Develop Reference

Python-threading: separate logging of individual threads

I would like individual .log file for each thread. Unfortunately, after using logging.basicConfig, many different files are created for logs, but finally all logs end up in the last declared file.
What should threads do to have independent log files?
import logging
import threading
import time
from datetime import datetime
def test_printing(name):
logging.basicConfig(
format="%(asctime)s, %(levelname)-8s | %(filename)-23s:%(lineno)-4s | %(threadName)15s: %(message)s", # noqa
datefmt="%Y-%m-%d:%H:%M:%S",
level=logging.INFO,
force=True,
handlers=[
logging.FileHandler(f"{name}.log"),
logging.StreamHandler()])
logging.info(f"Test {name}")
time.sleep(20)
logging.info(f"Test {name} after 20s")
def function_thread():
timestamp = datetime.now().strftime("%Y%m%d_%H%M%S")
thread = threading.Thread(
target=test_printing,
kwargs={"name": timestamp}
)
thread.start()
for i in range(5):
time.sleep(1)
function_thread()

From https://docs.python.org/3/library/logging.html#logger-objects
Note that Loggers should NEVER be instantiated directly, but always through the module-level function logging.getLogger(name).
So you have to create and configure a new logger inside each thread:
logger = logging.getLogger(name)
logger.basicConfig(...)
more info at: https://docs.python.org/3/howto/logging.html#logging-from-multiple-modules
Edit: Use already defined name as logger identifier, instead of __name__
Edit:
You cannot use logging.basicConfig, instead you need to configure each thread logger on its own.
Full code provided and tested:
import logging
import threading
import time
from datetime import datetime
def test_printing(name):
logger = logging.getLogger(name)
logger.setLevel(logging.INFO)
formatter = logging.Formatter(
fmt="%(asctime)s, %(levelname)-8s | %(filename)-23s:%(lineno)-4s | %(threadName)15s: %(message)s",
datefmt="%Y-%m-%d:%H:%M:%S")
sh = logging.StreamHandler()
fh = logging.FileHandler(f"{name}.log")
sh.setFormatter(formatter)
fh.setFormatter(formatter)
logger.addHandler(sh)
logger.addHandler(fh)
logger.info(f"Test {name}")
time.sleep(20)
logger.info(f"Test {name} after 20s")
def function_thread():
timestamp = datetime.now().strftime("%Y%m%d_%H%M%S")
thread = threading.Thread(
target=test_printing,
kwargs={"name": timestamp}
)
thread.start()
for i in range(5):
time.sleep(1)
function_thread()

Logging multiple scripts from the same directory in python

I have two python scripts in the same directory. I try to catch logging messages from both of them:
#script.py
import requests
import logging
logger = logging.getLogger(__name__)
class Downloader:
def __init__(self, url):
self.url = url
def download(self):
logger.debug(f'Downloading {self.url}')
req = requests.get(self.url, timeout=1)
return req
#main.py
import logging
from script import Downloader
logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)
handler = logging.StreamHandler()
handler.setLevel(logging.DEBUG)
logger.addHandler(handler)
logger.debug('create object')
d = Downloader('https://www.google.com')
res = d.download()
Basically I want to get rid of the debug-messages from the requests-module, so using logging.basicConfig() is not an option. But the way I do it, I do not get the debug-message from the imported script. Apparently because in script.py __name__ is not main.script.
How can I achieve this without hard coding anything to a string?

In a different module (e.g. logger.py):
import logging
def setup_logger(name, logfile, formatter, stream_handler=False, level=logging.DEBUG):
"""Function to create loggers."""
file_handler = logging.FileHandler(log_file)
stdout_handler = logging.StreamHandler()
file_handler.setFormatter(formatter)
stdout_handler.setFormatter(formatter)
logger = logging.getLogger(name)
if not logger.handlers:
logger.setLevel(level)
logger.addHandler(file_handler)
if stream_handler:
logger.addHandler(stdout_handler)
return logger
# Example formatter
formatter = logging.Formatter('%(asctime)s - %(levelname)s -> %(message)s\n')
# Generate the log object
log = setup_logger('logger_name', 'path_to_logfile', formatter)
Import this log object from your other modules to use it: from logger import log

How to limit log file size in python

I am using windows 7 and python 2.7.
I want to limit my log file size to 5MB.
My app, when it starts, writes to log file, and then the app terminates.
When my app starts again, it will write in same log file. So app is not continuously running.
App initiates, processes and terminates.
My code for logging is:
import logging
import logging.handlers
logging.basicConfig(filename=logfile.log, level="info", format='%(asctime)s %(levelname)s %(funcName)s(%(lineno)d) %(message)s')
logging.info("*************************************************")
I tried with RotatingFileHandler but it didn't work
logging.handlers.RotatingFileHandler(logFile, mode='a', maxBytes=5*1024*1024, backupCount=2, encoding=None, delay=0)
So, how can I enforce a file size limit in python?

