One class instance used automatically throughout p

Question

The python logging module once imported and instantiated will be so across the process, inside all modules and threads. How did they achieve that effect?

Example:

myapp.py

import logging
import mylib

def main():
    logging.basicConfig(filename='myapp.log', level=logging.INFO)
    logging.info('Started')
    mylib.do_something()
    logging.info('Finished')

if __name__ == '__main__':
    main()

mylib.py

import logging

def do_something():
    logging.info('Doing something')

myapp.log

INFO:root:Started
INFO:root:Doing something
INFO:root:Finished

Answer 1

When a module is imported for the first time, the loaded module object is put into sys.modules. Later imports will then find the module object and not reload the module.

The logging module has a bunch of module attributes which hold the state of logging configuration after the first import.