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Logging

The logging library is part of The Python Standard Library and allows users to implement a flexible event logging system for applications and libraries. Logging is typically deployed for more complex applications that run for extended periods time. Logging allows the programmer to monitor the application during runtime and afterwards for post-mortem analysis. Credits to realpython for inspiration in some of these examples on how to use this library.

Basic usage

To start using the logger, simply import logging and begin inserting logging messages into your code as such.

Python
>>> import logging
>>> logging.warning("Remain calm!")
WARNING:root:Remain calm!

How does this differ from just using print statements? In its most basic form it does not. However, in more complex programs logging provides additional functionality that simplifies event logging.

Logging levels

Log Level Function Description
DEBUG logging.debug() Provides detailed information that’s valuable to you as a developer.
INFO logging.info() Provides general information about what’s going on with your program.
WARNING logging.warning() Indicates that there’s something you should look into.
ERROR logging.error() Alerts you to an unexpected problem that’s occurred in your program.
CRITICAL logging.critical() Tells you that a serious error has occurred and may have crashed your app.

The logger can be configured to different levels to choose which level of log messages you want to record. For example, setting the logging level to WARNING (the default logging level) will result in WARNING and all higher level log messages to be recorded.

Python
>>> logging.debug("This is a debug message")

>>> logging.info("This is an info message")

>>> logging.warning("This is a warning message")
WARNING:root:This is a warning message

>>> logging.error("This is an error message")
ERROR:root:This is an error message

>>> logging.critical("This is a critical message")
CRITICAL:root:This is a critical message

Logging configuration

The logger can be configured using the logging.basicConfig method. See the example below.

Python
# foo.py

from datetime import datetime
import logging

def main():
    logging.info("I'm doing some stuff")
    logging.error("Oh no! Something went wrong.")

if __name__ == "__main__":
    current_datetime = datetime.now().strftime("%Y-%m-%d_%H-%M-%S") # (1)
    logging.basicConfig(filename=f"logs/foo_{current_datetime}.log", # (2)!
                    filemode='w', # (3)!
                    level=logging.DEBUG, # (4)!
                    format='%(asctime)s - %(levelname)s - %(message)s') # (5)!
    logging.info("START PROGRAM")
    main()
    logging.info("END PROGRAM")
  1. Get the current date and time
  2. Configure the logger using logging.basicConfig. We can specify the filename to save the output of the logger to.
  3. Set the filemode to write.
  4. Set the logging level to DEBUG.
  5. Format the logger to include the time, level name, and then write the message.

This will generate the output seen below. Depending on your editor, the messages will also be colored.

Text Only
2024-09-19 15:18:27,397 - INFO - START PROGRAM
2024-09-19 15:18:27,397 - INFO - I'm doing some stuff
2024-09-19 15:18:27,397 - ERROR - Oh no! Something went wrong.
2024-09-19 15:18:27,397 - INFO - END PROGRAM
Tip - Instantiating your own logger

Different modules in your application can have their own loggers with different configurations. Below is an example of instantiating a logger class. Using this you can also see which module different logs are coming.

Python
>>> import logging
>>> logger = logging.getLogger(__name__) # (1)!
>>> logger.warning("Look at my logger!")
Look at my logger!
  1. This will be the name of the module from which your code is in. Likely "__main__", at least in this case.
Example

If you don't want to use the logger or are working in different language, you can still write stdout (standard output) and stderr (standard error) to a file with the following bash code:

Bash
some_command > file.log 2>&1

  1. some_command: Executes a command (which could be any valid program or script).

  2. >file.log: Redirects the standard output (stdout) of the command to a file named file.log. If file.log doesn't exist, it is created; if it exists, its contents are overwritten.

  3. 2>&1: Redirects the standard error (stderr) (represented by file descriptor 2) to the same location as standard output (stdout) (represented by file descriptor 1). Essentially, this ensures that both the output and errors from the command are written to file.log.