Writing a locustfile

A locustfile is a normal python file. The only requirement is that it declares at least one class - let’s call it the locust class - that inherits from the class Locust.

The Locust class

A locust class represents one user (or a swarming locust if you will). Locust will spawn (hatch) one instance of the locust class for each user that is being simulated. There are a few attributes that a locust class should typically define.

The task_set attribute

The task_set attribute should point to a TaskSet class which defines the behaviour of the user and is described in more details below.

The min_wait and max_wait attributes

Additionally to the task_set attribute, one usually want to declare the min_wait and max_wait attributes. These are the minimum and maximum time, in milliseconds, that a simulated user will wait between executing each task. min_wait and max_wait defaults to 1000, and therefore a locust will always wait 1 second between each task if min_wait and max_wait is not declared.

With the following locustfile, each user would wait between 5 and 15 seconds between tasks:

from locust import Locust, TaskSet, task

class MyTaskSet(TaskSet):
    def my_task(l):
        print "executing my_task"

class MyLocust(Locust):
    task_set = MyTaskSet
    min_wait = 5000
    max_wait = 15000

The min_wait and max_wait attributes can also be overrided in a TaskSet class.

The weight attribute

You can run two locusts from the same file like so:

locust -f locust_file.py WebUserLocust MobileUserLocust

If you wish to make one of these locusts execute more often you can set a weight attribute on those classes. Say for example, web users are three times more likely than mobile users:

class WebUserLocust(Locust):
    weight = 3

class MobileUserLocust(Locust):
    weight = 1

The host attribute

The host attribute is a URL prefix (i.e. “http://google.com”) to the host that is to be loaded. Usually, this is specified on the command line, using the –host option, when locust is started. If one declares a host attribute in the locust class, it will be used in the case when no –host is specified on the command line.

TaskSet class

If the Locust class represents a swarming locust, you could say that the TaskSet class represents the brain of the locust. Each Locust class must have a task_set attribute set, that points to a TaskSet.

A TaskSet is, like it’s name suggests, a collection of tasks. These tasks are normal python callables and - if we were loadtesting an auction website - could do stuff like “loading the start page”, “searching for some product” and “making a bid”.

When a load test is started, each instance of the spawned Locust classes will start executing their TaskSet. What happens then is that each TaskSet will pick one of it’s tasks and call it. It will then wait a number of milliseconds, chosen at random between the Locust class’ min_wait and max_wait attributes (unless min_wait/max_wait has been defined directly under the TaskSet, in which case it will use it’s own values instead). Then it will again pick a new task which will be called, then wait again, and so on.

Declaring tasks

The typical way of declaring tasks for a TaskSet it to use the task decorator.

Here is an example:

from locust import Locust, TaskSet, task

class MyTaskSet(TaskSet):
    def my_task(self):
        print "Locust instance (%r) executing my_task" % (self.locust)

class MyLocust(Locust):
    task_set = MyTaskSet

@task takes an optional weight argument that can be used to specify the tasks’ execution ratio. In the following example task2 will be executed twice as much as task1:

from locust import Locust, TaskSet, task

class MyTaskSet(TaskSet):
    min_wait = 5000
    max_wait = 15000

    def task1(self):

    def task2(self):

class MyLocust(Locust):
    task_set = MyTaskSet

task attribute

Using the @task decorator to declare tasks is a convenience, and usually that’s the best way to do it. However, it’s also possible to define the tasks of a TaskSet by setting the tasks attribute (using the @task decorator will actually just populate the tasks attribute).

The tasks attribute which is either a list of python callables, or a <callable : int> dict. The tasks are python callables that recieve one argument - the TaskSet class instance that is executing the task. Here is an extremely simple example of a locustfile (this locsutfile won’t actually load test anything):

from locust import Locust, TaskSet

def my_task(l):

class MyTaskSet(TaskSet):
    tasks = [my_task]

class MyLocust(Locust):
    task_set = MyTaskSet

If the tasks attribute is specified as a list, each time a task is to be performed, it will be randomly chosen from the tasks attribute. If however, tasks is a dict - with callables as keys and ints as values - the task that is to be executed will be chosen at random but with the int as ratio. So with a tasks that looks like this:

{my_task: 3, another_task:1}

my_task would be 3 times more likely to be executed than another_task.

TaskSets can be nested

A very important property of TaskSets are that they can be nested, because real websites are usually built up in an hierarchical way, with multiple sub sections. Nesting TaskSets will therefore allow us to define a behaviour that simulates users in a more realistic way. For example we could define TaskSets with the following structure:

  • Main user behaviour
  • Index page
  • Forum page
  • Read thread
  • Reply
  • New thread
  • View next page
  • Browse categories
  • Watch movie
  • Filter movies
  • About page

The way you nest TaskSets is just like when you specify a task using the tasks attribute, but instead of refering to a python function, you point it to another TaskSet:

class ForumPage(TaskSet):
    def read_thread(self):

    def new_thread(self):

    def stop(self):

class UserBehaviour(TaskSet):
    tasks = {ForumPage:10}

    def index(self):

So in above example, if the ForumPage would get selected for execution when the UserBehaviour TaskSet is executing, is. that the ForumPage TaskSet would start executing. The ForumPage TaskSet would then pick one of it’s own task, execute it, then wait, and so on.

