Dictionary used to build a Triple Store

S. Chris Colbert sccolbert at gmail.com
Thu Jan 7 19:39:42 CET 2010


> Definitely a newbie question, so please bear with  me.
> 
> I'm reading "Programming the Semantic Web" by Segaran, Evans, and Tayor.
> 
> It's about the Semantic Web BUT it uses python to build a "toy" triple
> store claimed to have good performance in the "tens of thousands" of
> triples.
> 
> Just in case anybody doesnt know what an RDF triple is (not that it
> matters for my question) think of it as an ordered 3 tuple representing
> a Subject, a Predicate, and an Object eg: (John, loves, Mary) (Mary,
> has-a, lamb) {theSky, has-color,blue}
> 
> To build the triple store entirely in Python, the authors recommend
> using the Python hash. Three hashes actually (I get that. You
> want to have a hash with the major index being the Subject in one hash,
> the Predicate in another hash, or the Object for the third hash)
> 
> He creates a class SimpleGraph which initializes itself by setting the
> three hashes names _spo, _pos, and _osp thus
> 
> class SimpleGraph;
>    def __init__(self);
>       self._spo={};
>       self._pos=();
>       self._osp={};
> 
> So far so good. I get the convention with the double underbars for the
> initializer but
> 
> Q1: Not the main question but while I'm here....I'm a little fuzzy on
> the convention about the use of the single underbar in the definition of
> the hashes. Id the idea to "underbar" all objects and methods that
> belong to the class? Why do that?
> 
> But now the good stuff:
> 
> Our authors define the hashes thus: (showing only one of the three
> hashes because they're all the same idea)
> 
> self._pos = {predicate:{object:set( [subject] ) }}
> 
> Q2: Wha? Two surprises ...
>     1) Why not {predicate:{object:subject}} i.e.
> pos[predicate][object]=subject....why the set( [object] ) construct?
> putting the object into a list and turning the list into a set to be the
> "value" part of a name:value pair. Why not just use the naked subject
> for the value?
> 
because the argument has to be iterable.

In [1]: set(1)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)

/home/brucewayne/<ipython console> in <module>()

TypeError: 'int' object is not iterable

>     2) Why not something like pos[predicate][object][subject] = 1
> .....or any constant. The idea being to create the set of three indexes.
> If the triple exists in the hash, its "in" your tripple store. If not,
> then there's no such triple.
> 
I can't really answer that, I imagine there is a better way to code what is 
trying to be accomplished. But I'm no Steven D'Aprano and I'm already a few 
beers in ;)



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