Developer Guide

This guide is intended for people who want to contribute to the code, documentation, and test coverage of the core COMPAS packages:

Note, however, that the general procedure is applicable to all COMPAS package development.


To set up a developer environment

  1. Fork the repository and clone the fork.

  2. Create a virtual environment using your tool of choice (e.g. virtualenv, conda, etc).

    conda create -n compas-dev python=3.8 cython --yes
    conda activate compas-dev
  3. Install development dependencies:

    cd path/to/compas
    pip install -r requirements-dev.txt
  4. Make sure all tests pass and the code is free of lint:

    invoke lint
    invoke test
  5. Create a branch for your contributions.

    git branch title-proposed-changes
    git checkout title-proposed-changes
  6. Start making changes!

Submitting a PR

Once you are done making changes, you have to submit your contribution through a pull request (PR). The procedure for submitting a PR is the following.

  1. Make sure all tests still pass, the code is free of lint, and the docstrings compile correctly:

    invoke lint
    invoke test
    invoke docs
  2. Add yourself to

  3. Summarize the changes you made in

  4. Commit your changes and push your branch to GitHub.

  5. Create a pull request.

Style guide

Please run black -l 180 <path_to_source_file> to auto-format your code.

The command invoke lint runs the entire codebase through flake8. As described in the docs, flake8 includes lint checks provided by the PyFlakes project, PEP-0008 inspired style checks provided by the PyCodeStyle project, and McCabe complexity checking provided by the McCabe project.

The list of potential error codes issued by flake8 are available here:

The PEP-0008 style guide is available here:

Note that the maximum line length is set to 180 rather 79 in the setup.cfg of the repo.

Naming conventions

We (intend to) use the following naming conventions.

  1. variables, functions, methods, attributes use “snake_case”: they are written in lowercase and spaces between words are replaced by underscores.

  2. class names use (Upper) “CamelCase”: The are written in lowercase, with the first letter of each word capitalized and spaces between words removed.

  3. module or package level variables are in uppercase and with spaces between words replaced by underscores.


Ideally, we would use the following conventions for quotes.

  1. Double quotation marks for multiline statements and docstrings. For example, """Calculate the sum of two numbers."""

  2. Single quotation marks for strings that are used “as variables”. For example, config['param'] = 1.

  3. Double quotation marks for strings that are meant to be used as text. For examples, message = "Select one or more points."


The documentation of COMPAS is generated with Sphinx. This means that code docstrings and general documentation pages have to be written in RestructuredText.

Each function, method, and class should have a docstring describing its behaviour. We use sphinx.ext.napoleon to allow for human-readable docstrings, and prefer Numpy-style docstring formatting rules.

To include a new function or class in the documentation, it should be added to the API docstring in of the main package it belongs to.

For example, if you add a function somewhere in the geometry package, make sure to include it in the docstring of

Type hints

Type hints should be added to stub files at the public API level of the main packages (see Code structure). This allows the type hints to be written using Python 3 style annotations while maintaining compatibility with Python 2.7 for Rhino/GH.

For example, the type hints for compas.datastructures should be defined in compas.datastructures.__init__.pyi.

Code structure

Each of the core packages is divided into subpackages that group functionality into logical components. For example, compas is divided into:

The API of each subpackage is documented in the docstring of its file using basic RestructuredText. From outside of these packages, functionality should be imported directly from the subpackage level, regardless of the code structure underneath.

For example, in some

from compas.datastructures import Mesh
from compas.datastructures import Network

from compas.geometry import add_vectors
from compas.geometry import oriented_bounding_box_numpy
from compas.geometry import Polygon
from compas.geometry import Transformation

from compas.numerical import pca_numpy
from compas.numerical import fd_numpy

To allow the public API of the modules and packages contained in a subpackage to reach the subpackage level, each module should declare the classes, functions and variables of its public API in the module’s __all__ variable. Per package, the APIs of the contained module are collected in the __all__ variable of the package (in the

__all__ = [_ for _ in dir() if not _.startswith('_')]


More info coming soon…


Although we still have a significant backlog of existing functionality not yet covered by unit tests, at least all newly added functionality should have a corresponding test.

We use pytest as a testing framework. The tests are in the tests folder at the root of the repo.

More info coming soon…

Grasshopper components

Grasshopper user objects need to be built using COMPAS Github Action componentizer.

  1. Apply your changes to the component source code (src/compas_ghpython/components).

  2. Rebuild them:

    invoke build-ghuser-components
  3. Install them on Rhino/Grasshopper as usual:

    python -m compas_rhino.install

The install step does not copy them, but creates a symlink to the location in which they are built, so after the first installation, it is usually not required to reinstall them, only rebuild them (unless a new component is added).


