Querying with SPARQL

Run a Query

The RDFLib comes with an implementation of the SPARQL 1.1 Query and SPARQL 1.1 Update languages.

Queries can be evaluated against a graph with the rdflib.graph.Graph.query() method, and updates with rdflib.graph.Graph.update().

The query method returns a rdflib.query.Result instance. For SELECT queries, iterating over this return rdflib.query.ResultRow instances, each containing a set of variable bindings. For CONSTRUCT/DESCRIBE queries, iterating over the result object gives the triples. For ASK queries, iterating will yield the single boolean answer, or evaluating the result object in a boolean-context (i.e. bool(result))

Continuing the example...

import rdflib

g = rdflib.Graph()

# ... add some triples to g somehow ...

qres = g.query(
    """SELECT DISTINCT ?aname ?bname
       WHERE {
          ?a foaf:knows ?b .
          ?a foaf:name ?aname .
          ?b foaf:name ?bname .

for row in qres:
    print("%s knows %s" % row)

The results are tuples of values in the same order as your SELECT arguments. Alternatively, the values can be accessed by variable name, either as attributes, or as items: row.b and row["b"] is equivalent.

Timothy Berners-Lee knows Edd Dumbill
Timothy Berners-Lee knows Jennifer Golbeck
Timothy Berners-Lee knows Nicholas Gibbins
Timothy Berners-Lee knows Nigel Shadbolt
Dan Brickley knows binzac
Timothy Berners-Lee knows Eric Miller
Drew Perttula knows David McClosky
Timothy Berners-Lee knows Dan Connolly

As an alternative to using PREFIX in the SPARQL query, namespace bindings can be passed in with the initNs kwarg, see namespace_and_bindings.

Variables can also be pre-bound, using initBindings kwarg can be used to pass in a dict of initial bindings, this is particularly useful for prepared queries, as described below.

Prepared Queries

RDFLib lets you prepare queries before execution, this saves re-parsing and translating the query into SPARQL Algebra each time.

The method rdflib.plugins.sparql.prepareQuery() takes a query as a string and will return a rdflib.plugins.sparql.sparql.Query object. This can then be passed to the rdflib.graph.Graph.query() method.

The initBindings kwarg can be used to pass in a dict of initial bindings:

q = prepareQuery(
        'SELECT ?s WHERE { ?person foaf:knows ?s .}',
        initNs = { "foaf": FOAF })

g = rdflib.Graph()

tim = rdflib.URIRef("http://www.w3.org/People/Berners-Lee/card#i")

for row in g.query(q, initBindings={'person': tim}):
        print row

Custom Evaluation Functions

For experts, it is possible to override how bits of SPARQL algebra are evaluated. By using the setuptools entry-point rdf.plugins.sparqleval, or simply adding to an entry to rdflib.plugins.sparql.CUSTOM_EVALS, a custom function can be registered. The function will be called for each algebra component and may raise NotImplementedError to indicate that this part should be handled by the default implementation.

See examples/custom_eval.py