What are plugins?

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