SPARQLStore query parameter initBindings does not have the same behavior as the other stores

[Tpt]

RDFLib SPARQL query processor considers the initBindings parameter of the Store/Graph query method as a set of binds to do at the beginning of the query, inside of the SELECT expression. However SPARQLStore considers it as a join to be done after the query have been processed using a final VALUES clause when it generates a SPARQL query to send to the remote store.

For example the call

store.query("SELECT ?s WHERE { FILTER(BOUND(?s)) }", initBindings={Variable("s"): URIRef("http://example.com/foo")})

will be evaluated by the Memory store like

SELECT ?s WHERE { VALUES ?s { <http://example.com/foo> } FILTER(BOUND(?s)) }

and by the SPARQLStore like:

SELECT ?s WHERE { FILTER(BOUND(?s)) } VALUES ?s { <http://example.com/foo> }

This leads the Memory store to return one solution with the assignation ?s -> <http://example.com/foo> and the SPARQLStore to return no solution.

It would be great to harmonize the behavior. I encountered this issue while working on oxrdflib that currently follows the SPARQLStore behavior, breaking some usages relying on the Memory store behavior.

I can confirm the issue. FWIW, I found what might be a workaround, you can set the use_store_provided kwarg to False and get the desired behaviour.
Using a Fuseki instance primed with:

INSERT DATA { 
    <http://example.org/book/book1>  <http://purl.org/dc/elements/1.1/title>  "SPARQL Tutorial" .
    <http://example.org/book/book1>  <http://example.org/ns#price>  42 .
    <http://example.org/book/book2>  <http://purl.org/dc/elements/1.1/title>  "The Semantic Web" .
    <http://example.org/book/book2>  <http://example.org/ns#price>  23 .
}

The following test passes:

from rdflib import Graph, Literal, URIRef, XSD
def test_bindings():
    g = Graph("SPARQLStore")
    g.open("http://localhost:3030/db/query")
    bindings = {Variable("book"): URIRef("http://example.org/book/book1")}
    query = "SELECT ?book WHERE { FILTER(BOUND(?book)) }"
    res = g.query(query, initBindings=bindings, use_store_provided=False)
    assert list(res) == [(rdflib.term.URIRef('http://example.org/book/book1'),)]