fix: Prevent Collection from add 'rdf:nil rdf:rest rdf:nil.' triples

Creating 'Collection(graph, RDF.nil)' used to add a 'rdf:nil rdf:rest rdf:nil.'
triple to the graph, which turned the empty list into an infinite list.

This was a side-effect of unconditionally appending 'seq' to the collection,
which is not possible when the collection is 'nil' without ill side-effects.

This commit makes the append conditional; and also checks the collection
is non-empty in case the user explicitly tries to add item to the empty list.

Author: progval

rdflib/collection.py

@@ -49,12 +49,16 @@ class Collection:
     True
     >>> c.index(Literal(2)) == 1
     True
+
+    The collection is immutable if ``uri`` is the empty list
+    (``http://www.w3.org/1999/02/22-rdf-syntax-ns#nil``).
     """
 
     def __init__(self, graph: Graph, uri: Node, seq: List[Node] = []):
         self.graph = graph
         self.uri = uri or BNode()
-        self += seq
+        if seq:
+            self += seq
 
     def n3(self) -> str:
         """
@@ -208,6 +212,8 @@ def __iter__(self) -> Iterator[Node]:
         return self.graph.items(self.uri)
 
     def _end(self) -> Node:
+        if self.uri == RDF.nil:
+            raise ValueError("Cannot append to empty list")
         # find end of list
         container = self.uri
         while True:

test/test_misc/test_collection.py

@@ -2,7 +2,7 @@
 
 import pytest
 
-from rdflib import BNode, Graph, Literal
+from rdflib import BNode, Graph, Literal, URIRef
 from rdflib.collection import Collection
 
 
@@ -39,3 +39,14 @@ def test_scenario() -> None:
     c.clear()
 
     assert len(c) == 0
+
+
+def test_empty_list() -> None:
+    nil = URIRef("http://www.w3.org/1999/02/22-rdf-syntax-ns#nil")
+    g = Graph()
+
+    c = Collection(g, nil)
+
+    assert set(g) == set(), "Collection changed the graph"
+
+    assert len(c) == 0