How to write to a file, using the logging Python module?

http://docs.python.org/library/logging.handlers.html#filehandler

The FileHandler class, located in the core logging package, sends logging output to a disk file.

http://docs.python.org/library/logging.html#logging.basicConfig

logging.basicConfig(filename='/path/to/your/log', level=....)

Taken from the “logging cookbook“:

# create logger with 'spam_application'
logger = logging.getLogger('spam_application')
logger.setLevel(logging.DEBUG)
# create file handler which logs even debug messages
fh = logging.FileHandler('spam.log')
fh.setLevel(logging.DEBUG)
logger.addHandler(fh)

And you’re good to go.

P.S. Make sure to read the logging HOWTO as well.

I prefer to use a configuration file. It allows me to switch logging levels, locations, etc without changing code when I go from development to release. I simply package a different config file with the same name, and with the same defined loggers.

import logging.config
if __name__ == '__main__':
    # Configure the logger
    # loggerConfigFileName: The name and path of your configuration file
    logging.config.fileConfig(path.normpath(loggerConfigFileName))

    # Create the logger
    # Admin_Client: The name of a logger defined in the config file
    mylogger = logging.getLogger('Admin_Client')

    msg='Bite Me'
    myLogger.debug(msg)
    myLogger.info(msg)
    myLogger.warn(msg)
    myLogger.error(msg)
    myLogger.critical(msg)

    # Shut down the logger
    logging.shutdown()

Here is my code for the log config file

#These are the loggers that are available from the code
#Each logger requires a handler, but can have more than one
[loggers]
keys=root,Admin_Client


#Each handler requires a single formatter
[handlers]
keys=fileHandler, consoleHandler


[formatters]
keys=logFormatter, consoleFormatter


[logger_root]
level=DEBUG
handlers=fileHandler


[logger_Admin_Client]
level=DEBUG
handlers=fileHandler, consoleHandler
qualname=Admin_Client
#propagate=0 Does not pass messages to ancestor loggers(root)
propagate=0


# Do not use a console logger when running scripts from a bat file without a console
# because it hangs!
[handler_consoleHandler]
class=StreamHandler
level=DEBUG
formatter=consoleFormatter
args=(sys.stdout,)# The comma is correct, because the parser is looking for args


[handler_fileHandler]
class=FileHandler
level=DEBUG
formatter=logFormatter
# This causes a new file to be created for each script
# Change time.strftime("%Y%m%d%H%M%S") to time.strftime("%Y%m%d")
# And only one log per day will be created. All messages will be amended to it.
args=("D:\Logs\PyLogs\" + time.strftime("%Y%m%d%H%M%S")+'.log', 'a')


[formatter_logFormatter]
#name is the name of the logger root or Admin_Client
#levelname is the log message level debug, warn, ect 
#lineno is the line number from where the call to log is made
#04d is simple formatting to ensure there are four numeric places with leading zeros
#4s would work as well, but would simply pad the string with leading spaces, right justify
#-4s would work as well, but would simply pad the string with trailing spaces, left justify
#filename is the file name from where the call to log is made
#funcName is the method name from where the call to log is made
#format=%(asctime)s | %(lineno)d | %(message)s
#format=%(asctime)s | %(name)s | %(levelname)s | %(message)s
#format=%(asctime)s | %(name)s | %(module)s-%(lineno) | %(levelname)s | %(message)s
#format=%(asctime)s | %(name)s | %(module)s-%(lineno)04d | %(levelname)s | %(message)s
#format=%(asctime)s | %(name)s | %(module)s-%(lineno)4s | %(levelname)-8s | %(message)s

format=%(asctime)s | %(levelname)-8s | %(lineno)04d | %(message)s


#Use a separate formatter for the console if you want
[formatter_consoleFormatter]
format=%(asctime)s | %(levelname)-8s | %(filename)s-%(funcName)s-%(lineno)04d | %(message)s

import sys
import logging

from util import reducer_logfile
logging.basicConfig(filename=reducer_logfile, format='%(message)s',
                    level=logging.INFO, filemode='w')

This example should work fine. I have added streamhandler for console. Console
log and file handler data should be similar.

