Changelog Highlights

For full details of the Locust changelog, please see










































  • Fix dependency: Dont use latest Jinja2 because it has breaking changes




  • Fix issue with permissions in docker image


  • Further optimize docker image (60MB compressed)


  • Shrink docker image significantly (95MB compressed size for x64 instead of 358MB) by basing the image on python3-slim instead of python3

  • Fix empty tasks section in UI and static report bug (really)






  • Documentation fixes only.









  • Accidentally increased version to 2.4 directly so there is no 2.3…






  • Fix docker builds (2.0 never got pushed to Docker Hub)

  • Bump dependency on pyzmq to fix out of memory issue on Windows

  • Use 1 as default for user count and spawn rate in web UI start form

  • Various documentation updates


User ramp up/down and User type selection is now controlled by the master instead of autonomously by the workers

This has allowed us to fix some issues with incorrect/skewed User type selection and undesired stepping of ramp up. The issues were especially visible when running many workers and/or using LoadShape:s. This change also allows redistribution of Users if a worker disconnects during a test. This is a major change internally in Locust so please let us know if you encounter any problems (particularly regarding ramp up pace, User distribution, CPU usage on master, etc)

Other potentially breaking API changes

  • Change the default User weight to 1 instead of 10 (the old default made no sense)

  • Fire test_start and test_stop events on workers too (previously they were only fired on master/standalone instances)

  • Workers now send their version number to master. Master will warn about version differences, and pre 2.0-versions will not be allowed to connect at all (because they would not work anyway)

  • Update Flask dependency to 2.0

Significant merged PR:s (and prerelease version they were introduced in)

Some of these are not really that significant and may be removed from this list at a later time, once 2.0 has stabilised.






  • Unify request_success/request_failure into a single event called request (the old ones are deprecated but still work)

  • Add the response object and context as parameters to the request event. context is used to forward information to the request event handler (can be used for things like username, tags etc)



  • Fix bug that broke the tooltips for charts in the Web UI





  • List Python 3.9 as supported in the package/on PyPi

  • Fix XSS vulnerability in the web UI (sounds important but really isn’t, as Locust UI is not meant to be exposed to outside users)


  • Bump minimum required gevent version to 20.9.0 (latest), as the previous ones had sneaky binary incompatibilities with the latest version of greenlet (“RuntimeWarning: greenlet.greenlet size changed, may indicate binary incompatibility. Expected 144 from C header, got 152 from PyObject”)


  • Breaking change: Remove step-load feature (now that we have LoadTestShape it is no longer needed)

  • More type hints to enable better code completion and linting of locustfiles

Bug fixes:





  • Rename hatch rate to spawn rate (the –hatch-rate parameter is only deprecated, but the hatch_complete event has been renamed spawning_complete)

  • Ability to generate any custom load shape with LoadTestShape class

  • Allow ramping down of users

  • Ability to use save custom percentiles

  • Improve command line stats output

  • Bug fixes (excessive precision of metrics in losust csv stats, negative response time when system clock has changed, issue with non-string failure messages, some typos etc)

  • Documentation improvements


  • –run-time flag is not respected if there is an exception in a test_stop listener

  • FastHttpUser: Handle stream ended at an unexpected time and UnicodeDecodeError. Show bad/error status codes on failures page.

  • Improve logging when locust master port is busy


  • The official Docker image is now based on the python:3.8 image instead of python:3.8-alpine. This should make it easier to install other python packages when extending the locust docker image.

  • Allow Users to stop the runner by calling self.environment.runner.quit() (without deadlocking sometimes)

  • Cut to only 5% free space on the top of the graphs

  • Use csv module to generate csv data (solves issues with sample names that need escaping in csv)

  • Various documentation improvements


  • Ability to control the exit code of the Locust process by setting Environment.process_exit_code

  • FastHttpLocust: Change dependency to use original geventhttpclient (now that releases can be made there) instead of geventhttpclient-wheels

  • Fix search on readthedocs


  • Check for low open files limit (ulimit) and try to automatically increase it from within the locust process.

  • Other various bug fixes as improvements

1.0, 1.0.1

This version contains some breaking changes.

Locust class renamed to User

We’ve renamed the Locust and HttpLocust classes to User and HttpUser. The locust attribute on TaskSet instances has been renamed to user.

The parameter for setting number of users has also been changed, from -c / --clients to -u / --users.

