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Developer Guidelines

Contributors to the Chaos Toolkit are always welcome. This guide describes the general elements you probably need to know to get started. Once past those elements, you should dive into the code of each project and come discuss on our Slack.


Programming Environment

The programming environment really is up to you. However, since the Chaos Toolkit is implemented in Python so make sure to have the right tooling for it.

The most basics are:

  • Python 3.8+ installed.
  • A virtual environment so you can deploy the dependencies in a specific environment

If you’re not familiar with Python, you will find a few helpful books online, such The Hitchhiker’s Guide to Python.

Historically, Chaos Toolkit used pip to manage its projects from a Python dependencies and packages perspectives. However, we have now migrated to PDM which provides a better all-in-one experience.


The Chaos Toolkit projects are hosted on GitHub. If you wish to contribute, you will need to have an account there.

The general workflow is to fork the project you wish to contribute to, make your changes in a dedicated branch, rebase against the original master and finally submit a pull-request to the project with a clear description of the what and why.

Code Contributions Good Practices

Whether you contribute documentation or code, you should try follow the high-level rules here:

  • One PR per functional change. Try to keep your PR focused

  • A PR should always have the following:

  • Signed commits (as per the DCO) to notify the project you are allowed to submit this code change:

    git commit --signoff -m "...." -a
  • A CHANGELOG entry. Do not set the version or date on the entry

  • Tests wherever possible to control non-regression down the road and help with maintainance
  • Linted code. If the project uses pdm, you can run pdm run format and pdm run lint usually
  • Code must have the appropriate typing annotation

  • What to look for in a contribution:

  • Try to draw inspiration from other modules in the same extension or from other extensions. Respect the style as much as possible and use the linter to ensure you follow it
  • If you had an entirely new module containing actions/probes, make sure to make it discoverable by adding it to the module of the extension package. It usually already has existing entries.
  • Make actions/probes not too complex, better to have more specific actions sometimes. Otherwise, this increases the risk of breaking compatibility when they change
  • Be conservative in changes you make to existing actions/probes. We thrive for backward compatibility
  • Keep simple types for actions/probes arguments: string, integer, floats…
  • Raise a Chaos Toolkit exception when you want to notify the user about a particular error/edge case that will prevent the function from working correctly
  • Use the logger as much as needed, it helps figuring out what is going on. However think of who will read these messages in the future, likely not yourself
  • The returned value of an action/probe must be JSON serializable by the Python json module
  • Try to respect a width of 80 characters for code and documentation, this should be covered by the code formatter
  • Sort imports as follows:

    # Python stdlib imports
    # Third-party imports
    # Local package imports

    This should be dealt with by the code formatter

Here is a typical action:

import logging
from typing import Any, Dict

from chaoslib.exceptions import ActivityFailed
from chaoslib.types import Configuration, Secrets

__all__ = ["change_the_system"]

logger = logging.getLogger("chaostoolkit")

def change_the_system(
    address: str,
    value: int,
    configuration: Configuration = None,
    secrets: Secrets = None
) -> Dict[str, Any]:
    Change the system in funny ways and return the information about
    what changed
    if value < 0:
        logger.debug(f"change_the_system got {value}")
        raise ActivityFailed("value cannot be negative")

    # do something to your system with the given arguments

    return {}


Always finish the signature with the configuration and secrets arguments. This is merely code readability convention as neither Python nor Chaos Toolkit will care. However, this will be consistent across the entire Chaos Toolkit code base.

Since we have just added this action in a new module, we’ll also make the module discoverable as follows:


This is done in the top-level module of the extension in the discover function. Usually, it already contains this function and you can simply add the line above.

A typical CHANGELOG entry for this new action would be:

## [Unreleased][]


### Added

* The `extension.actions.change_the_system` action to change the system. Use
  it like this:

  "method": [
      "name": "do-something",
      "type": "action",
      "provider": {
        "type": "python",
        "module": "extension.actions",
        "func": "change_the_system",
        "arguments": {
          "address": "",
          "value": 56