Locust

Locust

Locust is an easy-to-use, scriptable, and scalable performance testing tool. You define the behavior of your users in regular Python code.

When to Use

• Python Teams: Scripting complexity is zero if you know Python.
• Distributed Testing: Easy to scale out using a Master/Worker architecture to simulate millions of users.
• Web UI: Comes with a nice web interface to start tests and view graphs in real-time.

Quick Start

from locust import HttpUser, task, between

class WebsiteUser(HttpUser):
    wait_time = between(1, 5)

    @task
    def index(self):
        self.client.get("/")

    @task(3)
    def view_item(self):
        item_id = randint(1, 10000)
        self.client.get(f"/item?id={item_id}", name="/item")

Run locust -f locustfile.py.

Core Concepts

User Class

Represents a user. Attributes like wait_time simulate "think time" between actions.

Tasks

Decorator @task marks methods as user actions. You can weight them (@task(3) runs 3x more often than @task(1)).

Web UI

Accessible at http://localhost:8089. Allows you to set the user count and spawn rate dynamically.

Best Practices (2025)

Do:

• Use FastHttpUser: Locust's standard HttpUser uses requests (slow). FastHttpUser uses geventhttpclient which is much faster for load generation.
• Group Requests: Use the name argument in client.get(url, name="my-endpoint"). Otherwise, every unique URL (with differing IDs) shows as a separate entry in reports.

Don't:

• Don't block the Greenlet: Locust uses Gevent. Do not perform CPU-intensive work or blocking IO (standard time.sleep) in your tasks, or you pause the whole user runner.

References

• Locust Documentation