From: Jeffrey Yasskin <jyasskin@gmail.com>
Date: Mon, 17 Mar 2008 16:31:21 +0000 (+0000)
Subject: Make isinstance(OldstyleClass, NewstyleClass) return False instead of raising
X-Git-Tag: v2.6a2~307
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=b9e15f7555ed6596fa0af15f457389827ecab8f6;p=python

Make isinstance(OldstyleClass, NewstyleClass) return False instead of raising
an exception. Issue reported by Joseph Armbruster.
---

diff --git a/Lib/abc.py b/Lib/abc.py
index 5da2590e4f..5e90bf5f1a 100644
--- a/Lib/abc.py
+++ b/Lib/abc.py
@@ -116,18 +116,18 @@ class ABCMeta(type):
 
     def __instancecheck__(cls, instance):
         """Override for isinstance(instance, cls)."""
-        # Inline the cache checking for new-style classes.
-        subclass = instance.__class__
+        # Inline the cache checking when it's simple.
+        subclass = getattr(instance, '__class__', None)
         if subclass in cls._abc_cache:
             return True
         subtype = type(instance)
-        if subtype is subclass:
+        if subtype is subclass or subclass is None:
             if (cls._abc_negative_cache_version ==
                 ABCMeta._abc_invalidation_counter and
-                subclass in cls._abc_negative_cache):
+                subtype in cls._abc_negative_cache):
                 return False
             # Fall back to the subclass check.
-            return cls.__subclasscheck__(subclass)
+            return cls.__subclasscheck__(subtype)
         return (cls.__subclasscheck__(subclass) or
                 cls.__subclasscheck__(subtype))
 
diff --git a/Lib/test/test_abc.py b/Lib/test/test_abc.py
index f1c0c3b79f..8fed220d22 100644
--- a/Lib/test/test_abc.py
+++ b/Lib/test/test_abc.py
@@ -70,6 +70,17 @@ class TestABC(unittest.TestCase):
         self.assertFalse(issubclass(OldstyleClass, A))
         self.assertFalse(issubclass(A, OldstyleClass))
 
+    def test_isinstance_class(self):
+        class A:
+            __metaclass__ = abc.ABCMeta
+        class OldstyleClass:
+            pass
+        self.assertFalse(isinstance(OldstyleClass, A))
+        self.assertTrue(isinstance(OldstyleClass, type(OldstyleClass)))
+        self.assertFalse(isinstance(A, OldstyleClass))
+        # This raises a recursion depth error, but is low-priority:
+        # self.assertTrue(isinstance(A, abc.ABCMeta))
+
     def test_registration_basics(self):
         class A:
             __metaclass__ = abc.ABCMeta