Pull request for issue 2957 - subselect fails under optional

test/test_sparql/test_subselectOptional.py

@@ -0,0 +1,97 @@
+# -*- coding: utf-8 -*-
+"""
+Created on Sat Apr  6 18:21:28 2024
+
+See issue https://github.com/RDFLib/rdflib/issues/2957.
+
+@author: Administrator
+"""
+from rdflib import Graph, Literal, URIRef, Variable
+import os
+
+current_dir = os.getcwd()
+
+GraphString = '''
+
+prefix ex:  <https://www.example.org/>
+
+ex:document ex:subject "Nice cars" .
+
+ex:document ex:theme "Car" .
+
+'''
+
+someGraph = Graph()
+
+someGraph.parse(data=GraphString , format="turtle")
+
+# This query states first a normal triple pattern and then an optional clause with a subselect query.    
+Query1 = someGraph.query('''
+
+prefix ex:  <https://www.example.org/> 
+
+select ?subject ?theme
+
+where  {
+
+    ?doc1 ex:subject ?subject.
+
+    OPTIONAL
+     {
+       select ?theme
+       where {
+              ?doc1 ex:theme ?theme.       
+       }
+      }
+    }
+''')
+
+filepath = current_dir+"/output1.txt"
+
+# Print the results with variable names and their bindings
+print ('\nResults for Query1: \n')
+for row in Query1:
+        print ("Result 'subject: "+ str(row.subject) + "'\nResult should show 'subject: Nice cars' \n")
+        print ("Result 'theme: " + str(row.theme) + "'\nResult should show 'theme: Car'")
+
+        with open(filepath, 'w', encoding='utf-8') as file:
+           file.write(str(row))
+# Result should yield two rows:
+#   subject: Nice cars
+#   theme: Car
+
+
+# This query states first an optional clause with a subselect query and then a normal triple pattern.
+Query2 = someGraph.query('''
+
+prefix ex:  <https://www.example.org/> 
+
+select ?subject ?theme
+
+where  {
+
+    OPTIONAL
+     {
+       select ?theme
+       where {
+              ?doc1 ex:theme ?theme.       
+       }
+      }
+     ?doc1 ex:subject ?subject.
+    }
+''')
+
+filepath = current_dir+"/output2.txt"
+
+# Print the results with variable names and their bindings
+print ('\nResults for Query2: \n')
+for row in Query2:
+        print ("Result 'subject: "+ str(row.subject) + "'\nResult should show 'subject: Nice cars' \n")
+        print ("Result 'theme: " + str(row.theme) + "'\nResult should show 'theme: Car'")
+
+        with open(filepath, 'w', encoding='utf-8') as file:
+           file.write(str(row))
+
+# Result should yield two rows:
+#   subject: Nice cars
+#   theme: Car