Ability to declare @task directly under the User class

It’s now possible to declare tasks directly under a User class like this:

class WebUser(User):
    def some_task(self):

In tasks declared under a User class (e.g. some_task in the example above), self refers to the User instance, as one would expect. For tasks defined under a TaskSet class, self would refer to the TaskSet instance.

The task_set attribute on the User class (previously Locust class) has been removed. To declare a User class with a single TaskSet one would now use the tasks attribute instead:

class MyTaskSet(TaskSet):

class WebUser(User):
    tasks = [MyTaskSet]

Task tagging

A new tag feature has been added that makes it possible to include/exclude tasks during a test run.

Tasks can be tagged using the @tag decorator:

class WebUser(User):
    @tag("tag1", "tag2")
    def my_task(self):

And tasks can then be specified/excluded using the --tags/-T and --exclude-tags/-E command line arguments.

Environment variables changed

The following changes has been made to the configuration environment variables

  • LOCUST_MASTER has been renamed to LOCUST_MODE_MASTER (in order to make it less likely to get variable name collisions when running Locust in Kubernetes/K8s which automatically adds environment variables depending on service/pod names).

  • LOCUST_SLAVE has been renamed to LOCUST_MODE_WORKER.



  • CSVFILEBASE has been renamed to LOCUST_CSV.

See the Configuration documentation for a full list of available environment variables.

Other breaking changes

  • The master/slave terminology has been changed to master/worker. Therefore the command line arguments --slave and --expect-slaves has been renamed to --worker and --expect-workers.

  • The option for running Locust without the Web UI has been renamed from --no-web to --headless.

  • Removed Locust.setup, Locust.teardown, TaskSet.setup and TaskSet.teardown hooks. If you want to run code at the start or end of a test, you should instead use the test_start and test_stop events:

    from locust import events
    def on_test_start(**kw):
        print("test is starting")
    def on_test_start(**kw):
        print("test is stopping")
  • TaskSequence and @seq_task has been replaced with SequentialTaskSet.

  • A User count column has been added to the history stats CSV file. The column order and column names has been changed.

  • The official docker image no longer uses a shell script with a bunch of special environment variables to configure how how locust is started. Instead, the locust command is now set as ENTRYPOINT of the docker image. See Running in Docker for more info.

  • Command line option --csv-base-name has been removed, since it was just an alias for --csv.

  • The way Locust handles logging has been changed. We no longer wrap stdout (and stderr) to automatically make print statements go into the log. print() statements now only goes to stdout. To add custom entries to the log, one should now use the Python logging module:

    import logging"custom logging message)

    For more info see Logging

Web UI improvements

  • It’s now possible to protect the Web UI with Basic Auth using the --web-auth command line argument.

  • The Web UI can now be served over HTTPS by specifying a TLS certificate and key with the --tls-cert and --tls-key command line arguments.

  • If the number of users and hatch rate are specified on command line, it’s now used to pre-populate the input fields in the Web UI.

Other fixes and improvements

  • Added --config command line option for specifying a configuration file path

  • The code base has been refactored to make it possible to run Locust as a python lib.

  • It’s now possible to call response.failure() or response.success() multiple times when using the catch_response=True in the HTTP clients. Only the last call to success/failure will count.

  • The --help output has been improved by grouping related options together.


  • Fix bug when running with latest Gevent version, and pinned the latest version


  • Drop Python 2 and Python 3.5 support!

  • Continuously measure CPU usage and emit a warning if we get a five second average above 90%

  • Show CPU usage of slave nodes in the Web UI

  • Fixed issue when running Locust distributed and new slave nodes connected during the hatching/ramp-up phase (


Various minor fixes, mainly regarding FastHttpLocust.


Identical to previous version, but now built & deployed to Pypi using Travis.



  • Fixed bug that broke the Web UI’s response time graph


  • Fixed crash bug on Python 3.8.0

  • Various other bug fixes and improvements.



  • Added –skip-log-setup to disable Locust’s default logging setup.

  • Added –stop-timeout to allow tasks to finish running their iteration before stopping

  • Added 99.9 and 99.99 percentile response times to csv output

  • Allow custom clients to set request response time to None. Those requests will be excluded when calculating median, average, min, max and percentile response times.

  • Renamed the last row in statistics table from “Total” to “Aggregated” (since the values aren’t a sum of the individual table rows).

  • Some visual improvements to the web UI.

  • Fixed issue with simulating fewer number of locust users than the number of slave/worker nodes.

