What is Locust?

Locust is an open source performance/load testing tool for HTTP and other protocols. Its developer friendly approach lets you to define your tests in regular Python code.

Locust tests can be run from command line or using its web-based UI. Throughput, response times and errors can be viewed in real time and/or exported for later analysis.

You can import regular Python libraries into your tests, and with Locust’s pluggable architecture it is infinitely expandable. Unlike when using most other tools, your test design will never be limited by a GUI or domain-specific language.

To start using Locust, go to Installation


  • Write test scenarios in plain old Python

If you want your users to loop, perform some conditional behaviour or do some calculations, you just use the regular programming constructs provided by Python. Locust runs every user inside its own greenlet (a lightweight process/coroutine). This enables you to write your tests like normal (blocking) Python code instead of having to use callbacks or some other mechanism. Because your scenarios are “just python” you can use your regular IDE, and version control your tests as regular code (as opposed to some other tools that use XML or binary formats)

  • Distributed and scalable - supports hundreds of thousands of concurrent users

Locust makes it easy to run load tests distributed over multiple machines. It is event-based (using gevent), which makes it possible for a single process to handle many thousands concurrent users. While there may be other tools that are capable of doing more requests per second on a given hardware, the low overhead of each Locust user makes it very suitable for testing highly concurrent workloads.

  • Web-based UI

Locust has a user friendly web interface that shows the progress of your test in real-time. You can even change the load while the test is running. It can also be run without the UI, making it easy to use for CI/CD testing.

  • Can test any system

Even though Locust primarily works with web sites/services, it can be used to test almost any system or protocol. Just write a client for what you want to test, or explore some created by the community.

  • Hackable

Locust is small and very flexible and we intend to keep it that way. If you want to send reporting data to that database & graphing system you like, wrap calls to a REST API to handle the particulars of your system or run a totally custom load pattern, there is nothing stopping you!

Name & background

Locust was born out of a frustration with existing solutions. No existing load testing tool was well equipped to generate realistic load against a dynamic website where most pages had different content for different users. Existing tools used clunky interfaces or verbose configuration files to declare the tests. In Locust we took a different approach. Instead of configuration formats or UIs you’d get a python framework that would let you define the behaviour of your users using Python code.

Locust takes its name from the grasshopper species, known for their swarming behaviour.

The history of Locust


Many thanks to our other great contributors!


Open source licensed under the MIT license (see LICENSE file for details).