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# Utilities & convenience functions
For RDF programming, RDFLib and Python may not be the fastest tools, but we try hard to make them the easiest and most convenient to use and thus the *fastest* overall!
This is a collection of hints and pointers for hassle-free RDF coding.
## Functional properties
Use [`value()`][rdflib.graph.Graph.value] and [`set()`][rdflib.graph.Graph.set] to work with *functional property* instances, i.e. properties than can only occur once for a resource.
```python
from rdflib import Graph, URIRef, Literal, BNode
from rdflib.namespace import FOAF, RDF
g = Graph()
g.bind("foaf", FOAF)
# Add demo data
bob = URIRef("http://example.org/people/Bob")
g.add((bob, RDF.type, FOAF.Person))
g.add((bob, FOAF.name, Literal("Bob")))
g.add((bob, FOAF.age, Literal(38)))
# To get a single value, use 'value'
print(g.value(bob, FOAF.age))
# prints: 38
# To change a single of value, use 'set'
g.set((bob, FOAF.age, Literal(39)))
print(g.value(bob, FOAF.age))
# prints: 39
```
## Slicing graphs
Python allows slicing arrays with a `slice` object, a triple of `start`, `stop` and `step-size`:
```python
for i in range(20)[2:9:3]:
print(i)
# prints:
# 2, 5, 8
```
RDFLib graphs override `__getitem__` and we pervert the slice triple to be a RDF triple instead. This lets slice syntax be a shortcut for [`triples()`][rdflib.graph.Graph.triples], [`subject_predicates()`][rdflib.graph.Graph.subject_predicates], [`__contains__()`][rdflib.graph.Graph.__contains__], and other Graph query-methods:
```python
from rdflib import Graph, URIRef, Literal, BNode
from rdflib.namespace import FOAF, RDF
g = Graph()
g.bind("foaf", FOAF)
# Add demo data
bob = URIRef("http://example.org/people/Bob")
bill = URIRef("http://example.org/people/Bill")
g.add((bob, RDF.type, FOAF.Person))
g.add((bob, FOAF.name, Literal("Bob")))
g.add((bob, FOAF.age, Literal(38)))
g.add((bob, FOAF.knows, bill))
print(g[:])
# same as
print(iter(g))
print(g[bob])
# same as
print(g.predicate_objects(bob))
print(g[bob: FOAF.knows])
# same as
print(g.objects(bob, FOAF.knows))
print(g[bob: FOAF.knows: bill])
# same as
print((bob, FOAF.knows, bill) in g)
print(g[:FOAF.knows])
# same as
print(g.subject_objects(FOAF.knows))
```
See [`examples.slice`][examples.slice] for a complete example.
!!! warning "Slicing Caution"
Slicing is convenient for run-once scripts for playing around
in the Python `REPL`, however since slicing returns
tuples of varying length depending on which parts of the
slice are bound, you should be careful using it in more
complicated programs. If you pass in variables, and they are
`None` or `False`, you may suddenly get a generator of
different length tuples back than you expect.
## SPARQL Paths
[SPARQL property paths](http://www.w3.org/TR/sparql11-property-paths/) are possible using overridden operators on URIRefs. See [`examples.foafpaths`][examples.foafpaths] and [`rdflib.paths`][rdflib.paths].
## Serializing a single term to N3
For simple output, or simple serialisation, you often want a nice
readable representation of a term. All terms (URIRef, Literal etc.) have a
`n3`, method, which will return a suitable N3 format:
```python
from rdflib import Graph, URIRef, Literal
from rdflib.namespace import FOAF
# A URIRef
person = URIRef("http://xmlns.com/foaf/0.1/Person")
print(person.n3())
# prints: <http://xmlns.com/foaf/0.1/Person>
# Simplifying the output with a namespace prefix:
g = Graph()
g.bind("foaf", FOAF)
print(person.n3(g.namespace_manager))
# prints foaf:Person
# A typed literal
l = Literal(2)
print(l.n3())
# prints "2"^^<http://www.w3.org/2001/XMLSchema#integer>
# Simplifying the output with a namespace prefix
# XSD is built in, so no need to bind() it!
l.n3(g.namespace_manager)
# prints: "2"^^xsd:integer
```
## Parsing data from a string
You can parse data from a string with the `data` param:
```python
from rdflib import Graph
g = Graph().parse(data="<a:> <p:> <p:>.")
for r in g.triples((None, None, None)):
print(r)
# prints: (rdflib.term.URIRef('a:'), rdflib.term.URIRef('p:'), rdflib.term.URIRef('p:'))
```
## Command Line tools
RDFLib includes a handful of commandline tools, see [`rdflib.tools`][rdflib.tools].
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