fixes for turtle/trig namespace handling

raise exception when trying to rebind a prefix to another ns.
fix broken rebinding when generating prefixes

This fixes #679 - but actually it's more like a work-around. The
underlying problem is confusion about context and graph objects (#167)

rdflib/plugins/serializers/turtle.py

@@ -47,6 +47,8 @@ def __init__(self, store):
         self.reset()
 
     def addNamespace(self, prefix, uri):
+        if prefix in self.namespaces and self.namespaces[prefix]!=uri:
+            raise Exception("Trying to override namespace prefix %s => %s, but it's already bound to %s"%(prefix, uri, self.namespaces[prefix]))
         self.namespaces[prefix] = uri
 
     def checkSubject(self, subject):
@@ -195,7 +197,8 @@ def addNamespace(self, prefix, namespace):
                     p = "p" + p
                 self._ns_rewrite[prefix] = p
 
-        prefix = self._ns_rewrite.get(prefix, prefix)
+            prefix = self._ns_rewrite.get(prefix, prefix)
+
         super(TurtleSerializer, self).addNamespace(prefix, namespace)
         return prefix
 

test/test_trig.py

@@ -2,6 +2,7 @@
 import rdflib
 import re
 
+from nose import SkipTest
 from rdflib.py3compat import b
 
 TRIPLE = (rdflib.URIRef("http://example.com/s"),
@@ -118,7 +119,7 @@ def testGraphParsing(self):
 
     def testRoundTrips(self):
 
-        raise unittest.SkipTest('skipped until 5.0')
+        raise SkipTest('skipped until 5.0')
 
         data = """
 <http://example.com/thing#thing_a> <http://example.com/knows> <http://example.com/thing#thing_b> .
@@ -153,7 +154,7 @@ def testDefaultGraphSerializesWithoutName(self):
         g.parse(data=data, format='trig')
         data = g.serialize(format='trig')
 
-        self.assertTrue('None' not in data)
+        self.assertTrue(b('None') not in data)
 
     def testPrefixes(self):
 
@@ -173,6 +174,6 @@ def testPrefixes(self):
         cg.parse(data=data, format='trig')
         data = cg.serialize(format='trig')
 
-        self.assert_('ns2: <http://ex.org/docs/' in data, data)
-        self.assert_('<ns2:document1>' not in data, data)
-        self.assert_('ns2:document1' in data, data)
+        self.assert_(b('ns2: <http://ex.org/docs/') in data, data)
+        self.assert_(b('<ns2:document1>') not in data, data)
+        self.assert_(b('ns2:document1') in data, data)