    # MUTHUKUMAR_TIME_DATE.py #>>>>>>>> file name(module)

    import sys
    import logging
    import logging.config
    # ================== Logger ================================
    def Logger(file_name):
        formatter = logging.Formatter(fmt='%(asctime)s %(module)s,line: %(lineno)d %(levelname)8s | %(message)s',
                                      datefmt='%Y/%m/%d %H:%M:%S') # %I:%M:%S %p AM|PM format
        logging.basicConfig(filename = '%s.log' %(file_name),format= '%(asctime)s %(module)s,line: %(lineno)d %(levelname)8s | %(message)s',
                                      datefmt='%Y/%m/%d %H:%M:%S', filemode = 'w', level = logging.INFO)
        log_obj = logging.getLogger()
        log_obj.setLevel(logging.DEBUG)
        # log_obj = logging.getLogger().addHandler(logging.StreamHandler())

        # console printer
        screen_handler = logging.StreamHandler(stream=sys.stdout) #stream=sys.stdout is similar to normal print
        screen_handler.setFormatter(formatter)
        logging.getLogger().addHandler(screen_handler)

        log_obj.info("Logger object created successfully..")
        return log_obj
    # =======================================================


MUTHUKUMAR_LOGGING_CHECK.py #>>>>>>>>>>> file name
# calling **Logger** function
file_name = 'muthu'
log_obj =Logger(file_name)
log_obj.info("yes   hfghghg ghgfh".format())
log_obj.critical("CRIC".format())
log_obj.error("ERR".format())
log_obj.warning("WARN".format())
log_obj.debug("debug".format())
log_obj.info("qwerty".format())
log_obj.info("asdfghjkl".format())
log_obj.info("zxcvbnm".format())
# closing file
log_obj.handlers.clear()

OUTPUT:
2019/07/13 23:54:40 MUTHUKUMAR_TIME_DATE,line: 17     INFO | Logger object created successfully..
2019/07/13 23:54:40 MUTHUKUMAR_LOGGING_CHECK,line: 8     INFO | yes   hfghghg ghgfh
2019/07/13 23:54:40 MUTHUKUMAR_LOGGING_CHECK,line: 9 CRITICAL | CRIC
2019/07/13 23:54:40 MUTHUKUMAR_LOGGING_CHECK,line: 10    ERROR | ERR
2019/07/13 23:54:40 MUTHUKUMAR_LOGGING_CHECK,line: 11  WARNING | WARN
2019/07/13 23:54:40 MUTHUKUMAR_LOGGING_CHECK,line: 12    DEBUG | debug
2019/07/13 23:54:40 MUTHUKUMAR_LOGGING_CHECK,line: 13     INFO | qwerty
2019/07/13 23:54:40 MUTHUKUMAR_LOGGING_CHECK,line: 14     INFO | asdfghjkl
2019/07/13 23:54:40 MUTHUKUMAR_LOGGING_CHECK,line: 15     INFO | zxcvbnm

Thanks, 

here’s a simpler way to go about it. this solution doesn’t use a
config dictionary and uses a rotation file handler, like so:

import logging
from logging.handlers import RotatingFileHandler
     
logging.basicConfig(handlers=[RotatingFileHandler(filename=logpath+filename,
                     mode='w', maxBytes=512000, backupCount=4)], level=debug_level,
                     format='%(levelname)s %(asctime)s %(message)s', 
                    datefmt='%m/%d/%Y%I:%M:%S %p')
     
logger = logging.getLogger('my_logger')

or like so:

import logging
from logging.handlers import RotatingFileHandler
     
handlers = [ RotatingFileHandler(filename=logpath+filename, 
            mode='w', 
            maxBytes=512000, 
            backupCount=4)
           ]
logging.basicConfig(handlers=handlers, 
                    level=debug_level, 
                    format='%(levelname)s %(asctime)s %(message)s', 
                    datefmt='%m/%d/%Y%I:%M:%S %p')
     
logger = logging.getLogger('my_logger')

the handlers variable needs to be an iterable. logpath+filename and debug_level are just variables holding the
respective info. of course, the values for the function params are up
to you.

the first time i was using the logging module i made the mistake of writing the following, which generates an OS file lock error (the
above is the solution to that):

import logging
from logging.handlers import RotatingFileHandler
     
logging.basicConfig(filename=logpath+filename, 
       level=debug_level, 
       format='%(levelname)s %(asctime)s %(message)s', 
       datefmt='%m/%d/%Y%I:%M:%S %p')
     
logger = logging.getLogger('my_logger')
logger.addHandler(RotatingFileHandler(
       filename=logpath+filename, 
       mode='w', 
       maxBytes=512000, 
       backupCount=4))

Here is two examples, one print the logs (stdout) the other write the logs to a file:

import logging
import sys

logger = logging.getLogger()
logger.setLevel(logging.INFO)
formatter = logging.Formatter('%(asctime)s | %(levelname)s | %(message)s')

stdout_handler = logging.StreamHandler(sys.stdout)
stdout_handler.setLevel(logging.DEBUG)
stdout_handler.setFormatter(formatter)

file_handler = logging.FileHandler('logs.log')
file_handler.setLevel(logging.DEBUG)
file_handler.setFormatter(formatter)


logger.addHandler(file_handler)
logger.addHandler(stdout_handler)

With this example, all logs will be printed and also be written to a file named logs.log

Use example:

logger.info('This is a log message!')
logger.error('This is an error message.')

