How to convert an OWL ontology to YAML

OSP-core requires an ontology in YAML format for installation. This document explains how an ontology in OWL format can be converted to YAML.


  • We use owlready2 for the conversion. Therefore, the ontology must be in one of the following formats: RDF/XML, OWL/XML, NTriples.
  • Make sure you ran the reasoner and exported the inferred axioms.
  • OSP-core must be installed


This section will explain in detail how you can convert an OWL ontology to YAML. We will use the EMMO as an example. The document doc/ explains how you can get the EMMO.

  1. The script for the conversion is called owl2yml. Call it with the -h option to print help:

    $ owl2yml -h
    usage: owl2yml [-h] --namespace NAMESPACE [--conversion_options_file CONVERSION_OPTIONS_FILE] [--version VERSION] [--output-file OUTPUT_FILE] input-file
    Convert an ontology in OWL format to an ontology in YAML format.
    positional arguments:
      input-file            The path to the input owl file
    optional arguments:
      -h, --help            show this help message and exit
      --namespace NAMESPACE, -n NAMESPACE
                            The namespace for the resulting YAML file in UPPERCASE
                            Path to a file explaining how the ontology should be transformed, s.t. it is compatible with OSP-core
      --version VERSION, -v VERSION
                            The version string for the resulting YAML file
      --output-file OUTPUT_FILE, -o OUTPUT_FILE
                            Where the output file should be saved
  2. You have to specify the owl input file and the namespace, with which the entities in the owl file should be accessible in OSP-core:

    $ owl2yml -n MY_NAMESPACE path/to/owlfile.owl
  3. Optionally, you can specify a version string:

    $ owl2yml -n MY_NAMESPACE path/to/owlfile.owl -v 0.0.1
  4. Optionally, you can specify where the resulting yaml file should be stored:

    $ owl2yml -n MY_NAMESPACE ../../owlfile.owl -o result.yml
  5. It is recommended that you extend the ontology, such that it is easier to use with OSP-core. For that you have to create a yaml file in the following format:

    default_rel: <iri_of_default_relationship>
    active_relationships: [list_of_iris]
        datatype: FLOAT
    • For EMMO, this file is provided and located in osp/core/tools/conversion_options/emmo.conversion_options.yml
    • You can specify a default relationship with the default_rel keyword. You have to provide the IRI of the relationship. In OSP-core, when you add one CUDS objects to another without specifying a relationship, this relationship will be used.
    • CUDS objects are containers conceptually. Therefore, you need to specify which relationships encode a containment in a CUDS objects. We call these relationships active relationships. Let’s say you have a relationship contains in the ontology. You want to specify that every cuds object related to cuds object A with contains should be in the container of A. Then contains should be marked as an active relationship. Note that the edge-induced graph of the cuds objects by only considering active relationships must be acyclic at any point in time. Active relationships must not be symmetric. See doc/ for more details. You can specify active relationships with the active_relationships keyword. You need to provide a list of IRIs of relationships that should become an active relationship.
    • If you are not satisfied with your result, you can manually modify the resulting yml file. You can also use the keywords insert_entities or update_entities to modify the resulting yml automatically.