Lose basicConfig() and use RotatingFileHandler:
import logging
from logging.handlers import RotatingFileHandler
log_formatter = logging.Formatter('%(asctime)s %(levelname)s %(funcName)s(%(lineno)d) %(message)s')
logFile = 'C:\\Temp\\log'
my_handler = RotatingFileHandler(logFile, mode='a', maxBytes=5*1024*1024,
backupCount=2, encoding=None, delay=0)
my_handler.setFormatter(log_formatter)
my_handler.setLevel(logging.INFO)
app_log = logging.getLogger('root')
app_log.setLevel(logging.INFO)
app_log.addHandler(my_handler)
while True:
app_log.info("data")

When you use logging.basicConfig with a file, the log is attached with a file handler to handle writing to the file.
afterwards you created another file handler to the same file with
logging.handlers.RotatingFileHandler
Now, once a rotate is needed, RotatingFileHandler is trying to remove the old file but it can't becuase there is an open file handler
this can be seen if you look directly at the log file handlers -
import logging
from logging.handlers import RotatingFileHandler
log_name = 'c:\\log.log'
logging.basicConfig(filename=log_name)
log = logging.getLogger()
handler = RotatingFileHandler(log_name, maxBytes=1024, backupCount=1)
log.addHandler(handler)
[<logging.FileHandler object at 0x02AB9B50>, <logging.handlers.RotatingFileHandler object at 0x02AC1D90>]

To use BasicConfig and RotatingFileHandler, add RotatingFileHandler as Handler in BasicConfig.
main.py:
import logging
rfh = logging.handlers.RotatingFileHandler(
filename='foo.log',
mode='a',
maxBytes=5*1024*1024,
backupCount=2,
encoding=None,
delay=0
)
logging.basicConfig(
level=logging.DEBUG,
format="%(asctime)s %(name)-25s %(levelname)-8s %(message)s",
datefmt="%y-%m-%d %H:%M:%S",
handlers=[
rfh
]
)
logger = logging.getLogger('main')
logger.debug("test")
other.py
import logging
class Other():
def __init(self):
self.logger = logging.getLogger('other')
self.logger.info("test2")
"test" will be written into foo.log with the tag 'main'
"test2" will be written into foo.log with the tag 'other'

python's logging module is not logging to a log file

I am trying out python's logger module to log info both in console and in a log and it works fine.I wanted that script to get invoked during machine star-tup and hence invoked my script from rc.local.
And my script gets invoked during boot-up, but the problem is, only console logging is working at that time. Logging to a file is not happening. Any clue on what could be the issue.
Is it anything related to syslog daemon?
import time
import logging
import datetime
global logger
global LOG_FILE
def initialize():
logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)
time = datetime.datetime.now()
LOG_FILE = "sample.log." + time.strftime("%Y%m%d")
log_file_handler = logging.FileHandler(LOG_FILE)
log_file_handler.setLevel(logging.DEBUG)
log_file_formatter = logging.Formatter('%(message)s')
log_file_handler.setFormatter(log_file_formatter)
logger.addHandler(log_file_handler)
log_file_handler = logging.StreamHandler()
log_file_handler.setLevel(logging.INFO)
log_file_formatter = logging.Formatter('%(message)s')
log_file_handler.setFormatter(log_file_formatter)
logger.addHandler(log_file_handler)
if __name__ == '__main__':
initialize()
logger.info("This should go both in console and log")
logger.debug("This should go only to Log")