  • Fixed bugs in the web UI related to the fact that the stats table is truncated at 500 entries.

  • Various other bug fixes and improvements.


  • Added new FastHttpLocust class that uses a faster HTTP client, which should be 5-6 times faster than the normal HttpLocust class. For more info see the documentation on increasing performance.

  • Added ability to set the exit code of the locust process when exceptions has occurred within the user code, using the --exit-code-on-error parameter.

  • Added TCP keep alive to master/slave communication sockets to avoid broken connections in some environments.

  • Dropped support for Python 3.4

  • Numerous other bug fixes and improvements.


  • Python 3.7 support

  • Added a status page to the web UI when running Locust distributed showing the status of slave nodes and detect down slaves using heartbeats

  • Numerous bugfixes/documentation updates (see detailed changelog)



  • Updated pyzmq version, and changed so that we don’t pin a specific version. This makes it easier to install Locust on Windows.


  • Python 3 support

  • Dropped support for Python 2.6

  • Added --no-reset-stats option for controlling if the statistics should be reset once the hatching is complete

  • Added charts to the web UI for requests per second, average response time, and number of simulated users.

  • Updated the design of the web UI.

  • Added ability to write a CSV file for results via command line flag

  • Added the URL of the host that is currently being tested to the web UI.

  • We now also apply gevent’s monkey patching of threads. This fixes an issue when using Locust to test Cassandra (

  • Various bug fixes and improvements


  • Use version 1.1.1 of gevent. Fixes an install issue on certain versions of python.



  • Fixed bug where POST requests (and other methods as well) got incorrectly reported as GET requests, if the request resulted in a redirect.

  • Added ability to download exceptions in CSV format. Download links has also been moved to its own tab in the web UI.


  • Locust now returns an exit code of 1 when any failed requests were reported.

  • When making an HTTP request to an endpoint that responds with a redirect, the original URL that was requested is now used as the name for that entry in the statistics (unless an explicit override is specified through the name argument). Previously, the last URL in the redirect chain was used to label the request(s) in the statistics.

  • Fixed bug which caused only the time of the last request in a redirect chain to be included in the reported time.

  • Fixed bug which caused the download time of the request body not to be included in the reported response time.

  • Fixed bug that occurred on some linux dists that were tampering with the python-requests system package (removing dependencies which requests is bundling). This bug only occurred when installing Locust in the python system packages, and not when using virtualenv.

  • Various minor fixes and improvements.


  • Exceptions that occurs within TaskSets are now caught by default.

  • Fixed bug which caused Min response time to always be 0 after all locusts had been hatched and the statistics had been reset.

  • Minor UI improvements in the web interface.

  • Handle messages from “zombie” slaves by ignoring the message and making a log entry in the master process.


HTTP client functionality moved to HttpLocust

Previously, the Locust class instantiated a HttpSession under the client attribute that was used to make HTTP requests. This functionality has now been moved into the HttpLocust class, in an effort to make it more obvious how one can use Locust to load test non-HTTP systems.

To make existing locust scripts compatible with the new version you should make your locust classes inherit from HttpLocust instead of the base Locust class.

msgpack for serializing master/slave data

Locust now uses msgpack for serializing data that is sent between a master node and its slaves. This addresses a possible attack that can be used to execute code remote, if one has access to the internal locust ports that are used for master-slave communication. The reason for this exploit was due to the fact that pickle was used.


Anyone who uses an older version should make sure that their Locust machines are not publicly accessible on port 5557 and 5558. Also, one should never run Locust as root.

Anyone who uses the report_to_master and slave_report events, needs to make sure that any data that is attached to the slave reports is serializable by msgpack.

requests updated to version 2.2

Locust updated requests to the latest major release.


Requests 1.0 introduced some major API changes (and 2.0 just a few). Please check if you are using any internal features and check the documentation: Migrating to 1.x and Migrationg to 2.x

gevent updated to version 1.0

gevent 1.0 has now been released and Locust has been updated accordingly.

Big refactoring of request statistics code

Refactored RequestStats.

  • Created StatsEntry which represents a single stats entry (URL).

Previously the RequestStats was actually doing two different things:

  • It was holding track of the aggregated stats from all requests

  • It was holding the stats for single stats entries.

