From 97f59b96a215d81f2bfff72f05a6cce1df20eef0 Mon Sep 17 00:00:00 2001
From: Jozef <knaperek@users.noreply.github.com>
Date: Thu, 9 Mar 2017 09:33:28 +0100
Subject: [PATCH] Fix regression in 0.13 that breaks old-style classes

This is causing crashes with other (unrelated) classes that are old-style.
---
 bidict/_common.py | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/bidict/_common.py b/bidict/_common.py
index 2b52e4a..034504b 100644
--- a/bidict/_common.py
+++ b/bidict/_common.py
@@ -43,7 +43,11 @@ class BidirectionalMapping(Mapping):
         Causes conforming classes to be virtual subclasses automatically.
         """
         if cls is BidirectionalMapping:
-            mro = C.__mro__
+            try:
+                mro = C.__mro__
+            except AttributeError:
+                # Old-style classes in Python 2.7 do not have the __mro__ attribute!
+                return False
             return all(any(B.__dict__.get(i) for B in mro) for i in cls._subclsattrs)
         return NotImplemented