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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',
],
}
"""
import rdflib
from rdflib.plugins.sparql.evaluate import evalBGP
from rdflib.namespace import FOAF
inferredSubClass = \
rdflib.RDFS.subClassOf * '*' # 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] == rdflib.RDF.type:
bnode = rdflib.BNode()
triples.append((t[0], t[1], bnode))
triples.append((bnode, inferredSubClass, 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.load("foaf.rdf")
# Add the subClassStmt so that we can query for it!
g.add((FOAF.Person,
rdflib.RDFS.subClassOf,
FOAF.Agent))
# Find all FOAF Agents
for x in g.query(
'PREFIX foaf: <%s> SELECT * WHERE { ?s a foaf:Agent . }' % FOAF):
print(x)
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