Literal total ordering

rdflib/term.py

@@ -812,6 +812,9 @@ def __gt__(self, other):
                     return self.language > other.language
 
             if self.value != None and other.value != None:
+                if type(self.value) in _NO_TOTAL_ORDER_TYPES:
+                    comparator = _NO_TOTAL_ORDER_TYPES[type(self.value)]
+                    return comparator(self.value) > comparator(other.value)
                 return self.value > other.value
 
             if text_type(self) != text_type(other):
@@ -1397,6 +1400,14 @@ def _writeXML(xmlnode):
     _XSD_DECIMAL,
 )
 
+# these are not guranteed to sort because it is not possible
+# to calculate a total order over all valid members of the type
+# the function must partition the type into subtypes that do have total orders
+_NO_TOTAL_ORDER_TYPES = {
+    datetime:lambda value:bool(value.tzinfo),
+    time:lambda value:bool(value.tzinfo),
+    xml.dom.minidom.Document:lambda value:value.toxml(),
+}
 
 def _castPythonToLiteral(obj):
     """

test/test_term.py

@@ -55,6 +55,28 @@ def test_base64_values(self):
         self.assertEqual(lit.value, decoded_b64msg)
         self.assertEqual(str(lit), b64msg)
 
+    def test_total_order(self):
+        types = {
+            XSD.dateTime:('0001-01-01T00:00:00', '0001-01-01T00:00:00Z',
+                          '0001-01-01T00:00:00-00:00'),
+            XSD.date:('0001-01-01', '0001-01-01Z', '0001-01-01-00:00'),
+            XSD.time:('00:00:00', '00:00:00Z', '00:00:00-00:00'),
+            XSD.gYear:('0001', '0001Z', '0001-00:00'),  # interval
+            XSD.gYearMonth:('0001-01', '0001-01Z', '0001-01-00:00'),
+        }
+        literals = [Literal(literal, datatype=type)
+                    for type, literals in types.items()
+                    for literal in literals]
+        try:
+            sorted(literals)
+            orderable = True
+        except TypeError as e:
+            for l in literals:
+                print(repr(l), repr(l.value))
+            print(e)
+            orderable = False
+        self.assertTrue(orderable)
+
 
 class TestValidityFunctions(unittest.TestCase):