fix: dataset nquads serialization including RDFLib internal default graph identifier

rdflib/plugins/serializers/nquads.py

@@ -3,7 +3,7 @@
 import warnings
 from typing import IO, Any, Optional
 
-from rdflib.graph import ConjunctiveGraph, Graph
+from rdflib.graph import DATASET_DEFAULT_GRAPH_ID, ConjunctiveGraph, Graph
 from rdflib.plugins.serializers.nt import _quoteLiteral
 from rdflib.serializer import Serializer
 from rdflib.term import Literal
@@ -45,17 +45,18 @@ def serialize(
 
 
 def _nq_row(triple, context):
+    graph_name = context.n3() if context and context != DATASET_DEFAULT_GRAPH_ID else ""
     if isinstance(triple[2], Literal):
         return "%s %s %s %s .\n" % (
             triple[0].n3(),
             triple[1].n3(),
             _quoteLiteral(triple[2]),
-            context.n3(),
+            graph_name,
         )
     else:
         return "%s %s %s %s .\n" % (
             triple[0].n3(),
             triple[1].n3(),
             triple[2].n3(),
-            context.n3(),
+            graph_name,
         )

test/test_serializers/test_nquads_default_graph.py

@@ -0,0 +1,39 @@
+from rdflib import Dataset
+from rdflib.compare import isomorphic
+
+
+def test_nquads_default_graph():
+    data = """
+        @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
+        @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
+        @prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
+
+        {
+            <urn:test> <http://www.w3.org/ns/prov#generatedAtTime> "2012-04-09"^^xsd:date .
+        }
+
+        <urn:test> {
+            <http://greggkellogg.net/foaf#me> a <http://xmlns.com/foaf/0.1/Person> ;
+                <http://xmlns.com/foaf/0.1/knows> "http://manu.sporny.org/about#manu" ;
+                <http://xmlns.com/foaf/0.1/name> "Gregg Kellogg" .
+
+            <http://manu.sporny.org/about#manu> a <http://xmlns.com/foaf/0.1/Person> ;
+                <http://xmlns.com/foaf/0.1/knows> "http://greggkellogg.net/foaf#me" ;
+                <http://xmlns.com/foaf/0.1/name> "Manu Sporny" .
+        }
+    """
+
+    ds = Dataset()
+    ds.parse(data=data, format="trig")
+    output = ds.serialize(format="nquads")
+
+    # The internal RDFLib default graph identifier should not appear in the output.
+    assert "<urn:x-rdflib:default>" not in output
+
+    # Ensure dataset round-trip still works.
+    ds2 = Dataset()
+    ds2.parse(data=output, format="nquads")
+    for graph in ds.graphs():
+        assert isomorphic(graph, ds2.graph(graph.identifier)), print(
+            f"{graph.identifier} not isomorphic"
+        )