Rollbar is a hosted monitoring service that can be used with Python web applications to catch and report errors.
Rollbar monitoring of Celery in a Django app covers the Celery task queue and how to discover issues that happen in asynchronous worker processes.
How to Add Hosted Monitoring to Flask Web Applications adds Rollbar's error tracking to a Flask web application.
How to Monitor Python Web Applications uses the Bottle web application framework to show how hosted monitoring can be added to a Python web application.
The official Rollbar Python docs have tons of great integration examples for various Python web frameworks.