# Navigating Graphs

An RDF Graph is a set of RDF triples, and we try to mirror exactly this in RDFLib. The Python `Graph()` tries to emulate a container type.

## Graphs as Iterators

RDFLib graphs override `__iter__()` in order to support iteration over the contained triples:

    for s, p, o in someGraph:
        if not (s, p, o) in someGraph:
            raise Exception("Iterator / Container Protocols are Broken!!")

This loop iterates through all the subjects(s), predicates (p) & objects (o) in `someGraph`.

## Contains check

Graphs implement `__contains__()`, so you can check if a triple is in a graph with a `triple in graph` syntax:

    from rdflib import URIRef
    from rdflib.namespace import RDF

    bob = URIRef("http://example.org/people/bob")
    if (bob, RDF.type, FOAF.Person) in graph:
        print("This graph knows that Bob is a person!")

Note that this triple does not have to be completely bound:

    if (bob, None, None) in graph:
        print("This graph contains triples about Bob!")

## Set Operations on RDFLib Graphs

Graphs override several pythons operators: `__iadd__()`, `__isub__()`, etc. This supports addition, subtraction and other set-operations on Graphs:

operation | effect
--- | ---
`G1 + G2` | return new graph with union (triples on both)
`G1 += G2` | in place union / addition
`G1 - G2` | return new graph with difference (triples in G1, not in G2)
`G1 -= G2` | in place difference / subtraction
`G1 & G2` | intersection (triples in both graphs)
`G1 ^ G2` | xor (triples in either G1 or G2, but not in both)

Warning

Set-operations on graphs assume Blank Nodes are shared between graphs. This may or may not be what you want. See Merging graphs for details.

## Basic Triple Matching

Instead of iterating through all triples, RDFLib graphs support basic triple pattern matching with a `triples()` function. This function is a generator of triples that match a pattern given by arguments, i.e. arguments restrict the triples that are returned. Terms that are `None` are treated as a wildcard. For example:

    g.load("some_foaf.ttl")
    # find all subjects (s) of type (rdf:type) person (foaf:Person)
    for s, p, o in g.triples((None, RDF.type, FOAF.Person)):
        print(f"{s} is a person")

    # find all subjects of any type
    for s, p, o in g.triples((None,  RDF.type, None)):
        print(f"{s} is a {o}")

    # create a graph
    bobgraph = Graph()
    # add all triples with subject 'bob'
    bobgraph += g.triples((bob, None, None))

If you are not interested in whole triples, you can get only the bits you want with the methods `objects()`, `subjects()`, `predicates()`, `predicate_objects()`, etc. Each take parameters for the components of the triple to constraint:

    for person in g.subjects(RDF.type, FOAF.Person):
        print("{} is a person".format(person))

Finally, for some properties, only one value per resource makes sense (i.e they are functional properties, or have a max-cardinality of 1). The `value()` method is useful for this, as it returns just a single node, not a generator:

    # get any name of bob
    name = g.value(bob, FOAF.name)
    # get the one person that knows bob and raise an exception if more are found
    mbox = g.value(predicate = FOAF.name, object=bob, any=False)

## `Graph` methods for accessing triples

Here is a list of all convenience methods for querying Graphs:

Graph.triples(triple: Tuple[Optional[rdflib.term.Node], Union[None, rdflib.paths.Path, rdflib.term.Node], Optional[rdflib.term.Node]])

  Generator over the triple store

  Returns triples that match the given triple pattern. If triple pattern does not provide a context, all contexts will be searched.

Graph.value(subject=None, predicate=rdflib.term.URIRef('http://www.w3.org/1999/02/22-rdf-syntax-ns#value'), object=None, default=None, any=True)

  Get a value for a pair of two criteria

  Exactly one of subject, predicate, object must be None. Useful if one knows that there may only be one value.

  It is one of those situations that occur a lot, hence this ‘macro’ like utility

  Parameters: subject, predicate, object – exactly one must be None default – value to be returned if no values found any – if True, return any value in the case there is more than one, else, raise UniquenessError

Graph.subjects(predicate=None, object=None) → Iterable[rdflib.term.Node]

  A generator of subjects with the given predicate and object

Graph.objects(subject=None, predicate=None) → Iterable[rdflib.term.Node]

  A generator of objects with the given subject and predicate

Graph.predicates(subject=None, object=None) → Iterable[rdflib.term.Node]

  A generator of predicates with the given subject and object

Graph.subject_objects(predicate=None)

  A generator of (subject, object) tuples for the given predicate

Graph.subject_predicates(object=None)

  A generator of (subject, predicate) tuples for the given object

Graph.predicate_objects(subject=None)

  A generator of (predicate, object) tuples for the given subject