custom_eval.py

"""
This example shows how a custom evaluation function can be added to
handle certain SPARQL Algebra elements.

A custom function is added that adds `rdfs:subClassOf` "inference" when
asking for `rdf:type` triples.

Here the custom eval function is added manually, normally you would use
entry points to do it. See the [Plugins Usage Documentation](../plugins.md).
"""

from pathlib import Path

import rdflib
from rdflib.namespace import FOAF, RDF, RDFS
from rdflib.plugins.sparql.evaluate import evalBGP

EXAMPLES_DIR = Path(__file__).parent


inferred_sub_class = (
    RDFS.subClassOf * "*"  # type: ignore[operator]
)  # any number of rdfs.subClassOf


def customEval(ctx, part):  # noqa: N802
    """
    Rewrite triple patterns to get super-classes
    """

    if part.name == "BGP":
        # rewrite triples
        triples = []
        for t in part.triples:
            if t[1] == RDF.type:
                bnode = rdflib.BNode()
                triples.append((t[0], t[1], bnode))
                triples.append((bnode, inferred_sub_class, t[2]))
            else:
                triples.append(t)

        # delegate to normal evalBGP
        return evalBGP(ctx, triples)

    raise NotImplementedError()


if __name__ == "__main__":
    # add function directly, normally we would use setuptools and entry_points
    rdflib.plugins.sparql.CUSTOM_EVALS["exampleEval"] = customEval

    g = rdflib.Graph()
    g.parse(f"{EXAMPLES_DIR / 'foaf.n3'}")

    # Add the subClassStmt so that we can query for it!
    g.add((FOAF.Person, RDFS.subClassOf, FOAF.Agent))

    # Find all FOAF Agents
    for x in g.query(
        f"""
        PREFIX foaf: <{FOAF}>

        SELECT *
        WHERE {{
            ?s a foaf:Agent .
        }}
        """
    ):
        print(x)