How to change filehandle with Python logging on the fly with different classes and imports

I found an easier way than the above ‘accepted’ answer. If you have a reference to the handler, all you need to do is call the close() method and then set the baseFilename property. When you assign baseFilename, be sure to use os.path.abspath(). There’s a comment in the library source that indicates it’s needed. I keep my configuration stuff in a global dict() so it’s easy to keep the FileHandler reference objects. As you can see below, it only takes 2 lines of code to change a log filename for a handler on the fly.

import logging

def setup_logging():
  global config

  if config['LOGGING_SET']:
    config['LOG_FILE_HDL'].close()
    config['LOG_FILE_HDL'].baseFilename = os.path.abspath(config['LOG_FILE'])

    config['DEBUG_LOG_HDL'].close()
    config['DEBUG_LOG_HDL'].baseFilename = os.path.abspath(config['DEBUG_LOG'])
  else:
    format_str = '%(asctime)s - %(name)s - %(levelname)s - %(message)s'
    formatter = logging.Formatter(format_str)

    log = logging.getLogger()

    log.setLevel(logging.DEBUG)

    # add file mode="w" to overwrite
    config['LOG_FILE_HDL'] = logging.FileHandler(config['LOG_FILE'], mode='a')
    config['LOG_FILE_HDL'].setLevel(logging.INFO)
    config['LOG_FILE_HDL'].setFormatter(formatter)
    log.addHandler(config['LOG_FILE_HDL'])

    # the delay=1 should prevent the file from being opened until used.
    config['DEBUG_LOG_HDL'] = logging.FileHandler(config['DEBUG_LOG'], mode='a', delay=1)
    config['DEBUG_LOG_HDL'].setLevel(logging.DEBUG)
    config['DEBUG_LOG_HDL'].setFormatter(formatter)
    log.addHandler(config['DEBUG_LOG_HDL'])

    ch = logging.StreamHandler()
    ch.setLevel(logging.DEBUG)
    ch.setFormatter(formatter)
    log.addHandler(ch)
    config['LOGGING_SET'] = True

The answer provided by @Martijn Pieters works good. However, the code snipper removes all handlers and placed only the file handler back. This will be troublesome if your application has handlers added by other modules.

Hence, the below snippet is designed in such a way to replace only the file handler.

The line if isinstance(hdlr,logging.FileHandler) is the key.

import logging

filehandler = logging.FileHandler('/tmp/logfile', 'a')
formatter = logging.Formatter('%(asctime)-15s::%(levelname)s::%(filename)s::%(funcName)s::%(lineno)d::%(message)s')
filehandler.setFormatter(formatter)
log = logging.getLogger()  # root logger - Good to get it only once.
for hdlr in log.handlers[:]:  # remove the existing file handlers
    if isinstance(hdlr,logging.FileHandler):
        log.removeHandler(hdlr)
log.addHandler(filehandler)      # set the new handler
# set the log level to INFO, DEBUG as the default is ERROR
log.setLevel(logging.DEBUG)

I tried to implemented the suggestions on this page from @Martijn Pieters combined with @Arun Thundyill Saseendran. I’m too new to be allowed to comment so I have to post an adjusted answer. In the isinstance call, I had to use ‘logging’ instead of ‘log’ to get access to the types (log was an instance) and then the ‘FileHander’ should be ‘FileHandler’. I’m using Python 3.6.

import logging

filehandler = logging.FileHandler('/tmp/logfile', 'a')
formatter = logging.Formatter('%(asctime)-15s::%(levelname)s::%(filename)s::%(funcName)s::%(lineno)d::%(message)s')
filehandler.setFormatter(formatter)
log = logging.getLogger()  # root logger - Good to get it only once.
for hdlr in log.handlers[:]:  # remove the existing file handlers
    if isinstance(hdlr,logging.FileHandler): #fixed two typos here
        log.removeHandler(hdlr)
log.addHandler(filehandler)      # set the new handler
# set the log level to INFO, DEBUG as the default is ERROR
logging.setLevel(log.DEBUG)      

TL;DR

INDEX = 0

#: change to new file location
logger.handlers[INDEX].setStream( open('/path/to/new/log/file.log', 'a') )

#: change to stdout or stderr
import sys
logger.handlers[INDEX].setStream( sys.stdout ) # or sys.stderr

#: change to another device
logger.handlers[INDEX].setStream( open('/dev/ttys010', 'a') )

• after registering a logging.FileHandler to your logger, you can reach into its internals and change the stream it outputs to "on the fly".
• make sure INDEX accesses the right handler within the logger. if you only added a FileHandler, then it should be index 0.

