[Python] logging in your own module

Overview

When I created my own Python module, I made a sample of how to output log.

Processing content

Call a module called sample.py from main.py. I write the log output process in sample.py, but if root logger is not created in main.py, the logger in the module will not output anything.

Hands-on

Create module

Create inside the module. Write the processing of log output.

sample.py

# -*- coding:utf-8 -*-
from logging import getLogger, DEBUG, NullHandler


class Sample:
    def __init__(self):
        self._logger = getLogger(__name__)
        self._logger.addHandler(NullHandler())
        self._logger.setLevel(DEBUG)
        self._logger.propagate = True

    def main(self):
        self._logger.debug('Debug')
        self._logger.info('Info')
        self._logger.warn('Warn')
        self._logger.error('Error')

Create root logger

Create a root logger definition for use in main.py with the file name my_logger.py. (The following definition may be made in main.py, but it is in a separate file.)

my_logger.py

# -*- coding:utf-8 -*-
from logging import Formatter, handlers, StreamHandler, getLogger, DEBUG


def root_logger():
    #Get root logger
    logger = getLogger()

    #Create formatter
    formatter = Formatter('%(asctime)s %(name)s %(funcName)s [%(levelname)s]: %(message)s')

    #Create handler and set formatter
    handler = StreamHandler()
    handler.setFormatter(formatter)

    #Set handler in logger, set level for event capture
    logger.addHandler(handler)
    #set log level
    logger.setLevel(DEBUG)

    return logger

Create main.py

Create main.py that calls the Sample module. As you can see in the comment out below, call my_logger.py created earlier to create a root logger.

main.py

#!/usr/bin/env python
# -*- coding:utf-8 -*-
import my_logger
import sample
from logging import Formatter, handlers, StreamHandler, getLogger, INFO


if __name__ == '__main__':
    #Create root logger
    logger = my_logger.root_logger()
    logger.info('The root logger is created.')

    #Module call
    sample = sample.Sample()
    sample.main()

Run

When you execute main.py, the log will be output as shown below.

$ python main.py
2020-02-17 14:01:21,721 root <module> [INFO]: The root logger is created.
2020-02-17 14:01:21,721 sample main [DEBUG]: Debug
2020-02-17 14:01:21,721 sample main [INFO]: Info
2020-02-17 14:01:21,721 sample main [WARNING]: Warn
2020-02-17 14:01:21,721 sample main [ERROR]: Error

If you do not create a root logger

Check the behavior when root logger is not created.

Modify main.py

Comment out the part of main.py that creates the root logger.

main.py

#!/usr/bin/env python
# -*- coding:utf-8 -*-
import my_logger
import sample
from logging import Formatter, handlers, StreamHandler, getLogger, INFO


if __name__ == '__main__':
    #Comment out the following.
    # logger = my_logger.root_logger()
    # logger.info('The root logger is created.')

    #Module call
    sample = sample.Sample()
    sample.main()

Run

When I run main.py, nothing is output.

$ python main.py

that's all

reference

-Summary of log output in library using logging module in Python