Why my query doesn't work using RDFlib

Question

I've been trying to query an OWL data using RDFlib (SPARQL), but I didn't get why it doesn't work. I tested the same query in Protege (SPARQL query) and it works perfectly! This is my code:

import rdflib
from rdflib import plugin
from rdflib.graph import Graph

g = Graph()
g.parse("/localPath/a.owl")

from rdflib.namespace import Namespace
ns = Namespace("http://oaei.ontologymatching.org/2011/benchmarks/101/onto.rdf#")

plugin.register(
    'sparql', rdflib.query.Processor,
    'rdfextras.sparql.processor', 'Processor')
plugin.register(
    'sparql', rdflib.query.Result,
    'rdfextras.sparql.query', 'SPARQLQueryResult')
#
qres = g.query(
                """
                    SELECT  DISTINCT ?varClass ?varSubClass ?varSubClassComment ?varProperty ?varPropComment
                        WHERE {
                                {
                                    ?varClass rdf:type owl:Class .
                                    ?varProperty rdf:type owl:ObjectProperty ; rdfs:domain ?varClass . OPTIONAL{?varProperty rdfs:comment ?varPropComment} .
                                    OPTIONAL{?varSubClass rdfs:subClassOf ?varClass ; rdfs:comment ?varSubClassComment} .
                        }
                    UNION
                        {
                                    ?varClass rdf:type owl:Class .
                                    ?varProperty rdf:type owl:DatatypeProperty ; rdfs:domain ?varClass . OPTIONAL{?varProperty rdfs:comment ?varPropComment}.
                        }
                             }
                """
                , initNs=dict(
                                ns=Namespace("http://oaei.ontologymatching.org/2011/benchmarks/101/onto.rdf#")
                              )
               )

for row in qres.result:
    #print ("%s %s %s %s %s" % row) # %s represent the fields selected in the query
    print row

print (len(qres.result))

My result is nothing. There's no error, but the length of the result file is 0. What am I doing wrong? Does anyone can help me?

Answer 1

When I run this query (with prefixes defined) at sparql.org's query processor, I get a bunch of results:

PREFIX ns: <http://oaei.ontologymatching.org/2011/benchmarks/101/onto.rdf#>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX owl: <http://www.w3.org/2002/07/owl#>

SELECT  DISTINCT ?varClass ?varSubClass ?varSubClassComment ?varProperty ?varPropComment
FROM <http://oaei.ontologymatching.org/2011/benchmarks/101/onto.rdf>
WHERE {
  {
    ?varClass rdf:type owl:Class .
    ?varProperty rdf:type owl:ObjectProperty ; rdfs:domain ?varClass . OPTIONAL{?varProperty rdfs:comment ?varPropComment} .
    OPTIONAL{?varSubClass rdfs:subClassOf ?varClass ; rdfs:comment ?varSubClassComment} .
  }
  UNION
  {
    ?varClass rdf:type owl:Class .
    ?varProperty rdf:type owl:DatatypeProperty ; rdfs:domain ?varClass . OPTIONAL{?varProperty rdfs:comment ?varPropComment}.
  }
}

I'd note that you can significantly simplify this query with values since you're using union just to specify owl:ObjectProperty and owl:DatatypeProperty:

PREFIX ns: <http://oaei.ontologymatching.org/2011/benchmarks/101/onto.rdf#>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX owl: <http://www.w3.org/2002/07/owl#>

SELECT DISTINCT ?varClass ?varSubClass ?varSubClassComment ?varProperty ?varPropComment
FROM <http://oaei.ontologymatching.org/2011/benchmarks/101/onto.rdf>
WHERE {
  VALUES ?propertyType { owl:ObjectProperty owl:DatatypeProperty }

  ?varClass rdf:type owl:Class .
  ?varProperty rdf:type ?propertyType ;
               rdfs:domain ?varClass .
  OPTIONAL{ ?varProperty rdfs:comment ?varPropComment }
  OPTIONAL{ ?varSubClass rdfs:subClassOf ?varClass ;
                         rdfs:comment ?varSubClassComment }
}

I don't see any reason that you need to be defining any prefixes for http://oaei.ontologymatching.org/2011/benchmarks/101/onto.rdf in your query, since you don't use any such namespace in your query. I'm assuming that that's the dataset that you're trying to query against, or perhaps that you expect the results to begin with that prefix (so perhaps defining the namespace makes their printed output nicer).

This strongly suggests that the problem is that the graph you're actually querying, from /localPath/a.owl isn't the same as that dataset, or that you're running some outdated versions of RDFlib, rdfextras, or both. I was able to run your Python code locally with RDFlib version 4.0.1 and rdfextras 0.4.