import time
import logging
import datetime
def initialize():
# because logging is global module, so you can
# always get logger by logging.getLogger(__name__)
logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)
time = datetime.datetime.now()
# if you only use LOG_FILE in function, you don't
# have to declare it outside.
LOG_FILE = "sample.log." + time.strftime("%Y%m%d")
log_file_handler = logging.FileHandler(LOG_FILE)
log_file_handler.setLevel(logging.DEBUG)
log_file_formatter = logging.Formatter('%(message)s')
log_file_handler.setFormatter(log_file_formatter)
logger.addHandler(log_file_handler)
log_file_handler = logging.StreamHandler()
log_file_handler.setLevel(logging.INFO)
log_file_formatter = logging.Formatter('%(message)s')
log_file_handler.setFormatter(log_file_formatter)
logger.addHandler(log_file_handler)
if __name__ == '__main__':
initialize()
logger = logging.getLogger(__name__)
logger.info("This should go both in console and log")
logger.debug("This should go only to Log")

logger configuration to log to file and print to stdout

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!

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

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().

Adding a StreamHandler without arguments goes to stderr instead of stdout. If some other process has a dependency on the stdout dump (i.e. when writing an NRPE plugin), then make sure to specify stdout explicitly or you might run into some unexpected troubles.
Here's a quick example reusing the assumed values and LOGFILE from the question:
import logging
from logging.handlers import RotatingFileHandler
from logging import handlers
import sys
log = logging.getLogger('')
log.setLevel(logging.DEBUG)
format = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
ch = logging.StreamHandler(sys.stdout)
ch.setFormatter(format)
log.addHandler(ch)
fh = handlers.RotatingFileHandler(LOGFILE, maxBytes=(1048576*5), backupCount=7)
fh.setFormatter(format)
log.addHandler(fh)

Here is a complete, nicely wrapped solution based on Waterboy's answer and various other sources. It supports logging to both console and log file, allows for different log level settings, provides colorized output and is easily configurable (also available as Gist):
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
# -------------------------------------------------------------------------------
# -
# Python dual-logging setup (console and log file), -
# supporting different log levels and colorized output -
# -
# Created by Fonic <https://github.com/fonic> -
# Date: 04/05/20 - 02/07/23 -
# -
# Based on: -
# https://stackoverflow.com/a/13733863/1976617 -
# https://uran198.github.io/en/python/2016/07/12/colorful-python-logging.html -
# https://en.wikipedia.org/wiki/ANSI_escape_code#Colors -
# -
# -------------------------------------------------------------------------------
# Imports
import os
import sys
import logging
# Logging formatter supporting colorized output
class LogFormatter(logging.Formatter):
COLOR_CODES = {
logging.CRITICAL: "\033[1;35m", # bright/bold magenta
logging.ERROR: "\033[1;31m", # bright/bold red
logging.WARNING: "\033[1;33m", # bright/bold yellow
logging.INFO: "\033[0;37m", # white / light gray
logging.DEBUG: "\033[1;30m" # bright/bold black / dark gray
}
RESET_CODE = "\033[0m"
def __init__(self, color, *args, **kwargs):
super(LogFormatter, self).__init__(*args, **kwargs)
self.color = color
def format(self, record, *args, **kwargs):
if (self.color == True and record.levelno in self.COLOR_CODES):
record.color_on = self.COLOR_CODES[record.levelno]
record.color_off = self.RESET_CODE
else:
record.color_on = ""
record.color_off = ""
return super(LogFormatter, self).format(record, *args, **kwargs)
# Set up logging
def set_up_logging(console_log_output, console_log_level, console_log_color, logfile_file, logfile_log_level, logfile_log_color, log_line_template):
# Create logger
# For simplicity, we use the root logger, i.e. call 'logging.getLogger()'
# without name argument. This way we can simply use module methods for
# for logging throughout the script. An alternative would be exporting
# the logger, i.e. 'global logger; logger = logging.getLogger("<name>")'
logger = logging.getLogger()
# Set global log level to 'debug' (required for handler levels to work)
logger.setLevel(logging.DEBUG)
# Create console handler
console_log_output = console_log_output.lower()
if (console_log_output == "stdout"):
console_log_output = sys.stdout
elif (console_log_output == "stderr"):
console_log_output = sys.stderr
else:
print("Failed to set console output: invalid output: '%s'" % console_log_output)
return False
console_handler = logging.StreamHandler(console_log_output)
# Set console log level
try:
console_handler.setLevel(console_log_level.upper()) # only accepts uppercase level names
except:
print("Failed to set console log level: invalid level: '%s'" % console_log_level)
return False
# Create and set formatter, add console handler to logger
console_formatter = LogFormatter(fmt=log_line_template, color=console_log_color)
console_handler.setFormatter(console_formatter)
logger.addHandler(console_handler)
# Create log file handler
try:
logfile_handler = logging.FileHandler(logfile_file)
except Exception as exception:
print("Failed to set up log file: %s" % str(exception))
return False
# Set log file log level
try:
logfile_handler.setLevel(logfile_log_level.upper()) # only accepts uppercase level names
except:
print("Failed to set log file log level: invalid level: '%s'" % logfile_log_level)
return False
# Create and set formatter, add log file handler to logger
logfile_formatter = LogFormatter(fmt=log_line_template, color=logfile_log_color)
logfile_handler.setFormatter(logfile_formatter)
logger.addHandler(logfile_handler)
# Success
return True
# Main function
def main():
# Set up logging
script_name = os.path.splitext(os.path.basename(sys.argv[0]))[0]
if (not set_up_logging(console_log_output="stdout", console_log_level="warning", console_log_color=True,
logfile_file=script_name + ".log", logfile_log_level="debug", logfile_log_color=False,
log_line_template="%(color_on)s[%(created)d] [%(threadName)s] [%(levelname)-8s] %(message)s%(color_off)s")):
print("Failed to set up logging, aborting.")
return 1
# Log some messages
logging.debug("Debug message")
logging.info("Info message")
logging.warning("Warning message")
logging.error("Error message")
logging.critical("Critical message")
# Call main function
if (__name__ == "__main__"):
sys.exit(main())
NOTE regarding Microsoft Windows:
For colorized output to work within the classic Command Prompt of Microsoft Windows, some additional code is necessary. This does NOT apply to the newer Terminal app which supports colorized output out of the box.
There are two options:
1) Use Python package colorama (filters output sent to stdout and stderr and translates escape sequences to native Windows API calls; works on Windows XP and later):
import colorama
colorama.init()
2) Enable ANSI terminal mode using the following function (enables terminal to interpret escape sequences by setting flag ENABLE_VIRTUAL_TERMINAL_PROCESSING; more info on this here, here, here and here; works on Windows 10 and later):
# Imports
import sys
import ctypes
# Enable ANSI terminal mode for Command Prompt on Microsoft Windows
def windows_enable_ansi_terminal_mode():
if (sys.platform != "win32"):
return None
try:
kernel32 = ctypes.windll.kernel32
result = kernel32.SetConsoleMode(kernel32.GetStdHandle(-11), 7)
if (result == 0): raise Exception
return True
except:
return False