List of all built-in logging handlers https://docs.python.org/3/library/logging.handlers.html

import logging

from datetime import datetime

filename = datetime.now().strftime("%d-%m-%Y %H-%M-%S")#Setting the filename from current date and time
logging.basicConfig(filename=filename, filemode='a',
                    format="%(asctime)s, %(msecs)d %(name)s %(levelname)s [ %(filename)s-%(module)s-%(lineno)d ]  : %(message)s",
                    datefmt="%H:%M:%S",
                    level=logging.DEBUG)

• asctime

  %(asctime)s

  Human-readable time when the LogRecord was created. By default this
  is of the form ‘2003-07-08 16:49:45,896’ (the numbers after the comma
  are millisecond portion of the time).

• created

  %(created)f

  Time when the LogRecord was created (as returned by time.time()).

• exc_info

  You shouldn’t need to format this yourself.

  Exception tuple (à la sys.exc_info) or, if no exception has occurred,
  None.

• filename

  %(filename)s

  Filename portion of pathname.

• funcName

  %(funcName)s

  Name of function containing the logging call.

• levelname

  %(levelname)s

  Text logging level for the message (‘DEBUG’, ‘INFO’, ‘WARNING’,
  ‘ERROR’, ‘CRITICAL’).

• levelno

  %(levelno)s

  Numeric logging level for the message (DEBUG, INFO, WARNING, ERROR,
  CRITICAL).

• lineno

  %(lineno)d

  Source line number where the logging call was issued (if available).

• message

  %(message)s

  The logged message, computed as msg % args. This is set when
  Formatter.format() is invoked.

• module

  %(module)s

  Module (name portion of filename).

• msecs

  %(msecs)d

  Millisecond portion of the time when the LogRecord was created.

• msg

  You shouldn’t need to format this yourself.

  The format string passed in the original logging call. Merged with
  args to produce message, or an arbitrary object (see Using arbitrary
  objects as messages).

• name

  %(name)s

  Name of the logger used to log the call.

• pathname

  %(pathname)s

  Full pathname of the source file where the logging call was issued
  (if available).

• process

  %(process)d

  Process ID (if available).

• processName

  %(processName)s

  Process name (if available).

• relativeCreated

  %(relativeCreated)d

  Time in milliseconds when the LogRecord was created, relative to the
  time the logging module was loaded.

• stack_info

  You shouldn’t need to format this yourself.

  Stack frame information (where available) from the bottom of the
  stack in the current thread, up to and including the stack frame of
  the logging call which resulted in the creation of this record.

• thread

  %(thread)d

  Thread ID (if available).

• threadName

  %(threadName)s

  Thread name (if available).

Head over to official python3 page for more info regarding logging.

Although it is an old question, for people reaching this question these days, you can also use dictConfig. For example, for a file with info level and above :

logging.config.dictConfig({
    'version': 1,
    'formatters': {
        'default': {
            'format': '[%(asctime)s] %(message)s',
        }
    },
    'handlers': {
        'info': {
            'level': logging.INFO,
            'class': 'logging.FileHandler',
            'filename': 'info.log',
        },
    },
    "root": {
        "level": logging.INFO,
        "handlers": ["info"]
    }
})

Or another example to be more specific, with rotating file and in a specific directory :

today = datetime.date.today()
folder = './log'
Path(folder).mkdir(parents=True, exist_ok=False) # Create folder if not exists
logging.config.dictConfig({
        ...
        'info': {
            'level': logging.INFO,
            'class': 'logging.handlers.TimedRotatingFileHandler',
            'filename': f'{folder}/info-{today.month:02}-{today.year}.log',
            # Roll over on the first day of the weekday
            'when': 'W0',
            # Roll over at midnight
            'atTime': datetime.time(hour=0),
            # Number of files to keep.
            'backupCount': 8
        },
        ...

best and clean method.

    ds = datetime.now().strftime("%Y%m%d_%H%M%S")
    try:
        # py39 有force参数指定可能强制除去之前的handler，这里使用兼容写法，0708
        logging.getLogger().removeHandler(logging.getLogger().handlers[0])
        logging.getLogger().removeHandler(logging.getLogger().handlers[0])
    except:
        pass
    logging.basicConfig(
        # force=
        level=logging.INFO,
        format="%(asctime)s [%(levelname)s] %(message)s",
        handlers=[
            logging.FileHandler('log_%s_%s_%s.log' % (ds, create_net, os.path.basename(dataset_path))),
            logging.StreamHandler(sys.stdout)
        ]
    )
    logging.info('start train')