Hi William,
Have you looked at the Virtuoso ODBC extensions for SPASQL, which enable
applications to query meta information on SPASQL queries, as detailed at:
http://docs.openlinksw.com/virtuoso/virtodbcsparql.html#virtodbcsparqlexample
These extensions enable the determination of the types of returned literals as
indicated in the example provided ...
Best Regards
Hugh Williams
Professional Services
OpenLink Software
Web: http://www.openlinksw.com
Support: http://support.openlinksw.com
Forums: http://boards.openlinksw.com/support
Twitter: http://twitter.com/OpenLink
On 18 Sep 2010, at 11:53, William Waites wrote:
> Hello, I'm a new subscriber to this list, and have
> looked through the documentation and archives to no
> avail, so apologies if this has been answered before.
>
> I'm working on back-ends for both SQLAlchemy and
> RDFLib to support Virtuoso. The SQLAlchemy one is
> minimally functional [0], but I am having some trouble
> with RDFLib.
>
> The python API for RDFLib looks something like this,
>
> g = Graph(identifier=URIRef("xyz..."))
> for statement in g.triples((None, None, None)):
> print statement
>
> The call to g.triples() would translate into a SPARQL
> statement like,
>
> SELECT ?s ?p ?o WHERE {
> GRAPH <xyz...> { ?s ?p ?o }
> }
>
> which is more or less how I implemented it when I made
> a 4store back-end for RDFLib.
>
> The trouble is, we are expected to have the return
> values instantiated in python objects, URIRef, BNode,
> Literal, etc.. Resolving ?s and ?p unambiguously is
> easy enough, by doing 'define output:valmode "LONG"'
> and then doing __ro2sq on the result.
>
> However when ?o comes back and is a literal with a
> language or a datatype, it just comes as a VARCHAR
> and there is no way that is obvious to me to find out
> what its language and datatype are.
>
> This leads to a situation where I can insert something
> with "foo"@en into the store and get back "foo" which
> is not the same.
>
> One alternative would be to (manually) do
> sparql_to_sql_text() and tweak the resulting queries
> to also return the extra information but that seems
> to rely too much on what should be internal implementation
> details.
>
> A second alternative would be to use one of the
> serialisations but that means (1) queries must return
> all rows immediately instead of using a cursor and (2)
> I have to deal with deserialisation.
>
> I guess the problem might be solved if I could do
> 'define output:valmode "SHORT"' but this appears to
> no be supported.
>
> Any help or guidance appreciated.
>
> Cheers,
> -w
>
>
> [0] http://packages.python.org/virtuoso
> --
> William Waites <[email protected]>
> Mob: +44 789 798 9965
> Fax: +44 131 464 4948
> CD70 0498 8AE4 36EA 1CD7 281C 427A 3F36 2130 E9F5
>
> ------------------------------------------------------------------------------
> Start uncovering the many advantages of virtual appliances
> and start using them to simplify application deployment and
> accelerate your shift to cloud computing.
> http://p.sf.net/sfu/novell-sfdev2dev_______________________________________________
> Virtuoso-users mailing list
> [email protected]
> https://lists.sourceforge.net/lists/listinfo/virtuoso-users
