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"""
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
setuptools and entry_points to do it:
i.e. in your setup.py::
entry_points = {
'rdf.plugins.sparqleval': [
'myfunc = mypackage:MyFunction',
],
}
"""
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):
"""
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)
|