Now RequestStats should be instantiated and holds the global stats, as well as a dict of StatsEntry instances which holds the stats for single stats entries (URLs)

Removed support for avg_wait

Previously one could specify avg_wait to TaskSet and Locust that Locust would try to strive to. However this can be sufficiently accomplished by using min_wait and max_wait for most use-cases. Therefore we’ve decided to remove the avg_wait as its use-case is not clear or just too narrow to be in the Locust core.

Removed support for ramping

Previously one could tell Locust, using the –ramp option, to try to find a stable client count that the target host could handle, but it’s been broken and undocumented for quite a while so we’ve decided to remove it from the locust core and perhaps have it reappear as a plugin in the future.

Locust Event hooks now takes keyword argument

When Event hooks by listening to Event hooks, the listener functions should now expect the arguments to be passed in as keyword arguments. It’s also highly recommended to add an extra wildcard keyword arguments to listener functions, since they’re then less likely to break if extra arguments are added to that event in some future version. For example:

from locust import events

def on_request(request_type, name, response_time, response_length, **kw):
    print "Got request!" += on_request

The method and path arguments to request_success and request_failure are now called request_type and name, since it’s less HTTP specific.

Other changes

  • You can now specify the port on which to run the web host

  • Various code cleanups

  • Updated gevent/zmq libraries

  • Switched to unittest2 discovery

  • Added option –only-summary to only output the summary to the console, thus disabling the periodic stats output.

  • Locust will now make sure to spawn all the specified locusts in distributed mode, not just a multiple of the number of slaves.

  • Fixed the broken Vagrant example.

  • Fixed the broken events example (

  • Fixed issue where the request column was not sortable in the web-ui.

  • Minor styling of the statistics table in the web-ui.

  • Added options to specify host and ports in distributed mode using –master-host, –master-port for the slaves, –master-bind-host, –master-bind-port for the master.

  • Removed previously deprecated and obsolete classes WebLocust and SubLocust.

  • Fixed so that also failed requests count, when specifying a maximum number of requests on the command line


  • Made Locust compatible with gevent 1.0rc2. This allows user to step around a problem with running Locust under some versions of CentOS, that can be fixed by upgrading gevent to 1.0.

  • Added parent attribute to TaskSet class that refers to the parent TaskSet, or Locust, instance. Contributed by Aaron Daubman.


  • Fixed bug that was causing problems when setting a maximum number of requests using the -n or –num-request command line parameter.



This version comes with non backward compatible changes to the API. Anyone who is currently using existing locust scripts and want to upgrade to 0.6 should read through these changes.

SubLocust replaced by TaskSet and Locust class behavior changed

Locust classes does no longer control task scheduling and execution. Therefore, you no longer define tasks within Locust classes, instead the Locust class has a task_set attribute which should point to a TaskSet class. Tasks should now be defined in TaskSet classes, in the same way that was previously done in Locust and SubLocust classes. TaskSets can be nested just like SubLocust classes could.

So the following code for 0.5.1:

class User(Locust):
    min_wait = 10000
    max_wait = 120000

    def index(self):

    class AboutPage(SubLocust):
        min_wait = 10000
        max_wait = 120000

        def on_init(self):

        def team_page(self):

        def press_page(self):

        def stop(self):

Should now be written like:

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

    class AboutPage(TaskSet):
        def on_init(self):

        def team_page(self):

        def press_page(self):

        def stop(self):

class User(Locust):
    min_wait = 10000
    max_wait = 120000
    task_set = BrowsePage

Each TaskSet instance gets a locust attribute, which refers to the Locust class.

Locust now uses Requests

Locust’s own HttpBrowser class (which was typically accessed through self.client from within a locust class) has been replaced by a thin wrapper around the requests library ( This comes with a number of advantages. Users can now take advantage of a well documented, well written, fully fledged library for making HTTP requests. However, it also comes with some small API changes which will require users to update their existing load testing scripts.

Gzip encoding turned on by default

The HTTP client now sends headers for accepting gzip encoding by default. The –gzip command line argument has been removed and if someone want to disable the Accept-Encoding that the HTTP client uses, or any other HTTP headers you can do:

class MyWebUser(Locust):
    def on_start(self):
        self.client.headers = {"Accept-Encoding":""}

Improved HTTP client

Because of the switch to using python-requests in the HTTP client, the API for the client has also gotten a few changes.

  • Additionally to the get, post, put, delete and head methods, the HttpSession class now also has patch and options methods.

