Make sure the code doesn't fail for old-style classes

edx_lint/pylint/required_base_class.py

@@ -56,7 +56,12 @@ class RequiredBaseClassChecker(BaseChecker):
         if not self.class_map:
             return
 
-        all_bases = [usable_class_name(c) for c in node.mro()]
+        try:
+            all_bases = [usable_class_name(c) for c in node.mro()]
+        except NotImplementedError:
+            # Old-style 2.x classes have no mro.
+            all_bases = []
+
         for base in all_bases:
             required = self.class_map.get(base)
             if required is not None:

test/test_required_base_class.py

@@ -67,3 +67,14 @@ class RequiredBaseClassTestCase(CheckerTestCase):
         ''')
         with self.assertNoMessages():
             self.checker.visit_class(node)
+
+    @set_config(required_base_class=["unittest.case.TestCase:.MyTestMixin"])
+    def test_old_style_classes(self):
+        # We don't support base class checking on old-style classes, but we
+        # have to be sure not to fall over at least.
+        node = self.get_class_node('''
+            class MyClass:
+                pass
+        ''')
+        with self.assertNoMessages():
+            self.checker.visit_class(node)