLogger(function_name)
    logger.setLevel(logging.DEBUG) #By default, logs all messages

    if console_level != None:
        ch = logging.StreamHandler() #StreamHandler logs to console
        ch.setLevel(console_level)
        ch_format = logging.Formatter('%(asctime)s - %(message)s')
        ch.setFormatter(ch_format)
        logger.addHandler(ch)

    fh = logging.FileHandler("{0}.log".format(function_name))
    fh.setLevel(file_level)
    fh_format = logging.Formatter('%(asctime)s - %(lineno)d - %(levelname)-8s - %(message)s')
    fh.setFormatter(fh_format)
    logger.addHandler(fh)

    return logger

def f1():
    f1_logger = function_logger(logging.DEBUG, logging.ERROR)
    f1_logger.debug('debug message')
    f1_logger.info('info message')
    f1_logger.warn('warn message')
    f1_logger.error('error message')
    f1_logger.critical('critical message')

def f2():
    f2_logger = function_logger(logging.WARNING)
    f2_logger.debug('debug message')
    f2_logger.info('info message')
    f2_logger.warn('warn message')
    f2_logger.error('error message')
    f2_logger.critical('critical message')

def main():
    f1()
    f2()
    logging.shutdown()

main()

Since logger objects can have more than one handler, we can create multiple handlers that write to different places. In my code, the function_logger function creates a logger object specific to the function in which it's called.

The function f1() logs DEBUG level messages and above to a file f1.log, while writing ERROR level messages and above to the console, with different formatting for each.

The function f2(), however, logs nothing to the console and only logs WARNING level messages to its log file f2.log. Running this script once yields this output on the console:

2012-07-20 10:46:38,950 - f1  - error message
2012-07-20 10:46:38,953 - f1  - critical message

and this output in f1.log and f2.log, respectively:

f1.log:

2012-07-20 10:46:38,950 - 26 - DEBUG    - debug message
2012-07-20 10:46:38,950 - 27 - INFO     - info message
2012-07-20 10:46:38,950 - 28 - WARNING  - warn message
2012-07-20 10:46:38,950 - 29 - ERROR    - error message
2012-07-20 10:46:38,953 - 30 - CRITICAL - critical message

f2.log

2012-07-20 10:46:38,960 - 36 - WARNING  - warn message
2012-07-20 10:46:38,960 - 37 - ERROR    - error message
2012-07-20 10:46:38,960 - 38 - CRITICAL - critical message

Answer 2

No. It writes to file only. You should use logging module. See http://docs.python.org/library/logging.html