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.. _intro_to_parsing:
======================
Loading and saving RDF
======================
Reading an n-triples file
--------------------------
RDF data has various syntaxes (``xml``, ``n3``, ``ntriples``,
``trix``, ``JSON-LD``, etc) that you might want to read. The simplest format is
``ntriples``, a line-based format. Create the file :file:`demo.nt` in
the current directory with these two lines:
.. code-block:: Turtle
<http://bigasterisk.com/foaf.rdf#drewp> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://xmlns.com/foaf/0.1/Person> .
<http://bigasterisk.com/foaf.rdf#drewp> <http://example.com/says> "Hello world" .
You need to tell RDFLib what format to parse, use the ``format``
keyword-parameter to :meth:`~rdflib.graph.Graph.parse`, you can pass
either a mime-type or the name (a :doc:`list of available parsers
<plugin_parsers>` is available). If you are not sure what format your
file will be, you can use :func:`rdflib.util.guess_format` which will
guess based on the file extension.
In an interactive python interpreter, try this:
.. code-block:: python
from rdflib import Graph
g = Graph()
g.parse("demo.nt", format="nt")
print(len(g)) # prints 2
import pprint
for stmt in g:
pprint.pprint(stmt)
# prints :
(rdflib.term.URIRef('http://bigasterisk.com/foaf.rdf#drewp'),
rdflib.term.URIRef('http://example.com/says'),
rdflib.term.Literal('Hello world'))
(rdflib.term.URIRef('http://bigasterisk.com/foaf.rdf#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; and the
subjects, predicates, and objects are all rdflib types.
Reading remote graphs
---------------------
Reading graphs from the net is just as easy:
.. code-block:: python
g.parse("http://bigasterisk.com/foaf.rdf")
print(len(g))
# prints 42
The format defaults to ``xml``, which is the common format for .rdf
files you'll find on the net.
RDFLib will also happily read RDF from any file-like object,
i.e. anything with a ``.read()`` method.
Saving RDF
----------
To store a graph in a file use the :func:`rdflib.Graph.serialize` function:
.. code-block:: python
g.parse("http://bigasterisk.com/foaf.rdf")
with open("foaf.n3", "wb") as f:
g.serialize(f, format="n3")
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