Logging in Flask: 3 Practical Examples

Setting Up Logging in a Flask Application

Logging is an essential aspect of any web application, allowing developers to track events, errors, and other significant occurrences during an application’s lifecycle. In Flask, setting up logging can help you diagnose issues and understand user interactions. In this article, we’ll explore three diverse examples of setting up logging in a Flask application.

Example 1: Basic File Logging

Context

In this example, we will set up a basic logging configuration that writes logs to a file. This is useful for a simple application where you want to keep a record of all events and errors.

from flask import Flask
import logging

app = Flask(__name__)

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

@app.route('/')
def home():
    app.logger.info('Home page accessed')
    return 'Welcome to the Flask App!'

@app.route('/error')
def error():
    app.logger.error('Error page accessed')
    return 'This is an error page!'

if __name__ == '__main__':
    app.run(debug=True)

Notes

• In this setup, all log messages will be written to app.log in the same directory as your Flask application.
• The log messages include a timestamp, the log level (INFO or ERROR), and the actual message.
• You can adjust the log level to DEBUG, INFO, WARNING, ERROR, or CRITICAL depending on your needs.

Example 2: Logging with Different Levels

Context

This example demonstrates how to log messages at different severity levels. This is helpful when you want to categorize logs for easier monitoring and debugging.

from flask import Flask
import logging

app = Flask(__name__)

## Configure logging
logging.basicConfig(level=logging.DEBUG, 
                    format='%(asctime)s - %(levelname)s - %(message)s')

@app.route('/')
def home():
    app.logger.debug('Debug: Home page accessed')
    return 'Welcome to the Flask App!'

@app.route('/info')
def info():
    app.logger.info('Info: Info page accessed')
    return 'This is the info page!'

@app.route('/warn')
def warn():
    app.logger.warning('Warning: Warning page accessed')
    return 'This is a warning page!'

@app.route('/error')
def error():
    app.logger.error('Error: Error page accessed')
    return 'This is an error page!'

if __name__ == '__main__':
    app.run(debug=True)

Notes

• This setup allows you to log messages at different levels, providing more granularity in your logging.
• Adjust the logging level in basicConfig to see messages of different severity.
• Always remember to keep your logging level appropriate for the environment (e.g., DEBUG in development, WARNING or ERROR in production).

Example 3: Logging to External Service Using Flask-Logging

Context

In this advanced example, we will set up logging to an external service such as Loggly. This is particularly useful for applications that require centralized logging, making it easier to monitor and analyze logs from multiple sources.

from flask import Flask
import logging
import logging.handlers

app = Flask(__name__)

## Configure logging to Loggly
loggly_handler = logging.handlers.HTTPHandler(
    'logs-01.loggly.com',
    '/inputs/YOUR-LOGGLY-TOKEN',
    method='POST',
    secure=True
)

## Set logging level and format
loggly_handler.setLevel(logging.INFO)
formatter = logging.Formatter('%(asctime)s - %(levelname)s - %(message)s')
loggly_handler.setFormatter(formatter)
app.logger.addHandler(loggly_handler)

@app.route('/')
def home():
    app.logger.info('Home page accessed')
    return 'Welcome to the Flask App!'

if __name__ == '__main__':
    app.run(debug=True)

Notes

• Replace YOUR-LOGGLY-TOKEN with your actual Loggly token for this to work.
• This setup sends logs directly to Loggly, allowing you to monitor your application logs from anywhere.
• Ensure you have the requests library installed, as it may be required for HTTPHandler.

Conclusion

By setting up logging in your Flask application, you can gain valuable insights into its behavior and performance. Whether you need basic file logging, different log levels, or centralized logging with an external service, these examples provide a solid foundation to get you started. Happy coding!