My project is composed by a main.py main script and a module aaa.py . I've succesfully setup a log procedure by using the logging package in main.py, also specifying a log filename. 
Which is the cleanest/"correct" way to let the functions contained in aaa.py to write in the same log file?
If you call only a single python script at once (main.py)
then you simply can define your logging config once; for exemple:
logging.basicConfig(filename=logfilepath, format='%(levelname)s:%(message)s')
and call it wherever you want in the modules, for exemple
logging.<level>("log_string")
logging package - share log file between main and modules - python 3