Make Store.namespaces an empty generator (#1432)

Fixes #1431

Author: aucampia

rdflib/store.py

@@ -364,6 +364,11 @@ def namespace(self, prefix):
 
     def namespaces(self):
         """ """
+        # This is here so that the function becomes an empty generator.
+        # See https://stackoverflow.com/q/13243766 and
+        # https://www.python.org/dev/peps/pep-0255/#why-a-new-keyword-for-yield-why-not-a-builtin-function-instead
+        if False:
+            yield None  # type: ignore[unreachable]
 
     # Optional Transactional methods
 

test/test_store.py

@@ -0,0 +1,25 @@
+import unittest
+from rdflib import Graph
+from rdflib.store import Store
+from rdflib.namespace import NamespaceManager
+
+
+class TestAbstractStore(unittest.TestCase):
+    def test_namespaces(self):
+        """
+        This tests that Store.namespaces is an empty generator.
+        """
+        store = Store()
+        self.assertEqual(list(store.namespaces()), [])
+
+    def test_namespaces_via_manager(self):
+        """
+        This tests that NamespaceManager.namespaces works correctly with an
+        abstract Store.
+        """
+        namespace_manager = NamespaceManager(Graph(store=Store()))
+        self.assertEqual(list(namespace_manager.namespaces()), [])
+
+
+if __name__ == "__main__":
+    unittest.main()