Dictionary used to build a Triple Store

[Lee]

Lee wrote:
> 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?
> 
>    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.
> 
> 
OK, Thanx. That cleares things up.

I had forgotton that once you have a "Cell" in a 2D matrix to represent 
the first two components of the 3 tuple, you then want multiple values 
IN the cell for the possibly multi valued  third component.