Example 4: Statement matching

The getStatements() method of the connection object provides a simple way to perform unsophisticated queries. This method lets you enter a mix of required values and wildcards, and retrieve all matching triples. (If you need to perform sophisticated tests and comparisons you should use a SPARQL query instead.)

Below, we illustrate two kinds of getStatements() calls. The first mimics traditional RDF4J syntax, and returns a Statement object at each iteration. We will reuse data from in previous examples to create a connection object and populate the repository with four triples describing Bob and Alice. We’re going to search for triples that mention Alice, so we have to create an “Alice” URI to use in the search pattern:

from franz.openrdf.connect import ag_connect

conn = ag_connect('python-tutorial', create=True, clear=True)

conn.addData("""
    @base <http://example.org/> .

    <people/alice> a <ontology/Person> ;
                   <ontology/name> "Alice" .
    <people/bob> a <ontology/Person> ;
                 <ontology/name> "Bob" .
""")
alice = conn.createURI("http://example.org/people/alice")

Now we search for triples with Alice’s URI in the subject position. The None values are wildcards for the predicate and object positions of the triple.

statements = conn.getStatements(alice, None, None)

The getStatements() method returns a RepositoryResult object (bound to the variable statements in this case). This object can be iterated over, exposing one result statement at a time. It is sometimes desirable to screen the results for duplicates, using the enableDuplicateFilter() method. Note, however, that duplicate filtering can be expensive. Our example does not contain any duplicates, but it is possible for them to occur.

with statements:
    statements.enableDuplicateFilter()
    for statement in statements:
        print(statement)

This prints out the two matching triples for “Alice.”

(<http://example.org/people/alice>, <http://www.w3.org/1999/02/22-rdf-syntax-ns#type>, <http://example.org/ontology/Person>)
(<http://example.org/people/alice>, <http://example.org/ontology/name>, "Alice")

Notice how we used the with keyword to ensure that the RepositoryResult object is closed after the results are fetched. This is necessary to release resources used during result retrieval. The same goal could be accomplished by calling the RepositoryResult.close() method (preferably in a finally block to ensure exception safety).