allow setting of @base via prefix _base

Author: cmungall

linkml_runtime/dumpers/rdflib_dumper.py

@@ -47,8 +47,11 @@ def as_rdf_graph(self, element: YAMLRoot, schemaview: SchemaView, prefix_map: Di
         else:
             for prefix in schemaview.namespaces():
                 g.bind(prefix, URIRef(schemaview.namespaces()[prefix]))
-        if schemaview.namespaces()._base:
-            g.base = schemaview.namespaces()._base
+        # user can pass in base in prefixmap using '_base'. This gets set
+        # in namespaces as a plain dict assignment - explicitly call the setter
+        # to set the underlying "@base"
+        if "_base" in schemaview.namespaces():
+            schemaview.namespaces()._base = schemaview.namespaces()["_base"]
         self.inject_triples(element, schemaview, g)
         return g