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

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"