Re: What does Substitute() actually do?

[Andy Seaborne]

On 12/02/2025 21:03, Martynas Jusevičius wrote:

Hi Andy,

I understand the differences of how they are implemented.

I think what I'm missing is a sort of overview of when those different
approaches should be used, and also examples of when they produce
semantically different queries.

string template substitution can alter or break queries.

Replace ?X in {?s :p ?X} with the  '"text" . FILTER (?s != :hideMe )'

https://xkcd.com/327/

    Andy

Martynas

On Thu, Feb 6, 2025 at 5:59 PM Andy Seaborne <a...@apache.org> wrote:

On 06/02/2025 03:24, Nicholas Car wrote:

You could try running that with RDFlib and see what it does….

How is substitute() different to just string template substitution? Is it just 
a convenience function so substitution can be done on a Jena SPARQL object 
(perhaps algebra) rather than a string?

It does not work on a string - when working with Query objects (e.g.
service enhancer), manipulate is on Query objects.
It does does not have injection attacks.

There some places where you can't replace a variable by a value

BIND(123  AS ?X)

for example. QueryTransforOps checks.

Substitute.substitue is a collect of algebra to algebra changes - some
of which are used in execution.

Is that your goal for an RDFLib equivalent?

See also ParameterizedSparqlString in Jena which is injection-safe.
That's closer to string replacement.

Under the general concept of "substitute" there are different uses cases
on different objects.

      Andy

SPARQL string to/from SPARQL grammar object is handles in RDFLib fine so I 
guess as I think the code says, an RDFLib substitute() could work on either or 
both.

Nick

On Thu, Feb 6, 2025 at 10:05 am, Martynas Jusevičius <[marty...@atomgraph.com](mailto:On 
Thu, Feb 6, 2025 at 10:05 am, Martynas Jusevičius <<a href=)> wrote:

OK I simply fed Jena's source to ChatGPT and asked it to produce an
RDFLIb version :)
No idea if it's correct but I'll find out I guess.

from rdflib import Variable, BNode, Literal
from rdflib.plugins.sparql.algebra import BGP, Filter, Join, LeftJoin,
Union, Graph, Extend, Minus, OrderBy, Project

def substitute(algebra, binding):
if isinstance(algebra, BGP):
new_triples = []
for s, p, o in algebra.triples:
s = binding.get(s, s)
p = binding.get(p, p)
o = binding.get(o, o)
new_triples.append((s, p, o))
return BGP(new_triples)
elif isinstance(algebra, Filter):
new_p = substitute(algebra.p, binding)
new_expr = algebra.expr
for var, val in binding.items():
new_expr = new_expr.substitute(var, val)
return Filter(new_expr, new_p)
elif isinstance(algebra, Join):
return Join(substitute(algebra.p1, binding),
substitute(algebra.p2, binding))
elif isinstance(algebra, LeftJoin):
return LeftJoin(substitute(algebra.p1, binding),
substitute(algebra.p2, binding), algebra.expr)
elif isinstance(algebra, Union):
return Union(substitute(algebra.p1, binding),
substitute(algebra.p2, binding))
elif isinstance(algebra, Graph):
return Graph(algebra.term, substitute(algebra.p, binding))
elif isinstance(algebra, Extend):
new_p = substitute(algebra.p, binding)
new_expr = algebra.expr
for var, val in binding.items():
new_expr = new_expr.substitute(var, val)
return Extend(new_p, new_expr, algebra.var)
elif isinstance(algebra, Minus):
return Minus(substitute(algebra.p1, binding),
substitute(algebra.p2, binding))
elif isinstance(algebra, OrderBy):
return OrderBy(substitute(algebra.p, binding), algebra.expr)
elif isinstance(algebra, Project):
return Project(substitute(algebra.p, binding), algebra.PV)
else:
return algebra

On Thu, Feb 6, 2025 at 12:54 AM Martynas Jusevičius
<marty...@atomgraph.com> wrote:

Hi,

I was looking to implement something like substitute() using RDFLib.

I checked the note:
https://afs.github.io/substitute.htm
But I couldn't find a clear example.

For example, how would this query string (the template)

PREFIX dbo: <http://dbpedia.org/ontology/>
CONSTRUCT
WHERE { ?city dbo:populationTotal ?population }

would look like after substituting (?city,
<http://dbpedia.org/resource/Copenhagen>)?


Martynas