Add a cbd_subject() method

rdflib/graph.py

@@ -951,6 +951,12 @@ def triples_choices(
         ):
             yield s, p, o
 
+    def roots(self) -> Generator["_SubjectType", None, None]:
+        """A generator of subjects that are roots of the graph"""
+        for s in self.subjects(unique=True):
+            if (None, None, s) not in self:
+                yield s
+
     @overload
     def value(
         self,
@@ -1873,6 +1879,23 @@ def add_to_cbd(uri: _SubjectType) -> None:
 
         return subgraph
 
+    def cbd_subject(self) -> Optional[_SubjectType]:
+        """Determine the subject for which the graph is a Concise Bounded Description
+
+        :return: The subject of the CBD or None
+        """
+        roots = set(self.roots())
+        if len(roots) != 1:
+            # A CBD has exactly one root
+            return None
+        root = roots.pop()
+
+        real_cbd = self.cbd(root)
+        if not real_cbd.isomorphic(self):
+            return None
+
+        return root
+
 
 _ContextType = Graph
 

test/test_graph/test_graph_cbd.py

@@ -159,3 +159,27 @@ def test_cbd_target(rdfs_graph: Graph):
 
     assert result is target
     assert expected_result == set(result.triples((None, None, None)))
+
+
+def test_cbd_subject(get_graph):
+    g = get_graph
+
+    assert g.cbd(EX.R1).cbd_subject() == (
+        EX.R1
+    ), "cbd_subject() for CBD of EX.R1 should be EX.R1"
+    assert g.cbd(EX.R2).cbd_subject() == (
+        EX.R2
+    ), "cbd_subject() for CBD of EX.R2 should be EX.R2"
+    assert g.cbd(EX.R3).cbd_subject() == (
+        EX.R3
+    ), "cbd_subject() for CBD of EX.R3 should be EX.R3"
+    assert g.cbd(EX.R4).cbd_subject() == (
+        None
+    ), "cbd_subject() for CBD of EX.R4 should be None"
+
+    test_g = g.cbd(EX.R1)
+    test_g.add((EX.R2, EX.propOne, EX.P1))
+
+    assert test_g.cbd_subject() is (
+        None
+    ), "cbd_subject() of graph with an additional subject should be None"