Check and rewrite namespace prefixes starting with _ in turtle serial…

…izer. fixes #161

rdflib/plugins/serializers/turtle.py

@@ -150,11 +150,35 @@ def __init__(self, store):
         self.reset()
         self.stream = None
         self._spacious = _SPACIOUS_OUTPUT
+        self._ns_rewrite={}
+
+    def addNamespace(self, prefix, namespace):
+        # Turtle does not support prefix that start with _ 
+        # if they occur in the graph, rewrite to p_blah
+        # this is more complicated since we need to make sure p_blah
+        # does not already exist. And we register namespaces as we go, i.e.
+        # we may first see a triple with prefix _9 - rewrite it to p_9
+        # and then later find a triple with a "real" p_9 prefix 
+
+        # so we need to keep track of ns rewrites we made so far.
+
+        if prefix[0]=='_' or self.namespaces.get(prefix, namespace)!=namespace:
+
+            if prefix not in self._ns_rewrite:
+                p="p"+prefix
+                while p in self.namespaces:
+                    p="p"+p
+                self._ns_rewrite[prefix]=p
+
+        prefix=self._ns_rewrite.get(prefix,prefix)
+        super(TurtleSerializer, self).addNamespace(prefix, namespace)
+        return prefix
 
     def reset(self):
         super(TurtleSerializer, self).reset()
         self._shortNames = {}
         self._started = False
+        self._ns_rewrite={}
 
     def serialize(self, stream, base=None, encoding=None, spacious=None, **args):
         self.reset()
@@ -220,7 +244,8 @@ def getQName(self, uri, gen_prefix=True):
         # Local parts with '.' will mess up serialization
         if '.' in local:
             return None
-        self.addNamespace(prefix, namespace)
+        prefix=self.addNamespace(prefix, namespace)
+
         return u'%s:%s' % (prefix, local)
 
     def startDocument(self):

test/test_issue161.py

@@ -1,28 +1,36 @@
 from unittest import TestCase
 from rdflib.graph import ConjunctiveGraph
-from nose.exc import SkipTest
+
 
 class EntityTest(TestCase):
 
     def test_turtle_namespace_prefixes(self):
-        raise SkipTest('Skipping: Turtle serializer preserves n3 prefixes (eg. "_9") that violate Turtle syntax.')
+
         g = ConjunctiveGraph()
         n3 = \
         """
         @prefix _9: <http://data.linkedmdb.org/resource/movie/> .
+        @prefix p_9: <urn:test:> .
         @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
 
+        p_9:a p_9:b p_9:c . 
+
         <http://data.linkedmdb.org/resource/director/1> a
         <http://data.linkedmdb.org/resource/movie/director>;
             rdfs:label "Cecil B. DeMille (Director)";
             _9:director_name "Cecil B. DeMille" ."""
+
+
         g.parse(data=n3, format='n3')
         turtle = g.serialize(format="turtle")
+
         # Check round-tripping, just for kicks.
         g = ConjunctiveGraph()
         g.parse(data=turtle, format='turtle')
         # Shouldn't have got to here
-        self.assert_('_9' not in g.serialize(format="turtle"))
+        s=g.serialize(format="turtle")
+
+        self.assert_('@prefix _9' not in s)