How to Log to File in Python: Simple Guide with Examples

Use Python's logging module to log messages to a file by configuring a FileHandler. Set up the logger with logging.basicConfig(filename='file.log', level=logging.INFO) to write logs to a file.

Syntax

The basic syntax to log to a file uses the logging.basicConfig function with the filename parameter to specify the log file. The level parameter sets the minimum severity of messages to log.

• filename: path to the log file.
• level: logging level like DEBUG, INFO, WARNING, etc.
• format: optional, defines the log message format.

python

import logging

logging.basicConfig(filename='app.log', level=logging.INFO, format='%(asctime)s - %(levelname)s - %(message)s')
logging.info('This is an info message')

Example

This example shows how to log different levels of messages to a file named app.log. It demonstrates logging an info message and an error message.

python

import logging

logging.basicConfig(filename='app.log', level=logging.DEBUG, format='%(asctime)s - %(levelname)s - %(message)s')

logging.debug('Debug message: Starting the program')
logging.info('Info message: Program is running')
logging.warning('Warning message: Low disk space')
logging.error('Error message: An error occurred')
logging.critical('Critical message: System failure')

print('Logging complete. Check app.log file.')

Output

Logging complete. Check app.log file.

Common Pitfalls

Common mistakes when logging to a file include:

• Not setting the filename in basicConfig, so logs go to the console instead.
• Calling basicConfig multiple times, which has no effect after the first call.
• Not setting an appropriate logging level, causing important messages to be missed.
• Forgetting to flush or close the log file when using custom handlers.

python

import logging

# Wrong: basicConfig called twice, second call ignored
logging.basicConfig(filename='first.log', level=logging.INFO)
logging.basicConfig(filename='second.log', level=logging.DEBUG)  # Ignored

logging.info('This will go to first.log only')

# Right way: configure once
# Note: basicConfig can only be called once per session
# To change configuration, restart the program or use handlers

Quick Reference

Summary tips for logging to a file in Python:

• Use logging.basicConfig(filename='file.log', level=logging.LEVEL) to set up file logging quickly.
• Choose the right level to control what messages are saved.
• Use format to customize log message appearance.
• Call basicConfig only once at the start of your program.
• Check the log file path and permissions if logs do not appear.

Key Takeaways

Use logging.basicConfig with filename to log messages to a file.

Set the logging level to control which messages are recorded.

Call basicConfig only once; multiple calls have no effect.

Customize log message format for clarity and detail.

Check file path and permissions if logs are missing.