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# Creating RDF triples

## Creating Nodes

RDF data is a graph where the nodes are URI references, Blank Nodes or Literals. In RDFLib, these node types are represented by the classes `URIRef`, `BNode`, and `Literal`. `URIRefs` and `BNodes` can both be thought of as resources, such a person, a company, a website, etc.

- A `BNode` is a node where the exact URI is not known - usually a node with identity only in relation to other nodes.
- A `URIRef` is a node where the exact URI is known. In addition to representing some subjects and predicates in RDF graphs, `URIRef`s are always used to represent properties/predicates
- `Literals` represent object values, such as a name, a date, a number, etc. The most common literal values are XML data types, e.g. string, int… but custom types can be declared too

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 rdflibliterals.

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

    from rdflib import Namespace

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

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

This is very useful for schemas where all properties and classes have the same URI prefix. RDFLib defines Namespaces for some common RDF/OWL schemas, including most W3C ones:

    from rdflib.namespace import CSVW, DC, DCAT, DCTERMS, DOAP, FOAF, ODRL2, ORG, OWL, \
                               PROF, PROV, RDF, RDFS, SDO, SH, SKOS, SOSA, SSN, TIME, \
                               VOID, XMLNS, XSD

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

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

    PROF.isProfileOf
    # == rdflib.term.URIRef("http://www.w3.org/ns/dx/prof/isProfileOf")

    SOSA.Sensor
    # == rdflib.term.URIRef("http://www.w3.org/ns/sosa/Sensor")

## Adding Triples to a graph

We already saw in intro_to_parsing, how triples can be added from files and online locations with the `parse()` function.

Triples can also be added within Python code directly, using the `add()` function:

`add()` takes a 3-tuple (a “triple”) of RDFLib nodes. Using the nodes and namespaces we defined previously:

    from rdflib import Graph, URIRef, Literal, BNode
    from rdflib.namespace import FOAF, RDF

    g = Graph()
    g.bind("foaf", FOAF)

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

    name = Literal("Bob")
    age = Literal(24)

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

    print(g.serialize())

outputs:

    @prefix foaf: <http://xmlns.com/foaf/0.1/> .
    @prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

    <http://example.org/people/Bob> a foaf:Person ;
        foaf:age 24 ;
        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 a max-cardinality of 1). The `set()` method is useful for this:

    from rdflib import Graph, URIRef, Literal
    from rdflib.namespace import FOAF

    g = Graph()
    bob = URIRef("http://example.org/people/Bob")

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

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

`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 graph-setops.

## Removing Triples

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

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 but `foaf:member_name` isn’t in FOAF’s namespace and perhaps they should have used `foaf:name`

To retrieve some LiveJournal data, add a `foaf:name` for every `foaf:member_name` and then remove the `foaf:member_name` values to ensure the data actually aligns with other FOAF data, we could do this:

    from rdflib import Graph
    from rdflib.namespace import FOAF

    g = Graph()
    # get the data
    g.parse("http://danbri.livejournal.com/data/foaf")

    # for every foaf:member_name, add foaf:name and remove foaf:member_name
    for s, p, o in g.triples((None, FOAF['member_name'], None)):
        g.add((s, FOAF['name'], o))
        g.remove((s, FOAF['member_name'], o))

Foaf member name

Since rdflib 5.0.0, using `foaf:member_name` is somewhat prevented in RDFlib since FOAF is declared as a `ClosedNamespace` class instance that has a closed set of members and `foaf:member_name` isn’t one of them! If LiveJournal had used RDFlib 5.0.0, an error would have been raised for `foaf:member_name` when the triple was created.

## Creating Containers & Collections

There are two convenience classes for RDF Containers & Collections which you can use instead of declaring each triple of a Containers or a Collections individually:

- `Container` (also `Bag`, `Seq` & `Alt`) and
- `Collection`

See their documentation for how.