fix urlopen usage in python2 when resolving refs

urlopen() result was used as a context manager in python2 too and this was
causing error.

Author: Gaetano Guerriero

jsonschema/compat.py

@@ -1,3 +1,4 @@
+import contextlib
 import operator
 import sys
 
@@ -27,7 +28,10 @@
         urljoin, urlunsplit, SplitResult, urlsplit as _urlsplit # noqa
     )
     from urllib import unquote  # noqa
-    from urllib2 import urlopen  # noqa
+    import urllib2  # noqa
+    def urlopen(*args, **kwargs):
+        return contextlib.closing(urllib2.urlopen(*args, **kwargs))
+
     str_types = basestring
     int_types = int, long
     iteritems = operator.methodcaller("iteritems")

jsonschema/tests/test_validators.py

@@ -4,7 +4,9 @@
 from io import BytesIO
 from unittest import TestCase
 import json
+import os
 import sys
+import tempfile
 import unittest
 
 from twisted.trial.unittest import SynchronousTestCase
@@ -22,6 +24,12 @@
 from jsonschema.tests.compat import mock
 
 
+if PY3:
+    from urllib.request import pathname2url
+else:
+    from urllib import pathname2url
+
+
 def startswith(validator, startswith, instance, schema):
     if not instance.startswith(startswith):
         yield ValidationError(u"Whoops!")
@@ -1341,6 +1349,15 @@ def fake_urlopen(url):
             pass
         self.assertEqual(resolved, 12)
 
+    def test_it_retrieves_local_refs_via_urlopen(self):
+        with tempfile.NamedTemporaryFile(delete=False, mode='wt') as tempf:
+            self.addCleanup(os.remove, tempf.name)
+            json.dump({'foo': 'bar'}, tempf)
+
+        ref = "file://{}#foo".format(pathname2url(tempf.name))
+        with self.resolver.resolving(ref) as resolved:
+            self.assertEqual(resolved, 'bar')
+
     def test_it_can_construct_a_base_uri_from_a_schema(self):
         schema = {"id": "foo"}
         resolver = validators.RefResolver.from_schema(