  • All arguments to the HTTP request methods, except for url and data should now be specified as keyword arguments. For example, previously one could specify headers using:

    client.get("/path", {"User-Agent":"locust"}) # this will no longer work

    And should now be specified like:

    client.get("/path", headers={"User-Agent":"locust"})
  • In general the whole HTTP client is now more powerful since it leverages on python-requests. Features that we’re now able to use in Locust includes file upload, SSL, connection keep-alive, and more. See the python-requests documentation for more details.

  • The new HttpSession class’ methods now return python-request Response objects. This means that accessing the content of the response is no longer made using the data attribute, but instead the content attribute. The HTTP response code is now accessed through the status_code attribute, instead of the code attribute.

HttpSession methods’ catch_response argument improved and allow_http_error argument removed

  • When doing HTTP requests using the catch_response argument, the context manager that is returned now provides two functions, success and failure that can be used to manually control what the request should be reported as in Locust’s statistics.

    class ResponseContextManager(response, request_event, request_meta)[source]

    A Response class that also acts as a context manager that provides the ability to manually control if an HTTP request should be marked as successful or a failure in Locust’s statistics

    This class is a subclass of Response with two additional methods: success and failure.


    Report the response as a failure.

    if exc is anything other than a python exception (like a string) it will be wrapped inside a CatchResponseError.


    with self.client.get("/", catch_response=True) as response:
        if response.content == b"":
            response.failure("No data")

    Report the response as successful


    with self.client.get("/does/not/exist", catch_response=True) as response:
        if response.status_code == 404:
  • The allow_http_error argument of the HTTP client’s methods has been removed. Instead one can use the catch_response argument to get a context manager, which can be used together with a with statement.

    The following code in the previous Locust version:

    client.get("/does/not/exist", allow_http_error=True)

    Can instead now be written like:

    with client.get("/does/not/exist", catch_response=True) as response:

Other improvements and bug fixes

  • Scheduled task callables can now take keyword arguments and not only normal function arguments.

  • SubLocust classes that are scheduled using locust.core.Locust.schedule_task() can now take arguments and keyword arguments (available in self.args and self.kwargs).

  • Fixed bug where the average content size would be zero when doing requests against a server that didn’t set the content-length header (i.e. server that uses Transfer-Encoding: chunked)

Smaller API Changes

  • The require_once decorator has been removed. It was an old legacy function that no longer fit into the current way of writing Locust tests, where tasks are either methods under a Locust class or SubLocust classes containing task methods.

  • Changed signature of locust.core.Locust.schedule_task(). Previously all extra arguments that was given to the method was passed on to the task when it was called. It no longer accepts extra arguments. Instead, it takes an args argument (list) and a kwargs argument (dict) which are be passed to the task when it’s called.

  • Arguments for request_success event hook has been changed. Previously it took an HTTP Response instance as argument, but this has been changed to take the content-length of the response instead. This makes it easier to write custom clients for Locust.


  • Fixed bug which caused –logfile and –loglevel command line parameters to not be respected when running locust without zeromq.


API changes

  • Web interface is now turned on by default. The –web command line option has been replaced by –no-web.

  • and now gets the HTTP method as the first argument.

Improvements and bug fixes

  • Removed –show-task-ratio-confluence and added a –show-task-ratio-json option instead. The –show-task-ratio-json will output JSON data containing the task execution ratio for the locust “brain”.

  • The HTTP method used when a client requests a URL is now displayed in the web UI

  • Some fixes and improvements in the stats exporting:

  • A file name is now set (using content-disposition header) when downloading stats.

  • The order of the column headers for request stats was wrong.

  • Thanks Benjamin W. Smith, Jussi Kuosa and Samuele Pedroni!


API changes

  • WebLocust class has been deprecated and is now called just Locust. The class that was previously called Locust is now called LocustBase.

  • The catch_http_error argument to HttpClient.get() and has been renamed to allow_http_error.

Improvements and bug fixes

  • Locust now uses python’s logging module for all logging

  • Added the ability to change the number of spawned users when a test is running, without having to restart the test.

  • Experimental support for automatically ramping up and down the number of locust to find a maximum number of concurrent users (based on some parameters like response times and acceptable failure rate).

  • Added support for failing requests based on the response data, even if the HTTP response was OK.

  • Improved master node performance in order to not get bottlenecked when using enough slaves (>100)

  • Minor improvements in web interface.

  • Fixed missing template dir in MANIFEST file causing locust installed with “ install” not to work.