Creating RDF triples

Creating Nodes

RDF is a graph where the nodes are URI references, Blank Nodes or Literals, in RDFLib represented by the classes URIRef, BNode, and Literal. URIRefs and BNodes can both be thought of as resources, such a person, a company, a web-site, etc. A BNode is a node where the exact URI is not known. URIRefs are also used to represent the properties/predicates in the RDF graph. Literals represent attribute values, such as a name, a date, a number, etc.

Nodes can be created by the constructors of the node classes:

from rdflib import URIRef, BNode, Literal

bob = URIRef("http://example.org/people/Bob")
linda = BNode() # a GUID is generated

name = Literal('Bob') # passing a string
age = Literal(24) # passing a python int
height = Literal(76.5) # passing a python float

Literals can be created from python objects, this creates data-typed literals, for the details on the mapping see Literals.

For creating many URIRefs in the same namespace, i.e. URIs with the same prefix, RDFLib has the rdflib.namespace.Namespace class:

from rdflib import Namespace

n = Namespace("http://example.org/people/")

n.bob # = rdflib.term.URIRef(u'http://example.org/people/bob')
n.eve # = rdflib.term.URIRef(u'http://example.org/people/eve')

This is very useful for schemas where all properties and classes have the same URI prefix, RDFLib pre-defines Namespaces for the most common RDF schemas:

from rdflib.namespace import RDF, FOAF

RDF.type
# = rdflib.term.URIRef(u'http://www.w3.org/1999/02/22-rdf-syntax-ns#type')

FOAF.knows
# = rdflib.term.URIRef(u'http://xmlns.com/foaf/0.1/knows')

Adding Triples

We already saw in Loading and saving RDF, how triples can be added with with the parse() function.

Triples can also be added with the add() function:

Graph.add((s, p, o))

Add a triple with self as context

add() takes a 3-tuple of RDFLib nodes. Try the following with the nodes and namespaces we defined previously:

from rdflib import Graph
g = Graph()

g.add( (bob, RDF.type, FOAF.Person) )
g.add( (bob, FOAF.name, name) )
g.add( (bob, FOAF.knows, linda) )
g.add( (linda, RDF.type, FOAF.Person) )
g.add( (linda, FOAF.name, Literal('Linda') ) )

print g.serialize(format='turtle')

outputs:

@prefix foaf: <http://xmlns.com/foaf/0.1/> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix xml: <http://www.w3.org/XML/1998/namespace> .

<http://example.org/people/Bob> a foaf:Person ;
        foaf:knows [ a foaf:Person ;
                        foaf:name "Linda" ] ;
        foaf:name "Bob" .

For some properties, only one value per resource makes sense (i.e they are functional properties, or have max-cardinality of 1). The set() method is useful for this:

g.add( ( bob, FOAF.age, Literal(42) ) )
print "Bob is ", g.value( bob, FOAF.age )
# prints: Bob is 42

g.set( ( bob, age, Literal(43) ) ) # replaces 42 set above
print "Bob is now ", g.value( bob, FOAF.age )
# prints: Bob is now 43

rdflib.graph.Graph.value() is the matching query method, it will return a single value for a property, optionally raising an exception if there are more.

You can also add triples by combining entire graphs, see Set Operations on RDFLib Graphs.

Removing Triples

Similarly, triples can be removed by a call to remove():

Graph.remove((s, p, o))

Remove a triple from the graph

If the triple does not provide a context attribute, removes the triple from all contexts.

When removing, it is possible to leave parts of the triple unspecified (i.e. passing None), this will remove all matching triples:

g.remove( (bob, None, None) ) # remove all triples about bob

An example

LiveJournal produces FOAF data for their users, but they seem to use foaf:member_name for a person’s full name. To align with data from other sources, it would be nice to have foaf:name act as a synonym for foaf:member_name (a poor man’s one-way owl:equivalentProperty):

from rdflib.namespace import FOAF
g.parse("http://danbri.livejournal.com/data/foaf")
for s,_,n in g.triples((None, FOAF['member_name'], None)):
    g.add((s, FOAF['name'], n))