feat: do not write prefix for empty graph id (#2160)

Since the names of empty graphs do not appear in the serialization, do not consider them for namespace issues.

2 files changed, 9 insertions, 0 deletions

diff --git a/rdflib/plugins/serializers/trig.py b/rdflib/plugins/serializers/trig.py
index 0fa62b71..d4052b29 100644
--- a/rdflib/plugins/serializers/trig.py
+++ b/rdflib/plugins/serializers/trig.py
@@ -34,6 +34,9 @@ class TrigSerializer(TurtleSerializer):
 
     def preprocess(self):
         for context in self.contexts:
+            # do not write unnecessary prefix (ex: for an empty default graph)
+            if len(context) == 0:
+                continue
             self.store = context
             self.getQName(context.identifier)
             self._subjects = {}
diff --git a/test/test_trig.py b/test/test_trig.py
index dbb1adbf..30bff634 100644
--- a/test/test_trig.py
+++ b/test/test_trig.py
@@ -192,3 +192,9 @@ def test_prefixes():
     assert "ns2: <http://ex.org/docs/".encode("latin-1") in data, data
     assert "<ns2:document1>".encode("latin-1") not in data, data
     assert "ns2:document1".encode("latin-1") in data, data
+
+
+def test_issue_2154():
+    ds = rdflib.Dataset()
+    sg = ds.serialize(format="trig")
+    assert "prefix" not in sg, sg