Either run basicConfig with stream=sys.stdout as the argument prior to setting up any other handlers or logging any messages, or manually add a StreamHandler that pushes messages to stdout to the root logger (or any other logger you want, for that matter).

Logging to stdout and rotating file with different levels and formats:
import logging
import logging.handlers
import sys
if __name__ == "__main__":
# Change root logger level from WARNING (default) to NOTSET in order for all messages to be delegated.
logging.getLogger().setLevel(logging.NOTSET)
# Add stdout handler, with level INFO
console = logging.StreamHandler(sys.stdout)
console.setLevel(logging.INFO)
formater = logging.Formatter('%(name)-13s: %(levelname)-8s %(message)s')
console.setFormatter(formater)
logging.getLogger().addHandler(console)
# Add file rotating handler, with level DEBUG
rotatingHandler = logging.handlers.RotatingFileHandler(filename='rotating.log', maxBytes=1000, backupCount=5)
rotatingHandler.setLevel(logging.DEBUG)
formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')
rotatingHandler.setFormatter(formatter)
logging.getLogger().addHandler(rotatingHandler)
log = logging.getLogger("app." + __name__)
log.debug('Debug message, should only appear in the file.')
log.info('Info message, should appear in file and stdout.')
log.warning('Warning message, should appear in file and stdout.')
log.error('Error message, should appear in file and stdout.')

After having used Waterboy's code over and over in multiple Python packages, I finally cast it into a tiny standalone Python package, which you can find here:
https://github.com/acschaefer/duallog
The code is well documented and easy to use. Simply download the .py file and include it in your project, or install the whole package via pip install duallog.

