Contributing Guidelines

This document describes the existing developer tooling we have in place (and what to expect of it), as well as our design and development philosophy.

See for a Python crash course.


Underneath the hood, lisa is just Pytest! Refer to its documentation.

Some useful CLI options are:

  • --help and --verbose, of course

  • --log-cli-level=DEBUG to enable live output of all DEBUG and above logs

  • --collect-only to show which tests would be selected (but don’t run them)

  • --setup-only to show in which order fixtures and tests would be setup (but don’t run them)

  • --flake8 --mypy -m "flake8 or mypy" to run the semantic analysis tools

YAML Schema

The registered playbook schema can be generated using --print-schema:

lisa --print-schema=playbooks/schema.json

This will create a file playbooks/schema.json with the JSON Schema for all the registered schemata (including those added in any local plugins). Note that this file is committed to the repo for the public schema, but you can generate your own (or update it) with the above command.

Using the LSP yaml-language-server you can setup almost any editor to lint the playbook files against the schema. Either add as a comment to the top of the playbook file:

# yaml-language-server: $schema=file:///path/to/playbooks/schema.json

Or set yaml.schemas as appropriate for your editor. Also ensure that yaml-language-server is installed, which you can do via:

npm install -g yaml-language-server

See for a crash course in YAML.


If you have ran lisa already, then you have installed and used the poetry tool. Poetry is a PEP 518 compliant and cross-platform build system which handles our Python dependencies and environment.

This project’s dependencies are found in the pyproject.toml file. This is similar to but more powerful than the familiar requirements.txt. With PEP 518 and PEP 621.


The first section, tool.poetry, defines the project’s metadata (name, version, description, authors, and license) which will be embedded in the final built package.

The chosen version follows Semantic Versioning, with the Python specific pre-release versioning suffix ‘.dev1’. Since this is “pytest-lisa” it seemed appropriate to set our version to ‘3.0.0.dev1’, that is, “the first development release of pytest-lisa.”

Package Dependencies

The next section, tool.poetry.dependencies, is where poetry add <package_name> records our required packages.

Poetry automatically creates and manages isolated environments.

From the documentation:

Poetry will first check if it’s currently running inside a virtual environment. If it is, it will use it directly without creating a new one. But if it’s not, it will use one that it has already created or create a brand new one for you.

On Linux, your initial run of poetry install will cause Poetry to automatically setup a new virtualenv using pyenv. If you are developing on Windows, you may need to setup your own, perhaps using Conda.

The path to the virtualenv used by Poetry can found with this command:

poetry env list --full-path

Use it to configure your editor.

Developer Dependencies

Similar to the previous section, is where poetry add --dev <package_name> records our developer packages. These are not necessary for LISAv3 to execute, but are used by developers to automatically adhere to our coding standards.

  • Black, the opinionated code formatter which settles all debates as to how our Python files should be formatted. It follows PEP 8, the official Python style guide, and where ambiguous makes the decision for us.

  • Flake8 (and integrations), the semantic analyzer, used to coordinate most of the other tools.

  • isort, the import sorter, which automatically splits imports into the expected, alphabetized sections.

  • mypy, the static type checker, which coupled with type annotations allows us to avoid the pitfalls of Python being a dynamically typed language.

  • python-language-server (and integrations), the de facto LSP server. While Microsoft is developing their own LSP servers, they do not integrate with the existing ecosystem of tools, and their latest tool, Pyright, simply does not support pyproject.toml. Since pyls is used far more widely, and supports every editor, we use it.

  • rope, to provide completions and renaming support to pyls.

With these packages installed and a correctly setup editor (see below, and feel free to reach out to us), your code should automatically follow all the standards which we could automate.

The final sections,, tool.isort, build-system, and files mypy.ini and .flake8 configure the tools per their recommendations.

Editor Setup

Visual Studio Code

First, click the Python version in the bottom left, then enter the path emitted by the command above. This will point Code to the Poetry virtual environment.

Make sure below settings are in root level of .vscode/settings.json

    "python.analysis.typeCheckingMode": "strict",
    "python.formatting.provider": "black",
    "python.linting.enabled": true,
    "python.linting.flake8Enabled": true,
    "python.linting.mypyEnabled": true,
    "python.linting.pylintEnabled": false,
    "editor.formatOnSave": true,
    "python.linting.mypyArgs": [
    "python.sortImports.path": "isort",
    "python.analysis.useLibraryCodeForTypes": false,
    "python.analysis.autoImportCompletions": false,
    "files.eol": "\n",


I recommend using the pyvenv package to have Emacs automatically use the correct Python venv (setup by Poetry), and the eglot package to provide LSP support. The below expects you already have an init.el with use-package.

(use-package pyvenv
  :ensure t
  :hook (python-mode . pyvenv-tracking-mode))

(use-package eglot
  (python-mode . eglot-ensure)
  (yaml-mode . eglot-ensure)
  (eglot-auto-display-help-buffer t)
  (eglot-autoshutdown t)
  (eglot-confirm-server-initiated-edits nil)
  (add-to-list 'eglot-server-programs '(yaml-mode . ("yaml-language-server" "--stdio")))
  (defun eglot-format-buffer-on-save ()
    (if (and (project-current) (eglot-managed-p))
        (add-hook 'before-save-hook #'eglot-format-buffer nil 'local)
      (remove-hook 'before-save-hook #'eglot-format-buffer 'local)))
  (add-hook 'eglot-managed-mode-hook #'eglot-format-buffer-on-save))

Then run M-x add-dir-local-variable RET python-mode RET pyvenv-activate RET <path/to/virtualenv> where the value is the path given by the command above. This will create a .dir-locals.el with this variable set for Python.