This step requires IronPython version 2.7 to be available on the system. The default behavior is to run the command ipy. If this command is not available or is the wrong version, the optional ironpython argument will replace it, eg invoke build-ghuser-components --ironpython="mono path/to/ipy.exe".


COMPAS has an extensible architecture based on plugins that allows to customize and extend the functionality of the core framework.

For a plugin to work, there needs to exist a counterpart to be connected to. This means there are two components involved:

Both of these components are declared using decorators:

def do_hard_stuff(input):

def do_hard_stuff_numpy(input):
    # NOTE: Here use the power of numpy to do hard stuff very fast
    # ..

Once these parts are implemented, the program could simply call the function do_hard_stuff and the appropriate plugin implementation using numpy would be called automatically.

Why are plugins important?

The example above is just a single code block, but the power of plugins comes from the ability to split those two parts -the compas.plugins.pluggable() and the compas.plugins.plugin()- into completely different files, folders or even entire projects and still work the same way.

Additionally, COMPAS is able to pick the most suitable plugin implementation for its current execution context. For instance, one could have two implementations of the same compas.plugins.pluggable() definition, one using numpy and another one using Rhino SDK and have the correct one automatically selected based on where your script is executing.

How to make plugins discoverable?

COMPAS plugin discovery is based on naming conventions. This is mainly due to the need to support IronPython inside Rhino, which lacks setuptools infrastructure. For more details, check these python guidelines.

A COMPAS plugin needs to fulfill two conditions:

  • Name: The package name should be prefixed with compas, eg. compas_cgal.

  • Metadata: The package should define a bit of metadata listing the modules that contain plugins. This is done declaring a variable called __all_plugins__, eg. __all_plugins__ = ['compas_cgal.booleans'].

COMPAS automatically discovers plugins searching over all available packages in the system, and picks up those prefixed with the compas word. All packages are included in the search: packages installed with pip, packages made available through the PYTHONPATH / IRONPYTHONPATH, local packages, etc.

Once a package is found, the metadata in __all_plugins__ is read and all modules listed are analyzed to look for functions decorated with the compas.plugins.plugin() decorator.

Two kinds of extension points

An extension point, or pluggable interface can be declared as being one of two types based on how they select which implementation to pick if there are multiple available.

  • selector='first_match': this type of extension point will pick the first plugin implementation that satisfies the requirements.

  • selector='collect_all': extension points defined with this selector will instead collect all plugin implementations and execute them all, collecting the return values into a list. An example of this is the Rhino install extension point: compas_rhino.install.installable_rhino_packages().

A complete example

Let’s explore a complete example to gain a better understanding.

Extension point

For the sake of example, we are going to assume that compas core defines the following compas.plugins.pluggable() interface in


def boolean_union_mesh_mesh(A, B):


Now let’s write a plugin that implements this interface:


__all_plugins__ = ['compas_plugin_sample.boolean_trimesh']


import trimesh

@plugin(category='booleans', requires=['trimesh'])
def boolean_union_mesh_mesh(A, B):
    va, fa = A
    at = trimesh.Trimesh(vertices=va, faces=fa)

    vb, fb = B
    bt = trimesh.Trimesh(vertices=vb, faces=fb)

    r = at.union(bt, engine='scad')

    return r.vertices, r.faces

Voilà! We have a trimesh-based boolean union plugin!

Advanced options

There are a few additional options that plugins can use:

  • requires: List of requirements. COMPAS will filter out plugins if their requirements list is not satisfied at runtime. This allows to have multiple implementations of the same operation and have them selected based on different criteria. The requirement can either be a package name string (e.g. requires=['scipy']) or a callable with a boolean return value, in which any arbitrary check can be implemented (e.g. requires=[lambda: is_rhino_active()]).

  • tryfirst and trylast: Plugins cannot control the exact priority they will have but they can indicate whether to try to prioritize them or demote them as fallback using these two boolean parameters.

  • pluggable_name: Usually, the name of the decorated plugin method matches that of the pluggable interface. When that is not the case, the pluggable name can be specified via this parameter.

  • domain: extension points are unambiguously identified by a URL that combines domain, category and pluggable name. All COMPAS core plugins use the same domain, but other packages could potentially decide to use a different domain to ensure collision-free naming of pluggable extension points.

While developing plugins, it is also possible to enable print output to understand what how plugin selection works behind the scenes. To enable that, set DEBUG flag accordingly:

from compas.plugins import plugin_manager
plugin_manager.DEBUG = True