Explained

Well first, following the logging documentation’s suggested idiom, you would get a new logger instance named after the __name__ of whatever your specific package, module, class, or function:

#: class
>>> class A:
        def __init__(self):
            self.logger = logging.getLogger(self.__class__.__name__)
>>>A().logger
<Logger A (WARNING)>

#: function
>>> def func():
        logger = logging.getLogger(func.__name__)
        print(logger)
>>> func()
<Logger func (WARNING)>

#: module
>>> logger = logging.getLogger( __name__ )
>>> logger
<Logger __main__ (WARNING)>

#: package (e.g. a package named 'pkg', write this in '__init__.py')
>>> logger = logging.getLogger( __package__ )
>>> logger
<RootLogger pkg (WARNING)>

Next, if you’ve registered a logging.FileHandler handler for your logger, like so:

logger.addHandler( logging.FileHandler('/tmp/logfile.log', 'a') )

then you can you can change the file it outputs to by replacing the stream it outputs to:

INDEX = 0  # you will have to find the index position of the `FileHandler` you 
           # registered to this logger. I justed listed them with: `logger.handlers`
           # and picked the one I needed. if you only register one handler
           # then it should be at index 0, i.e the first one

#: change to new file location
logger.handlers[INDEX].setStream( open('/path/to/new/log/file.log', 'a') )

#: change to stdout or stderr
import sys
logger.handlers[INDEX].setStream( sys.stdout ) # or sys.stderr

#: change to another device
logger.handlers[INDEX].setStream( open('/dev/ttys010', 'a') )

---

If your curious, found this in a few minutes, by doing a bit of digging like so (in ipython and python interpreters):

>>> import logging
>>> logger = logging.getLogger( __name__ )
>>> logger.addHandler( logging.FileHandler('/tmp/logfile', 'a') )

>>> globals()
>>> dir(logger)

#: found that the logger has 'handlers' attribute
>>> dir(logger.handlers)
>>> logger.handlers

#: found that the FileHandler I registered earlier is at index: 0
>>> logger.handlers[0]
>>> dir(logger.handlers[0])

#: found that FileHandler has a dictionary '__dict__'
>>> logger.handlers[0].__dict__

#: found that FileHandler dict has 'baseFilename' attribute with the filename
#: i had set when registering the file handler
>>> logger.handlers[0].__dict__['baseFilename']

#: tried changing the file it points to
>>> logger.handlers[0].__dict__['baseFilename'] = '/tmp/logfile.log'
#: tried logging
>>> logger.info(f'hello world')
#: didn't work

#: found another interesting perhaps relevant attribute 'stream' in the
#: FileHandler dict
>>> logger.handlers[0].__dict__['stream']
>>> dir(logger.handlers[0].__dict__['stream'])
>>> logger.handlers[0].__dict__['stream'].__dict__
#: tried replacing the stream altogether
>>> logger.handlers[0].__dict__['stream'] = open('/tmp/logfile.log','a')
#: tried logging
>>> logger.info(f'hello world again')
#: it worked
>>> logger.info(f'hey it worked')

#: found another interesting perhaps relevant method 'setStream'
>>> logger.handlers[0].setStream( open('/tmp/otherlogfile.log','a') )
#: tried logging
>>> logger.info(f'hello world again')
#: it worked
>>> logger.info(f'hey it worked')

• you can also change the name of the logger with:

  logger.name = 'bla'
  

• and more, see: dir(logger)