# HG changeset patch
# User Ed Catmur <ed@catmur.co.uk>
# Date 1371493767 -3600
#      Mon Jun 17 19:29:27 2013 +0100
# Branch singledispatch-mro-18244
# Node ID 22986b6763145c05d132e445b22c80ac6a9b198d
# Parent  ec4b0145f7e5e9d95c82b0b428d7daea2dde4a6b
Issue 18244: When virtual-inheriting ABCs at distinct levels in the MRO, the composed MRO should not depend on the haystack ordering.

diff -r ec4b0145f7e5 -r 22986b676314 Lib/functools.py
--- a/Lib/functools.py	Mon Jun 17 15:45:11 2013 +0200
+++ b/Lib/functools.py	Mon Jun 17 19:29:27 2013 +0100
@@ -376,14 +376,18 @@
         if (needle in bases or not hasattr(needle, '__mro__')
                             or not issubclass(cls, needle)):
             continue   # either present in the __mro__ already or unrelated
-        for index, base in enumerate(mro):
-            if not issubclass(base, needle):
+        for i, base in enumerate(mro):
+            if issubclass(base, needle):
+                index = i + 1
+            elif base in bases:
                 break
-        if base in bases and not issubclass(needle, base):
+        if not issubclass(needle, base):
             # Conflict resolution: put classes present in __mro__ and their
             # subclasses first. See test_mro_conflicts() in test_functools.py
             # for examples.
-            index += 1
+            for index, base in enumerate(mro[i + 1:], i + 1):
+                if not issubclass(base, needle):
+                    break
         mro.insert(index, needle)
     return mro
 
diff -r ec4b0145f7e5 -r 22986b676314 Lib/test/test_functools.py
--- a/Lib/test/test_functools.py	Mon Jun 17 15:45:11 2013 +0200
+++ b/Lib/test/test_functools.py	Mon Jun 17 19:29:27 2013 +0100
@@ -946,6 +946,15 @@
         m = mro(c.defaultdict, [c.Container, c.Sized, str])
         self.assertEqual(m, [c.defaultdict, dict, c.Sized, c.Container, object])
 
+        # This MRO should not depend on haystack ordering
+        class C(c.defaultdict):
+            def __call__(self):
+                pass
+        m = mro(C, [c.Callable, c.Container])
+        self.assertEqual(m, [C, c.defaultdict, c.Callable, dict, c.Container, object])
+        m = mro(C, [c.Container, c.Callable])
+        self.assertEqual(m, [C, c.defaultdict, c.Callable, dict, c.Container, object])
+
     def test_register_abc(self):
         c = collections
         d = {"a": "b"}