That's self.env.unset(k) and not env.unset(k) I was heading back to the problem.

Lib/test/test_urllib.py

@@ -103,7 +103,7 @@ class ProxyTests(unittest.TestCase):
         # Delete all proxy related env vars
         for k, v in os.environ.iteritems():
             if 'proxy' in k.lower():
-                env.unset(k)
+                self.env.unset(k)
 
     def tearDown(self):
         # Restore all proxy related env vars