Logging is means of tracking events that happen during software execution.

The logging module provides a set of convenience functions that are meant to be used in differnet scenarios

• logging.info(): Report events that occur during normal/expected operation of a program.

• logging.debug(): Report detailed otuput for diagnostic purposes.

• logging.warn(): Issue a warning due to a runtime event, use this if the situation is avoidable by the user to eliminate the warning.

• logging.warning(): Issue a warning due to a runtime event, use this if the user cannnot do anything about the situation, the the event should be notted.

• logging.error(): Report error without raising a python exception.

By default, messages that are not printed (due to the logging level threshold) are simply discarded and are not stored anywhere. The logging module only processes messages that meet or exceed the configured logging level.

In ex1.py one can find the following code

import logging
logging.warning('Warning: check this!')  # will print a message to the console
logging.info('Information logged')  # will not print anything

When running ex1.py the user will see:

WARNING:root:Warning: check this!

Note: Nothing about the logging.info() message is displayed..

Without configuration, only messages at WARNING level or higher will be displayed. The logging levels in order of increasing severity are:

DEBUG (10) < INFO (20) < WARNING (30) < ERROR (40) < CRITICAL (50)

Now in example 2 we will simply add logging.basicConfig(level=logging.INFO) to the code so that INFO messages are shown.

One can change what is printed in the terminal when running a script updating the level of logging using logging.basicConfig(level=logging.LEVEL) where LEVEL can be a string from [].

For example, if we add LEVEL=INFO, as one can see in ex2.py

import logging

logging.basicConfig(level=logging.INFO)

logging.warning('Warning: check this!')  # will be printed because `logging.basicConfig(level=logging.INFO)`
logging.info('Information logged')  # will be printed

When running python ex2.py the user will see:

WARNING:root:Warning: check this!
INFO:root:Information logged

Which includes the INFO details that did not appear in ex1.py.

The code in ex3.py shows how to add a handler that stores to disk the logged messages.

• To print in the console console_handler = logging.StreamHandler() is used.

• To print in a file file_handler = logging.FileHandler(log_file) is used.

import logging

# Create and configure handlers
console_handler = logging.StreamHandler()
console_handler.setLevel(logging.WARNING)  # Console shows only WARNING and above

file_handler = logging.FileHandler('all_messages.log')
file_handler.setLevel(logging.DEBUG)      # File stores everything

# Configure the root logger
logging.basicConfig(
    level=logging.DEBUG,  # Allow processing of all messages
    format='%(asctime)s - %(levelname)s - %(message)s',
    handlers=[console_handler, file_handler]
)

# Now test all levels
logging.debug("Debug message - in file only")
logging.info("Info message - in file only")
logging.warning("Warning message - in both file and console")
logging.error("Error message - in both file and console")

After running this:

• Console output will show only WARNING and ERROR messages

• The file ex3__all_messages.log will contain ALL messages (DEBUG, INFO, WARNING, and ERROR)

Example ex4.py shows how to create a logger that stores the logged items to disk with logging.FileHandler. The example also has a logger that takes care of printing to the console (we can use more than one logger!).

• To print in the console console_handler = logging.StreamHandler() is used.

• To print in a file file_handler = logging.FileHandler(log_file) is used.

import logging
import os
from datetime import datetime

# Create 'logged_messages' directory if it doesn't exist
log_dir = 'logged_messages'
if not os.path.exists(log_dir):
    os.makedirs(log_dir)

# Create log file path with timestamp
timestamp = datetime.now().strftime('%Y_%m_%d_%H_%M_%S')
log_file = os.path.join(log_dir, f'ex4__all_messages__{timestamp}.log')

# Create and configure handlers
console_handler = logging.StreamHandler()
console_handler.setLevel(logging.WARNING)  # Console shows only WARNING and above

file_handler = logging.FileHandler(log_file)
file_handler.setLevel(logging.INFO)      # File stores INFO and above (changed from DEBUG)

# Configure the root logger
logging.basicConfig(
    level=logging.DEBUG,  # Keep DEBUG as lowest level to allow all messages to be processed
    format='%(asctime)s - %(levelname)s - %(message)s',
    handlers=[console_handler, file_handler]
)

# Test all logging levels
logging.debug("Debug message - won't be stored anywhere")
logging.info("Info message - in file only")
logging.warning("Warning message - in both file and console")
logging.error("Error message - in both file and console")

After running this:

• Console output will show only WARNING and ERROR messages

• The file ex4__all_messages__2025_02_25_11_20_04.log will contain messages INFO, WARNING, and ERROR.