Example 11: Namespaces¶
A namespace is that portion of a URI that preceeds the last #,
/, or : character, inclusive. The remainder of a URI is called
the localname. For example, with respect to the URI
http://example.org/people/alice, the namespace is
http://example.org/people/ and the localname is alice. When
writing SPARQL queries, it is convenient to define prefixes or
nicknames for the namespaces, so that abbreviated URIs can be
specified. For example, if we define ex to be a nickname for
http://example.org/people/, then the string ex:alice is a
recognized abbreviation for http://example.org/people/alice. This
abbreviation is called a qname (qualified name).
In the SPARQL query discussed in this chapter we see two qnames,
rdf:type and ex:alice. Ordinarily, we would expect to see
PREFIX declarations in SPARQL that define namespaces for the
rdf and ex nicknames. However, the connection and query
machinery can do that job for you. The mapping of prefixes to
namespaces includes the built-in prefixes rdf, rdfs, xsd,
and owl. Hence, we can write rdf:type in a SPARQL query, and
the system already knows its meaning. In the case of the ex
prefix, we need to instruct it. The setNamespace() method of the
connection object registers a new namespace.
Note
It is legal, although not recommended, to redefine the built-in prefixes (RDF, XSD etc…).
We start by opening a connection
from franz.openrdf.connect import ag_connect
conn = ag_connect('python-tutorial', create=True, clear=True)
and creating two URIs. Note how createURI() allows us to compose
URIs from namespaces and local names.
exns = "http://example.org/people/"
alice = conn.createURI(namespace=exns, localname="alice")
person = conn.createURI(namespace=exns, localname="Person")
Now we can assert Alice’s RDF:TYPE triple.
from franz.openrdf.vocabulary.rdf import RDF
conn.add(alice, RDF.TYPE, person)
Now we register the exns namespace with the connection object, so
we can use it in a SPARQL query. The query looks for triples that have
rdf:type in the predicate position, and ex:Person in the
object position.
conn.setNamespace('ex', exns)
conn.executeTupleQuery("""
SELECT ?s ?p ?o WHERE {
?s ?p ?o .
FILTER (?p = rdf:type && ?o = ex:Person)
}""", output=True)
The output shows the single triple that we expected to find. This demonstrates that the qnames in the SPARQL query successfully matched the fully-expanded URIs in the triple. Note that the namespace prefix is also used in the table below.
-----------------------------------
| s | p | o |
===================================
| ex:alice | rdf:type | ex:Person |
-----------------------------------
It should be mentioned here that the prefix of a namespace can be an empty string. This allows the resulting qnames to be very concise and readable:
conn.setNamespace('', 'http://a-long-and-often-used-namespace/')
conn.executeUpdate('insert data { :this :looks :nice }')
conn.executeTupleQuery('select ?s { ?s :looks :nice }',
output=True)
---------
| s |
=========
| :this |
---------
