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.. _intro_to_parsing:
======================
Loading and saving RDF
======================
Reading RDF files
-----------------
RDF data can be represented using various syntaxes (``turtle``, ``rdf/xml``, ``n3``, ``n-triples``,
``trix``, ``JSON-LD``, etc.). The simplest format is
``ntriples``, which is a triple-per-line format.
Create the file :file:`demo.nt` in the current directory with these two lines in it:
.. code-block:: Turtle
<http://example.com/drewp> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://xmlns.com/foaf/0.1/Person> .
<http://example.com/drewp> <http://example.com/says> "Hello World" .
On line 1 this file says "drewp is a FOAF Person:. On line 2 it says "drep says "Hello World"".
RDFLib can guess what format the file is by the file ending (".nt" is commonly used for n-triples) so you can just use
:meth:`~rdflib.graph.Graph.parse` to read in the file. If the file had a non-standard RDF file ending, you could set the
keyword-parameter ``format`` to specify either an Internet Media Type or the format name (a :doc:`list of available
parsers <plugin_parsers>` is available).
In an interactive python interpreter, try this:
.. code-block:: python
from rdflib import Graph
g = Graph()
g.parse("demo.nt")
print(len(g))
# prints: 2
import pprint
for stmt in g:
pprint.pprint(stmt)
# prints:
# (rdflib.term.URIRef('http://example.com/drewp'),
# rdflib.term.URIRef('http://example.com/says'),
# rdflib.term.Literal('Hello World'))
# (rdflib.term.URIRef('http://example.com/drewp'),
# rdflib.term.URIRef('http://www.w3.org/1999/02/22-rdf-syntax-ns#type'),
# rdflib.term.URIRef('http://xmlns.com/foaf/0.1/Person'))
The final lines show how RDFLib represents the two statements in the
file: the statements themselves are just length-3 tuples ("triples") and the
subjects, predicates, and objects of the triples are all rdflib types.
Reading remote RDF
------------------
Reading graphs from the Internet is easy:
.. code-block:: python
from rdflib import Graph
g = Graph()
g.parse("http://www.w3.org/People/Berners-Lee/card")
print(len(g))
# prints: 86
:func:`rdflib.Graph.parse` can process local files, remote data via a URL, as in this example, or RDF data in a string
(using the ``data`` parameter).
Saving RDF
----------
To store a graph in a file, use the :func:`rdflib.Graph.serialize` function:
.. code-block:: python
from rdflib import Graph
g = Graph()
g.parse("http://www.w3.org/People/Berners-Lee/card")
g.serialize(destination="tbl.ttl")
This parses data from http://www.w3.org/People/Berners-Lee/card and stores it in a file ``tbl.ttl`` in this directory
using the turtle format, which is the default RDF serialization (as of rdflib 6.0.0).
To read the same data and to save it as an RDF/XML format string in the variable ``v``, do this:
.. code-block:: python
from rdflib import Graph
g = Graph()
g.parse("http://www.w3.org/People/Berners-Lee/card")
v = g.serialize(format="xml")
The following table lists the RDF formats you can serialize data to with rdflib, out of the box, and the ``format=KEYWORD`` keyword used to reference them within ``serialize()``:
.. csv-table::
:header: "RDF Format", "Keyword", "Notes"
"Turtle", "turtle, ttl or turtle2", "turtle2 is just turtle with more spacing & linebreaks"
"RDF/XML", "xml or pretty-xml", "Was the default format, rdflib < 6.0.0"
"JSON-LD", "json-ld", "There are further options for compact syntax and other JSON-LD variants"
"N-Triples", "ntriples, nt or nt11", "nt11 is exactly like nt, only utf8 encoded"
"Notation-3","n3", "N3 is a superset of Turtle that also caters for rules and a few other things"
"Trig", "trig", "Turtle-like format for RDF triples + context (RDF quads) and thus multiple graphs"
"Trix", "trix", "RDF/XML-like format for RDF quads"
"N-Quads", "nquads", "N-Triples-like format for RDF quads"
Working with multi-graphs
-------------------------
To read and query multi-graphs, that is RDF data that is context-aware, you need to use rdflib's
:class:`rdflib.Dataset` class. This an extension to :class:`rdflib.Graph` that
know all about quads (triples + graph IDs).
If you had this multi-graph data file (in the ``trig`` format, using new-style ``PREFIX`` statement (not the older
``@prefix``):
.. code-block:: Turtle
PREFIX eg: <http://example.com/person/>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
eg:graph-1 {
eg:drewp a foaf:Person .
eg:drewp eg:says "Hello World" .
}
eg:graph-2 {
eg:nick a foaf:Person .
eg:nick eg:says "Hi World" .
}
You could parse the file and query it like this:
.. code-block:: python
from rdflib import Dataset
from rdflib.namespace import RDF
g = Dataset()
g.parse("demo.trig")
for s, p, o, g in g.quads((None, RDF.type, None, None)):
print(s, g)
This will print out:
.. code-block::
http://example.com/person/drewp http://example.com/person/graph-1
http://example.com/person/nick http://example.com/person/graph-2
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