Add optional graph argument to Graph.cbd, use this new argument in ev…

…alDescribeQuery

rdflib/graph.py

@@ -1816,7 +1816,7 @@ def do_de_skolemize2(t: _TripleType) -> _TripleType:
 
         return retval
 
-    def cbd(self, resource: _SubjectType) -> Graph:
+    def cbd(self, resource: _SubjectType, target_graph: Optional[Graph] = None) -> Graph:
         """Retrieves the Concise Bounded Description of a Resource from a Graph
 
         Concise Bounded Description (CBD) is defined in [1] as:
@@ -1842,10 +1842,14 @@ def cbd(self, resource: _SubjectType) -> Graph:
         [1] https://www.w3.org/Submission/CBD/
 
         :param resource: a URIRef object, of the Resource for queried for
-        :return: a Graph, subgraph of self
+        :param target_graph: Optionally, a graph to add the CBD to; otherwise, a new graph is created for the CBD
+        :return: a Graph, subgraph of self if no graph was provided otherwise the provided graph
 
         """
-        subgraph = Graph()
+        if target_graph is None:
+            subgraph = Graph()
+        else:
+            subgraph = target_graph
 
         def add_to_cbd(uri: _SubjectType) -> None:
             for s, p, o in self.triples((uri, None, None)):

rdflib/plugins/sparql/evaluate.py

@@ -630,7 +630,7 @@ def evalDescribeQuery(ctx: QueryContext, query) -> Dict[str, Union[str, Graph]]:
     # Get a CBD for all resources identified to describe
     for resource in to_describe:
         # type error: Item "None" of "Optional[Graph]" has no attribute "cbd"
-        graph += ctx.graph.cbd(resource)  # type: ignore[union-attr]
+        ctx.graph.cbd(resource, graph)  # type: ignore[union-attr]
 
     res: Dict[str, Union[str, Graph]] = {}
     res["type_"] = "DESCRIBE"