Index: Lib/test/test_typechecks.py
===================================================================
--- Lib/test/test_typechecks.py	(revision 0)
+++ Lib/test/test_typechecks.py	(revision 0)
@@ -0,0 +1,45 @@
+"""Unit tests for __instancecheck__ and __subclasscheck__."""
+
+import unittest
+from test import test_support
+
+
+class ABC(type):
+
+    def __instancecheck__(cls, inst):
+        """Implement isinstance(inst, cls)."""
+        return any(cls.__subclasscheck__(c)
+                   for c in {type(inst), inst.__class__})
+
+    def __subclasscheck__(cls, sub):
+        """Implement issubclass(sub, cls)."""
+        candidates = cls.__dict__.get("__subclass__", set())
+        return any(c in candidates for c in sub.mro())
+
+
+class Integer(metaclass=ABC):
+
+    __subclass__ = {int}
+
+
+class TypeChecksTest(unittest.TestCase):
+
+    def testIsSubclassInternal(self):
+        self.assertEqual(Integer.__subclasscheck__(int), True)
+        self.assertEqual(Integer.__subclasscheck__(float), False)
+
+    def testIsSubclassBuiltin(self):
+        self.assertEqual(issubclass(int, Integer), True)
+        self.assertEqual(issubclass(float, Integer), False)
+
+    def testIsInstanceBuiltin(self):
+        self.assertEqual(isinstance(42, Integer), True)
+        self.assertEqual(isinstance(3.14, Integer), False)
+
+
+def test_main():
+    test_support.run_unittest(TypeChecksTest)
+
+
+if __name__ == "__main__":
+    unittest.main()

Property changes on: Lib/test/test_typechecks.py
___________________________________________________________________
Name: svn:keywords
   + Id
Name: svn:eol-style
   + native

Index: Objects/abstract.c
===================================================================
--- Objects/abstract.c	(revision 54985)
+++ Objects/abstract.c	(working copy)
@@ -2132,7 +2132,22 @@
 int
 PyObject_IsInstance(PyObject *inst, PyObject *cls)
 {
-    return recursive_isinstance(inst, cls, Py_GetRecursionLimit());
+	PyObject *t, *v, *tb;
+	PyErr_Fetch(&t, &v, &tb);
+	PyObject *checker = PyObject_GetAttrString(cls, "__instancecheck__");
+	PyErr_Restore(t, v, tb);
+	if (checker != NULL) {
+		PyObject *res = PyObject_CallFunctionObjArgs(checker, inst,
+							     NULL);
+		int ok = -1;
+		Py_DECREF(checker);
+		if (res != NULL) {
+			ok = PyObject_IsTrue(res);
+			Py_DECREF(res);
+		}
+		return ok;
+	}
+	return recursive_isinstance(inst, cls, Py_GetRecursionLimit());
 }
 
 static  int
@@ -2182,7 +2197,22 @@
 int
 PyObject_IsSubclass(PyObject *derived, PyObject *cls)
 {
-    return recursive_issubclass(derived, cls, Py_GetRecursionLimit());
+	PyObject *t, *v, *tb;
+	PyErr_Fetch(&t, &v, &tb);
+	PyObject *checker = PyObject_GetAttrString(cls, "__subclasscheck__");
+	PyErr_Restore(t, v, tb);
+	if (checker != NULL) {
+		PyObject *res = PyObject_CallFunctionObjArgs(checker, derived,
+							     NULL);
+		int ok = -1;
+		Py_DECREF(checker);
+		if (res != NULL) {
+			ok = PyObject_IsTrue(res);
+			Py_DECREF(res);
+		}
+		return ok;
+	}
+	return recursive_issubclass(derived, cls, Py_GetRecursionLimit());
 }