Extending Locust using event hooks

Locust comes with a number of event hooks that can be used to extend Locust in different ways.

Event hooks live on the Environment instance under the events attribute. However, since the Environment instance hasn’t been created when locustfiles are imported, the events object can also be accessed at the module level of the locustfile through the locust.events variable.

Here’s an example on how to set up an event listener:

from locust import events

def my_success_handler(request_type, name, response_time, response_length, **kw):
    print("Successfully made a request to: %s" % name)


It’s highly recommended that you add a wildcard keyword argument in your listeners (the **kw in the code above), to prevent your code from breaking if new arguments are added in a future version.

See also

To see all available events, please see Event hooks.

Adding Web Routes

Locust uses Flask to serve the web UI and therefore it is easy to add web end-points to the web UI. By listening to the init event, we can retrieve a reference to the Flask app instance and use that to set up a new route:

from locust import events

def on_locust_init(web_ui, **kw):
    def my_added_page():
        return "Another page"

You should now be able to start locust and browse to

Extending Web UI

As an alternative to adding simple web routes, you can use Flask Blueprints and templates to not only add routes but also extend the web UI to allow you to show custom data along side the built-in Locust stats. This is more advanced as it involves also writing and including HTML and Javascript files to be served by routes but can greatly enhance the utility and customizability of the web UI.

A working example of extending the web UI, complete with HTML and Javascript example files, can be found in the examples directory of the Locust source code.

Run a background greenlet

Because a locust file is “just code”, there is nothing preventing you from spawning your own greenlet to run in parallel with your actual load/Users.

For example, you can monitor the fail ratio of your test and stop the run if it goes above some threshold:

from locust import events
from locust.runners import STATE_STOPPING, STATE_STOPPED, STATE_CLEANUP, WorkerRunner

def checker(environment):
    while not environment.runner.state in [STATE_STOPPING, STATE_STOPPED, STATE_CLEANUP]:
        if environment.runner.stats.total.fail_ratio > 0.2:
            print(f"fail ratio was {environment.runner.stats.total.fail_ratio}, quitting")

def on_locust_init(environment, **_kwargs):
    # only run this on master & standalone
    if not isinstance(environment.runner, WorkerRunner):
        gevent.spawn(checker, environment)

More examples

See locust-plugins