I have handled redirecting logs and prints to a file on disk, stdout, and stderr simultaneously, via the following module (the Gist is also available here):
import logging
import pathlib
import sys
from ml.common.const import LOG_DIR_PATH, ML_DIR
def create_log_file_path(file_path, root_dir=ML_DIR, log_dir=LOG_DIR_PATH):
path_parts = list(pathlib.Path(file_path).parts)
relative_path_parts = path_parts[path_parts.index(root_dir) + 1:]
log_file_path = pathlib.Path(log_dir, *relative_path_parts)
log_file_path = log_file_path.with_suffix('.log')
# Create the directories and the file itself
log_file_path.parent.mkdir(parents=True, exist_ok=True)
log_file_path.touch(exist_ok=True)
return log_file_path
def set_up_logs(file_path, mode='a', level=logging.INFO):
log_file_path = create_log_file_path(file_path)
logging_handlers = [logging.FileHandler(log_file_path, mode=mode),
logging.StreamHandler(sys.stdout)]
logging.basicConfig(
handlers=logging_handlers,
format='%(asctime)s %(name)s %(levelname)s %(message)s',
level=level
)
class OpenedFileHandler(logging.FileHandler):
def __init__(self, file_handle, filename, mode):
self.file_handle = file_handle
super(OpenedFileHandler, self).__init__(filename, mode)
def _open(self):
return self.file_handle
class StandardError:
def __init__(self, buffer_stderr, buffer_file):
self.buffer_stderr = buffer_stderr
self.buffer_file = buffer_file
def write(self, message):
self.buffer_stderr.write(message)
self.buffer_file.write(message)
class StandardOutput:
def __init__(self, buffer_stdout, buffer_file):
self.buffer_stdout = buffer_stdout
self.buffer_file = buffer_file
def write(self, message):
self.buffer_stdout.write(message)
self.buffer_file.write(message)
class Logger:
def __init__(self, file_path, mode='a', level=logging.INFO):
self.stdout_ = sys.stdout
self.stderr_ = sys.stderr
log_file_path = create_log_file_path(file_path)
self.file_ = open(log_file_path, mode=mode)
logging_handlers = [OpenedFileHandler(self.file_, log_file_path,
mode=mode),
logging.StreamHandler(sys.stdout)]
logging.basicConfig(
handlers=logging_handlers,
format='%(asctime)s %(name)s %(levelname)s %(message)s',
level=level
)
# Overrides write() method of stdout and stderr buffers
def write(self, message):
self.stdout_.write(message)
self.stderr_.write(message)
self.file_.write(message)
def flush(self):
pass
def __enter__(self):
sys.stdout = StandardOutput(self.stdout_, self.file_)
sys.stderr = StandardError(self.stderr_, self.file_)
def __exit__(self, exc_type, exc_val, exc_tb):
sys.stdout = self.stdout_
sys.stderr = self.stderr_
self.file_.close()
Written as a context manager, you can simply add the functionality to your python script by adding an extra line:
from logger import Logger
...
if __name__ == '__main__':
with Logger(__file__):
main()

Although the question specifically asks for a logger configuration, there is an alternative approach that does not require any changes to the logging configuration, nor does it require redirecting stdout.
A bit simplistic, perhaps, but it works:
def log_and_print(message: str, level: int, logger: logging.Logger):
logger.log(level=level, msg=message) # log as normal
print(message) # prints to stdout by default
Instead of e.g. logger.debug('something') we now call log_and_print(message='something', level=logging.DEBUG, logger=logger).
We can also expand this a little, so it prints to stdout only when necessary:
def log_print(message: str, level: int, logger: logging.Logger):
# log the message normally
logger.log(level=level, msg=message)
# only print to stdout if the message is not logged to stdout
msg_logged_to_stdout = False
current_logger = logger
while current_logger and not msg_logged_to_stdout:
is_enabled = current_logger.isEnabledFor(level)
logs_to_stdout = any(
getattr(handler, 'stream', None) == sys.stdout
for handler in current_logger.handlers
)
msg_logged_to_stdout = is_enabled and logs_to_stdout
if not current_logger.propagate:
current_logger = None
else:
current_logger = current_logger.parent
if not msg_logged_to_stdout:
print(message)
This checks the logger and its parents for any handlers that stream to stdout, and checks if the logger is enabled for the specified level.
Note this has not been optimized for performance.

For 2.7, try the following:
fh = logging.handlers.RotatingFileHandler(LOGFILE, maxBytes=(1048576*5), backupCount=7)