Locust

Locust is an open source user load testing tool written in Python. The idea behind Locust is to swarm your web site or other systems with attacks from simulated users during a test, with each user behavior defined by you using Python code. This swarming process is then monitored from a web UI in real-time, and will help identify any bottlenecks in your code before real users can come in.

As it is completely event-based, Locust can have thousands or even millions of simultaneous users distributed over multiple machines swarming your system. Unlike other event-based apps, it doesn’t use callbacks but uses lightweight processes instead, so you can write very expressive scenarios in Python without complicating it with callbacks.