There is one important thing to note about the above example, and that is the call to self.interrupt() in the ForumPage’s stop method. What this does is essentially that it will stop executing the ForumPage task set and the execution will continue in the UserBehaviour instance. If we wouldn’t have had a call to the interrupt() method somewhere in ForumPage, the Locust would never stop running the ForumPage task once it has started. But by having the interrupt function, we can - together with task weighting - define how likely it is that a simulated user leaves the forum.

It’s also possible to declare a nested TaskSet, inline in a class, using the @task decorator, just like when declaring normal tasks:

class MyTaskSet(TaskSet):
    class SubTaskSet(TaskSet):
        def my_task(self):

The on_start function

A TaskSet class can optionally declare an on_start function. If so, that function is called when a simulated user starts executing that TaskSet class.

Referencing the Locust instance, or the parent TaskSet instance

A TaskSet instance will have the attribute locust point to it’s Locust instance, and the attribute parent point to it’s parent TaskSet (it will point to the Locust instance, in the base TaskSet).

Making HTTP requests

So far, we’ve only covered the task scheduling part of a Locust user. In order to actually load test a system we need to make HTTP requests. To help us do this, the HttpLocust class exists. When using this class, each instance gets a client attribute which will be an instance of HttpSession which can be used to make HTTP requests.

class HttpLocust

Represents an HTTP “user” which is to be hatched and attack the system that is to be load tested.

The behaviour of this user is defined by the task_set attribute, which should point to a TaskSet class.

This class creates a client attribute on instantiation which is an HTTP client with support for keeping a user session between requests.

client = None

Instance of HttpSession that is created upon instantiation of Locust. The client support cookies, and therefore keeps the session between HTTP requests.

When inheriting from the HttpLocust class, we can use it’s client attribute to make HTTP requests against the server. Here is an example of a locust file that can be used to load test a site with two urls; / and /about/:

from locust import HttpLocust, TaskSet, task

class MyTaskSet(TaskSet):
    def index(self):

    def about(self):

class MyLocust(HttpLocust):
    task_set = MyTaskSet
    min_wait = 5000
    max_wait = 15000

Using the above Locust class, each simulated user will wait between 5 and 15 seconds between the requests, and / will be requested twice as much as /about/.

The attentive reader will find it odd that we can reference the HttpSession instance using self.client inside the TaskSet, and not self.locust.client. We can do this because the TaskSet class has a convenience property called client that simply returns self.locust.client.

Using the HTTP client

Each instance of HttpLocust has an instance of HttpSession in the client attribute. The HttpSession class is actually a subclass of requests.Session and can be used to make HTTP requests, that will be reported to Locust’s statistics, using the get, post, put, delete, head, patch and options methods. The HttpSession instance will preserve cookies between requests so that it can be used to log in to websites and keep a session between requests. The client attribute can also be reference from the Locust instance’s TaskSet instances so that it’s easy to retrieve the client and make HTTP requests from within your tasks.

Here’s a simple example that makes a GET request to the /about path (in this case we assume self is an instance of a TaskSet or HttpLocust class:

response = self.client.get("/about")
print "Response status code:", response.status_code
print "Response content:", response.content

And here’s an example making a POST request:

response = self.client.post("/login", {"username":"testuser", "password":"secret"})

Safe mode

The HTTP client is configured to run in safe_mode. What this does is that any request that fails due to a connection error, timeout, or similar will not raise an exception, but rather return an empty dummy Response object. The request will be reported as a failure in Locust’s statistics. The returned dummy Response’s content attribute will be set to None, and it’s status_code will be 0.

Manually controlling if a request should be considered successful or a failure

By default, requests are marked as failed requests unless the HTTP response code is ok (2xx). Most of the time, this default is what you want. Sometimes however - for example when testing a URL endpoint that you expect to return 404, or testing a badly designed system that might return 200 OK even though an error occurred - there’s a need for manually controlling if locust should consider a request as a success or a failure.

One can mark requests as failed, even when the response code is okay, by using the catch_response argument and a with statement:

with client.get("/", catch_response=True) as response:
    if response.content != "Success":
        response.failure("Got wrong response")

Just as one can mark requests with OK response codes as failures, one can also use catch_response argument together with a with statement to make requests that resulted in an HTTP error code still be reported as a success in the statistics:

with client.get("/does_not_exist/", catch_response=True) as response:
    if response.status_code == 404:

Grouping requests to URLs with dynamic parameters

It’s very common for websites to have pages whose URLs contain some kind of dynamic parameter(s). Often it makes sense to group these URL’s together in Locust’s statistics. This can be done by passing a name argument to the HttpSession's different reqeust methods.


# Statistics for these requests will be grouped under: /blog/?id=[id]
for i in range(10):
    client.get("/blog?id=%i" % i, name="/blog?id=[id]")