sparqls optionals clause can now bind variables. with test. issue 2957

Author: WhiteGobo

rdflib/plugins/sparql/evaluate.py

@@ -180,7 +180,7 @@ def evalLeftJoin(
  for b in evalPart(c, join.p2):
  if _ebv(join.expr, b.forget(ctx)):
  ok = True
- yield b
+ yield b.merge(a)
  if not ok:
  # we've cheated, the ctx above may contain
  # vars bound outside our scope

test/test_sparql/test_optional.py

@@ -0,0 +1,37 @@
+from rdflib import Graph, Literal, URIRef, Variable
+
+
+def test_binding_with_optional_clause() -> None:
+ """
+ Optional clauses should bind variables if feasible.
+
+ See https://github.com/RDFLib/rdflib/issues/2957
+ """
+ g = Graph().parse(
+ data="""
+ prefix ex: <https://www.example.org/>
+ ex:document ex:subject "Nice cars" .
+ ex:someCar ex:type "Car" .
+ """
+ )
+ result = g.query(
+ """prefix ex: <https://www.example.org/>
+ select ?subject ?car
+ where {
+ $this ex:subject ?subject.
+ optional
+ {
+ # an offending subselect clause
+ select ?car
+ where {
+ ?car ex:type "Car".
+ }
+ }
+ }"""
+ )
+ assert len(result.bindings) == 1
+ (first,) = result.bindings
+ assert first.get(Variable("car")) == URIRef("https://www.example.org/someCar")
+ assert first.get(Variable("subject")) == Literal(
+ "Nice cars"
+ ), "optional clause didnt bind"