Refer to this .dir-locals.el for a complete setup:

;;; Directory Local Variables
;;; For more information see (info "(emacs) Directory Variables")

  . ((eglot-workspace-configuration ; an LSP implementation
      ;; Use `flake8’ instead of the default, and disable noisy plugins.
      . ((:pyls . (:configurationSources ["flake8"] :plugins (:pycodestyle (:enabled nil) :mccabe (:enabled nil))))))))
  . ((eglot-workspace-configuration
      ;; Set the `playbooks/schema.json’ as the schema for playbooks.
      . ((:yaml . (:schemas (:playbooks/schema.json "playbooks/*")))))))
 ;; Set `pyvenv’ to use the given venv for the whole project.
 (nil . ((pyvenv-activate . "~/.cache/pypoetry/virtualenvs/<venv name>"))))

Type Annotations

We are using mypy to enforce static type checking of our Python code. This may surprise you as Python is not a statically typed language. While dynamic typing can be useful, for a complex tool such as LISA it is more likely to introduce bugs that are found only at runtime (which the user experiences as a crash). For more information on why we (and others) do this, see Dropbox’s journey to type checking 4 million lines of Python. PEP 484 and PEP 526 (among others) introduced and defined type hints for the Python language. You can probably figuring out the syntax based on the surrounding code, but you can also see this Intro to Using Python Type Hints and mypy’s cheat sheet.

Python Naming Conventions

Naming conventions are not automatically enforced, so please read the naming conventions section of PEP 8, which describes what each of the different styles means. A short summary of the most important parts:

  • Modules (and hence files) should have short, all-lowercase names.

  • Class (and exception) names should normally use the CapWords convention (also known as CamelCase).

  • Function and variable names should be lowercase, with words separated by underscores as necessary to improve readability (also known as snake_case).

  • To avoid collisions with the standard library, an underscore can be appended, such as id_.

  • Always use self for the first argument to instance methods.

  • Always use cls for the first argument to class methods.

  • Use one leading underscore only for non-public methods and instance variables, such as _data. Do not activate name mangling with __ unless necessary.

  • If there is a pair of get_x and set_x methods, they should instead be a proper property, which is easy to do with the built-in @property decorator.

  • Constants should be CAPITALIZED_SNAKE_CASE.

  • When importing a function, try to avoid renaming it with import as because it introduces cognitive overhead to track yet another name.

When in doubt, adhere to existing conventions, or check the style guide.

Committing Guidelines

A best practice when using Git is to create a series of independent and well-documented commits. Each commit should “do one thing” and do it correctly. If a mistake is made (you need to fix a bug or adjust formatting), you should amend it (or use an interactive rebase to edit it). If you’re using Emacs, the Magit package makes all of this easy. Some of the reasons for making each commit polished is that it aids immensely in future debugging. It lets us use tools like git bisect to automatically find bugs, and understand why prior code was written. Although some of it has gone out of date, see this otherwise great essay on Git best practices. For how Git works, read Git from the Bottom Up.

For writing your commit messages, see this modification of Tim Pope’s example:

Capitalized, short (72 chars or less) summary

More detailed explanatory text, if necessary. Wrap it to about 72 characters or so. In some contexts, the first line is treated as the subject of an email and the rest of the text as the body. The blank line separating the summary from the body is critical (unless you omit the body entirely); tools like rebase can get confused if you run the two together.

Write your commit message in the imperative: “Fix bug” and not “Fixed bug” or “Fixes bug.” This convention matches up with commit messages generated by commands like git merge and git revert.

Further paragraphs come after blank lines.

  • Bullet points are okay, too

  • Typically a hyphen or asterisk is used for the bullet, followed by a single space, with blank lines in between, but conventions vary here

  • Use a hanging indent

You should also feel free to use Markdown in the commit messages, as our project is hosted on GitHub which renders it (and Markdown is human readable).

Design Patterns

The most important goal we are attempting to accomplish with LISAv3 is for it to be “simple, clean, and with a low maintenance cost.”

We should use caution when using Object Oriented Design, because when it is used without critical analysis, it creates unmaintainable code. A great talk on this subject is Stop Writing Classes, by Jack Diederich. As he says, “classes are great but they are also overused.”

This Python Design Patterns is a fantastic collection of material for writing maintainable Python code. It specifically details many of the common “Object Oriented” patterns from the Gang of Four book (which, in fact, were patterns geared toward languages like C++, and no longer apply to modern languages like Python), what lessons can be learned from them, and how to apply them (or their modern alternatives) today. It also serves as an easy-to-read guide to the Gang of Four book itself, as its principles still serve us well today.

Every time a developer chooses to use a design pattern, that person needs to reason through and document why it was chosen, and what alternatives were considered. We will recreate the problems with LISAv2 unless we take our time to carefully create a well-designed and maintainable framework.

Several popular patterns that actually do not work well in Python are:

Conversely, patterns that are a natural fit to Python include:

Finally, a high-level guide to all things Python is The Hitchhiker’s Guide to Python. It covers just about everything in the Python world. If you make it through even some of these guides, you will be well on your way to being a “Pythonista” (a Python developer) writing “Pythonic” (canonically correct Python) code left and right.

Generating Documentation

We use Sphinx to generate our documentation. To build it locally (and check all the links), use:

sphinx-build -b linkcheck . _build

Publishing Packages

Setup PyPI with Poetry’s repositories:

poetry config pypi-token.pypi my-token

Then for each package, build and publish it! Don’t forget to increment the version number afterwards. You should carefully test the package on the Test PyPI instance first, as you cannot overwrite uploaded packages. You can also bump the patch version if you made a small mistake.

poetry publish --build

Future Sections

Just a collection of reminders for the author to expand on later.